  Building with
Straw Bales

A practical manual for self-builders and architects

Barbara Jones

Photograph © Rae Parkinson

Acknowledgements

Very little in this world is totally new or innovative. I have been lucky enough to have been given some wonderful opportunities in my life to travel, to meet extraordinary people, to learn from the ways and cultures of other countries and the heritage of my own. Through these journeys and meetings I have had the good fortune to understand how we might build more simply, how the ideas of the past can be brought into the 21st century, and how ordinary people can be inspired and empowered to participate in the amazing experience of building, as we all used to do. So to everyone who has helped me on this path, I thank you. I extend an especial thank you to my partner, Rae Parkinson, for helping to make love, happiness and inspiration so alive in my life.

CHAPTER 1

Why build with straw bales?

Building with straw makes so much sense! No other building material stores as much carbon within itself, or provides so much insulation for so little cost, or is as easy to build with. So why aren’t there more straw-bale houses? Partly, there is still lack of awareness about this humble building material. Partly, there is fear of something so simple and straightforward. Partly, it’s because of lack of affordable land available for people of limited means but with determination and ability. But mostly, there is no profit to be made in the usual way of a business selling a product, because straw-bale building is as much about passion, about empowerment, about belief – a way of life – as it is about construction.

Photograph © Rae Parkinson

Straw-bale house, Hickling, Norfolk.
© Fran Bradshaw.

Straw-bale building offers us a radical way to solve many of the issues facing construction with respect to thermal efficiency, carbon footprint production, sustainable materials, durability, air quality and cost. It’s far more than just a wall-building technique; it’s a completely different approach to the process of building itself. Its background is grass-roots self-build: it is firmly based in that sustainable ‘green building’ culture that has brought to the construction industry many new and useful ideas about energy efficiency and environmental responsibility.

This method of building is now entering mainstream construction via co-housing projects, designers and architects, and community groups, which see its value in terms of cost-effectiveness, sustainability, ease of installation, air-tightness and energy efficiency. The building method itself is based on a block system, making the designs very easy to adapt from one project to another, and giving great flexibility in its use.

High plinth walls, self-draining foundations, and large overhangs to the roof – ‘a good hat and a good pair of boots’.
© Rae Parkinson.

The accessible nature of straw as a construction material means that even those unfamiliar with this building process can participate in it. This opens the door for interest groups to work together on joint projects. Housing associations, co-operatives, and local authorities are ideal managers for self-build straw projects, which are quick to build and which will engender an excitement and motivation that gets the job done.

The atmosphere on a straw-bale building site is qualitatively different from that found on the vast majority of other sites; it is woman-friendly, joyful, optimistic and highly motivated. Knowledge and skills are freely shared, and cooperation and teamwork predominate; all these factors have a positive effect on health and safety on site.

Working with straw is unlike working with any other material. It is simple, flexible, imprecise and organic. It will challenge your preconceptions about the nature of building and the correct way of doing things – and not everyone will be able to meet this challenge. The simplicity of straw can be disarming, or alarming. If you need complexity in order to feel secure, this may not be for you. Don’t be put off by nursery tales about the big bad wolf – we should be wise enough to realize that the wolf probably worked for the cement manufacturers! And there are thousands of examples of professionally finished buildings – so read on, and make up your own mind.

Straw-bale building enables you to be creative, to design and build the sort of shape and space that suits you. Photograph © Bee Rowan.

Straw as a building material excels in terms of its simple installation, affordability, energy efficiency, and the healthy air quality it produces. Straw-bale houses can be built to Passivhaus standards, and a two-bedroomed detached load-bearing* straw house can be built by a self-builder for £50,000. Due to the buildings’ super-insulation, huge savings in heating costs can be made. Potential savings of up to 75 per cent on long-term running costs can be achieved when compared with a conventional modern house. A typical plastered straw wall has a U-value† of 0.11, more than twice as insulating as Building Regulations require. And because straw houses can be built entirely of natural materials, there is no threat from toxic materials, and no harm to allergy sufferers.

This book is aimed at self-builders and architects as well as the construction industry. It is meant to give clear and straightforward information about how to build houses with bales of straw. Since this is a simple and accessible wall-building technique available to almost anyone, it is an ideal self-build manual; it also provides information for mainstream designers and builders to design with straw, and to write specifications for sustainable house building.

Throughout this book I will be attempting to encourage you towards the best possible way of doing things with a simple, straightforward and common-sense approach. One of the best features of straw-bale building is the opportunity it provides for creative fun, and the way it enables you to design and build the sort of shape and space you’d really like. It lends itself very well to curved and circular shapes, and can provide deep window seats, alcoves and niches due to the thickness of the bales. It is flexible, and combined with flexible foundations (not as unusual as you might think) provides excellent buildings to withstand earthquakes. Its forgiving nature means it can be knocked back into shape fairly easily during wall raising, doesn’t require absolute precision, and can make rounded as well as angular corners. Partly owing to its great insulation value and partly because of its organic nature, the inside of a straw-bale house feels very different from a brick or a stone one, with a cosy, warm quality to it and a pleasing look to the eye. The beauty of straw (apart from its aesthetic beauty) is that it combines very high insulation properties with great load-bearing ability: it is a material that provides building blocks and insulation all in one.

Different styles and opinions have grown up around the world as bale building has spread. What was suitable in one climate has not proved to be best practice in others, and availability and cost of materials varies from country to country. However, there have been wonderfully imaginative adaptations to conditions. Main design concerns are:

alt adequate foundations to deal with moisture and insulation while impacting on the earth as little as possible;

alt airtightness;

alt maximizing solar gain;

alt weather protection during construction;

alt weather protection in general.

By going back to basics, we are able to design more appropriate foundations than became common in the 20th century, which use natural rather than unnatural materials and achieve high levels of thermal efficiency. Differences to be found in the UK are also in the predominant use of load-bearing methods and the type of render (lime) used as a weatherproof coating. In Europe we have been able to draw on the rich knowledge of the past, using ideas that have been tried and tested over centuries. In many respects, the requirements of straw-bale buildings are essentially the same as those of traditional cob (earth) or wattle-and-daub buildings. They have high plinth walls, self-draining foundations and large overhangs to the roof – ‘a good hat and a good pair of boots’, as cob builders used to say. They are also constructed of breathable materials and must not be waterproofed (although they must be weatherproofed). Building with straw encompasses far more than a different wall-building system, however, as the whole building can be constructed of natural materials with very low embodied energy (the energy that was used in making the product) and a negative carbon footprint, all at an affordable price.

Straw is a flexible material and this requires us to work with it somewhat differently from the way we’d work if it were rigid. Accurate measurement and precision is impossible and unnecessary with straw, but working without these aids can be worrying to the novice, and alarming to those already used to building techniques developed in the last century. However, it is very important that you have the right attitude from the outset. You have to develop a feel for the straw. You have to give it time; absorb its flexibility. It is possible to be macho about it – to hurl bales around single-handedly and force them tightly into spaces – but this always has adverse consequences. Rushing the process, and working alone or competitively can mean that an adjoining section of wall is distorted and pushed out of shape – a section that someone else has spent time and care to get right. Straw-bale building is as much a personal learning process as it is about learning a new building technique. More than any other material (together with cob and clay), it is susceptible to your own spirit and that of the team. It is not something to do alone. It requires cooperation, skill-sharing and common sense. Many of the inspirational and artistic features are created in this atmosphere. It is empowering, expanding the world of opportunities for you and making possible what you thought to be impossible!

The atmosphere and environment in which we live is a matter of increasing concern to homeowners and designers alike. There is a growing body of knowledge on the harmful effects of living long-term with modern materials that give off minute but significant amounts of toxins. The more airtight a building is, the more concentrated are the toxins. Living in a straw house protects you from all that. It is a natural, breathable material that has no harmful effects. Hay-fever sufferers are not affected by straw, as it does not contain pollens. Asthmatics also find a straw house a healthier environment in which to live. Combined with a sensible choice of natural plasters and paints, it can positively enhance your quality of life.

When building a straw-bale house, many of the other elements of a conventional building remain. The installation of plumbing, electrics, interior carpentry, joinery and partition walls may be no different from the methods and materials you are used to, though of course they could also be rethought in terms of using natural, locally sourced and recycled materials. This book covers the environmental attributes of straw; how to design an affordable house; the different types of foundation you can build without needing cement; how to build walls with straw and stabilize them; how to protect walls from the weather and make them durable; how straw performs with humidity and how straw-bale buildings can easily meet Building Regulation / Code requirements. There is also a section on frequently asked questions (Appendix 1) and a reference section for further reading, research and contacts (Appendix 2).

History

Straw-bale buildings were first constructed in the USA in the late 1800s, when baling machines were invented. The white settlers on the plains of Nebraska were growing crops in an area without stone or timber with which to build, and while waiting for timber to arrive by wagon train, they built temporary houses out of what was, to them, a waste material – the baled-up straw stalks of the grain crop. They built directly with the bales as if they were giant building blocks, with the bales themselves forming the load-bearing structure. This is known as the Nebraskan or load-bearing style. The settlers discovered that these bale houses kept them warm throughout the very cold winter yet cool during the hot summer, with the additional sound-proofing benefits of protection from the howling winds. Their positive experience of building and living in straw-bale homes led to the building of permanent houses, some of which are still occupied dwellings today!

This early building method flourished until about 1940, when a combination of war (killing off many of the most experienced traditional builders) and the rise in the popularity and use of cement led to its virtual extinction. Then, in the late 1970s/80s, Judy Knox and Matts Myhrman, among other pioneers of the straw-bale revival, rediscovered some of those early houses and set about refining the building method and passing on this knowledge to an eager audience of environmental enthusiasts. Through the green and permaculture movements the ideas spread very rapidly, with most of the new buildings being this self-build, Nebraska / load-bearing style (see Chapter 4, page 35). Before long, new techniques were developed to improve the building method, and The Last Straw journal was founded in Arizona to disseminate ideas, promote good practice, and provide a forum within which owners and builders could network.

The first straw-bale building in the UK was built in 1994, the same year that I first learned how to build with straw on a two-day women’s course run by Jill Lorenzini in California. David Eisenberg of the Development Centre for Appropriate Technology (DCAT), based in Tucson, was invited to the Findhorn eco-community in Scotland in 1995 to help them build a straw-bale shed. My first straw building in the UK was in 1996, and in that year, among other buildings, I helped construct the UK’s very first straw-bale home with full planning permission. This was for Brian Stinchcombe, a farmer in Wales, and you can see a video of an interview with Brian on our website here: www.strawworks.co.uk/ourachievements/

I designed and helped build the first straw-bale building in Ireland in 1996, for the Steiner school in Holywood, Belfast, and there are now thousands of new structures being built annually all over the world. Just about every state in the USA has a straw-bale house, the same in Canada. There are probably close to a thousand in the UK and maybe a hundred or so in Ireland at the present time, most with full planning permission and Building Regulation approval. The UK began building with straw bales earlier than any other European country except France, which has one historic building dating from 1921. France now has 4,000 straw-bale homes with another 1,000 being built in 2014. France has overtaken the UK in the vast numbers of straw-bale houses it has because of its ample availability of affordable land, and relaxed planning laws (compared with the UK) for single self-built dwellings.

Some pioneering individuals and institutions in the UK have taken the risk to prove just what straw can do. In Stansted Mountfitchet, Essex, is a saleroom built for Sworders, a fine-art auctioneering company. It is a compressive frame style, pioneered by one of the directors, Robert Ward Booth. Originally 1,100m² (12,000ft²) it is the largest straw-bale building of its type in the UK. It won the East of England Royal Institute of Chartered Surveyors (RICS) Sustainability Award in 2009 and in 2013 was extended to 1,600m² (17,000ft²). It was built for the very competitive price of £950/m² (£88/ft²).

Award-winning saleroom for Sworders, Essex, fine-art auctioneers of Stanstead Mountfitchet. A compressive frame design, this is the largest straw-bale building in the UK, and now stands at 1600m² (17,000ft²) with an extensions built in 2013. Photograph © Rae Parkinson.

The largest UK straw prefabricated panel building was built with grant funding in Bradford, West Yorkshire, in 2011 with panels supplied by Modcell (see page 20). Designed by Waller and Partners, it is 2,787m² (30,000ft²) in size with 14 serviced offices and 14 managed work spaces, making it the largest building using straw bales in Europe. It was built with a £3.389 million grant funding plus £1 million from a mortgage so it cost £1,575/m2 (£146/ft²) to build, using 4,000 straw bales. In 2008 another innovative two-storey load-bearing straw-bale house, built with my help between 2002 and 2005, won the Grand Designs Eco-home of the Year award. In 2009 North Kesteven Council in Lincolnshire became the first local authority to build semi-detached council houses out of straw. Straw Works designed the world’s first load-bearing terraced houses, near Canterbury, built by self-builders in 2012/13.

Penwhilwr, voted Grand Designs Eco-home of 2008, was the first 2-storey load-bearing straw-bale house in the UK. Photograph © Barbara Jones.

Why use straw?

Sustainability

Straw is an annually renewable natural product, formed by photosynthesis, fuelled by the sun. Over 5.6 million tonnes are produced surplus to requirements each year in the UK. Using straw can mean there is less pressure to use other more environmentally damaging materials, and in the unlikely event that the building is no longer required, it could be composted afterwards. It is extremely low in embodied energy at 0.91MJ/kg compared with a concrete block with an embodied energy of 4.6-5.6MJ/kg‡ and has a seriously negative carbon footprint, storing carbon in its fabric for the lifetime of the building. For houses that last upwards of 100 years, this is extremely significant for helping revert the emission of greenhouse gases.

Energy efficiency and greenhouse gas emissions

Over 50 per cent of all greenhouse gases are produced worldwide by the construction industry and the transportation associated with it. If the surplus straw in the UK was baled and used for local building, we could build at least 640,000 houses per year. That’s more than half a million super-insulated homes, made with a material that takes in carbon dioxide and makes it into oxygen during its life cycle. Coupled with vastly reduced heating requirements, thereby further reducing carbon dioxide emissions from the burning of fossil fuels, straw-bale building can actually cause a net decrease in greenhouse gas emissions and hence have a negative carbon footprint. To improve the energy efficiency of houses is the design challenge of the 21st century. Straw-bale building designs are extremely airtight, can be built to Passivhaus standards, and easily reach the UK Sustainable Homes Code 5 and Code 6 by adding a green roof and solar panels.

Highly insulating

Straw provides super-insulation at an affordable cost. In walls typically over 450mm thick the U-value is 0.11W/m²K, two or three times lower (ie better) than contemporary materials, and much lower than any Building Regulations / Codes worldwide require (see Chapter 15). When used with a design with south-facing windows, incorporating some thermal mass to store heat and release it on a 24-hour cycle (eg clay or lime plaster, or earth, limecrete or brick floors), this maximizes solar gain. This will dramatically reduce the amount of fuel needed to heat a straw-bale house. Due to the monolithic compressed wall construction, particularly of the load-bearing style, straw-bale houses can achieve Passivhaus standards with an air change of 0.6/hour.§

Acoustically insulating

Straw-bale walls are also super-insulative acoustically. There are several recording studios in the USA built of straw bales for their sound-proofing quality and insulation, as well as one in Wales, the Strawdio. Straw-bale wall systems are increasingly being used near airport runways and motorways as sound barriers in all parts of the world, particularly using jumbo bales. Straw Works design load-bearing straw-bale party walls as thermal and acoustic barriers for semi-detached and terraced houses.

Low fire risk

Plastered straw-bale walls are less of a fire risk than traditional timber-framed walls. Research in the USA and Mexico has shown that “a straw-bale infill wall assembly is a far greater fire-resistive assembly than a wood frame wall assembly using the same finishes”.¶ In fact, there have been fire tests all over the world on plastered (and unplastered) straw-bale walls. All of them tell us the same thing – it’s hard to make straw-bale walls burn, because they do not contain enough oxygen, and especially if they are plastered and the sections where bales meet other features (eg windows) are built correctly. See Appendix 1 FAQs.

Affordable

Straw is currently produced surplus to requirements and a construction-grade bale costs about £3.50 + VAT delivered or £2 from the field. The walls of a 3-bedroomed, 2-storey house can be built with 350 bales. Also, because the building method is so straightforward, people without previous building experience can participate in the design and construction, thereby saving on labour costs.
By keeping designs simple and working on it yourself, it’s possible to build a house of 82m² for £50,000. There are some famous builds (see www.simondale.net) that have cost significantly less than this, but these generally do not meet Regulations/Codes, use freely sourced materials sourced for free, and may also be off-grid – so have no costs from connecting to services and utilities.

Straw-bale building: a sustainable, insulating, healthy and affordable way to create a home you really want to live in. Photograph © Barbara Jones.

The most significant saving on straw-bale houses is in the long-term fuel reductions owing to the high level of insulation. Heating costs can be reduced by up to 75 per cent annually compared with conventional-style housing, and the savings therefore continue to accrue throughout the life of the building.

Structurally sound

Bales of straw are more than adequate to carry typical loadings of floors, roofs and winter snow. They have passed load-bearing tests both in the laboratory and in practice, and are used to build houses of up to four storeys. If you find this hard to imagine, think of placing a new sheaf of photocopy paper on the floor and then standing on it. It squashes down with your weight. Now ask several friends to stand on it with you – it doesn’t squash down any more than it already has done. This is what straw does too. We know how much a densely packed construction grade bale will compress under load, and once we’ve made it do this, it won’t compress any more. In framework methods (see Chapter 4), bales are not used structurally; they simply infill the gaps between posts and it is the posts that carry the load (and take away an engineer’s anxiety) – the bales do not take weight (although they could!). Increasingly, as more research has been done on the strength of bales, they are used to provide racking strength for framework buildings, which means the sizes of timber can be reduced.

A healthy living environment

Straw, particularly organic straw, is a healthy alternative to modern materials. It is natural and harmless. It does not cause hay fever, since it’s not hay (ie it doesn’t contain pollen), and in fact it is the building material of choice for many allergy sufferers because it is so innocuous. Living within straw walls can enhance the quality of air we breathe, because it does not give off harmful fumes such as formaldehydes or dioxins, as many modern materials do, and because it is a breathable material, thereby helping to keep the inside air fresh. Coupled with the use of non-toxic organic finishes such as clay and natural pigments and paints, it can provide one of the safest and most comfortable atmospheres in which to live. Traditional lime plasters and limewashes are actually antibacterial, which is why they were used so much in the past for hygienic reasons. Lime washes will inhibit mould and other bacteria. Both clay and lime plasters deal effectively with humidity inside homes, absorbing excess moisture, for instance when you have a shower, and releasing it slowly as humidity levels decrease, keeping levels at the optimum for human health, all without causing condensation. Another health benefit is the ambience inside a straw-bale house, which is calm, cosy and peaceful. This is partly to do with the high level of sound insulation, partly to do with the air quality, and partly to do with the organic feel to the house – a beautiful, nurturing and safe environment to inhabit. Try it!

Empowering and fun!

The most unquantifiable aspect of a straw-bale house has to be the way that the building process itself empowers ordinary people. It is accessible to many people who are otherwise excluded from the design-and-build process, and enables them to transform their living environment, and their lives, in a very enjoyable way. Even mainstream builders, who are often sceptical when faced with a new material, can become firm believers in its qualities and performance.

* In a load-bearing straw-bale building, the bales themselves support the weight of the roof and any floors above.

† A U-value is a measure of heat loss in a building. A low U value indicates high levels of insulation.

‡ Offin, Maria. 2010. ‘Straw-bale Construction: Assessing and minimizing embodied energy.’ MSc diss, Queen’s University Canada. Available as a PDF:
https://qspace.library.queensu.ca/bitstream/1974/5409/1/
Offin_Maria_201001_MES.pdf

§ 0.6 of the air in the building is changed with air from the outside environment every hour.

¶ 1996 Report to the Construction Industries Division by Manuel A. Fernandez, State Architect and Head of Permitting and Plan Approval, CID, State of New Mexico, USA.

CHAPTER 2

Affordable houses

Straw-bale buildings compare favourably with conventional techniques in terms of build cost, and straw-bale costs will fall as the method becomes more mainstream. Simple designs, locally sourced and recycled materials, and enlisting volunteer labour can further help keep costs down. But don’t forget that affordability isn’t only about the build itself, but also about long-term maintenance and running costs (including energy bills for heating or cooling) as well as creating a place that will have a positive impact on your life.

Photograph © The Whisharts.

The environment in which we live affects our moods; the way we feel. It can make us feel confident, optimistic and motivated or demoralized, downtrodden and hopeless. However, when buying a house, the question ‘What would I feel like if I lived in this house?’ isn’t usually the first one we ask – it’s more likely to be ‘Can I afford it?’ We don’t ask whether the house will help us to grow, to be purposeful, to contribute to society, or whether it will make us feel small and mean. Imagine what our world would feel like if our homes were full of positive, optimistic and creative energy! We do have power to change things, we are able to make choices, but we live in a culture that’s driven by money and fear, and it can be hard to step outside that and envision a world for ourselves that nurtures and sustains us. But why should we accept anything less?

Quality, good design and well-being in 21st century Western culture

The majority of mass house building designs today are mediocre. They do not make use of the sun’s orientation to maximize solar gain, they do not use natural and sustainable materials, and they are not built to last. Developers try to squeeze as many houses into one plot as possible, and build as fast as possible, regardless of what it will feel like to live in a cramped, ill-designed house that has very little sunlight or outside space. Feeling good is vitally important to well-being, but it is rarely taken into account within design and construction. If we build homes that people love then the people who inhabit them will want to look after them and preserve them. How things are built, the energy and the intention with which they are built, becomes part of the fabric of that building. The way the construction industry operates does not take such feelings and processes into account; it values only economics and function.

Moreover, the system of piecework on building sites, where workers are paid by the number of doors they hang, or the use of subcontractors who are forced to submit low quotes in order to get a job encourages poor-quality work because everyone tries to work faster and cuts corners to earn more money. Competitive tendering is the same thing: companies are encouraged to submit a low price in order to win a contract, and then have to build as fast as possible with the cheapest materials in order to stay within the quoted price. Manual skills are not valued, training is not taken seriously, and the industry is not proud of itself. We are working within the myth of the free market economy where competition is supposed to deliver the best quality, but in fact has the opposite effect because it encourages greed and lack of trust, coupled with the myth of continual growth, which anyone with common sense can see is not physically possible.

Our current house building system values short-term economics above good design, above durability, above quality, above well-being.

The affordability of straw-bale buildings could enable more people to live in a home built to a high quality and good design that promotes well-being. Photograph © The Wisharts.

What does ‘affordable’ mean?

Building cheaply is not the same as building affordably, and does not benefit any of us in the long run. ‘Affordable’ means reasonably priced, whereas ‘cheap’ implies of little worth. We shouldn’t have to accept poor-quality design, materials and workmanship in return for shelter that is within our budget, but we have to sacrifice our own well-being in a market where choice is restricted by the limited range of what’s on offer. Short-term savings cost us more in the end, because mediocre building does not last, so has to be retrofitted with the features it lacks, such as adequate insulation, then knocked down and rebuilt. It also makes us unhappy and dissatisfied: unhappy people can be disruptive for the rest of society. We are making do with what’s available when we could be creating places to live and work that actually increase our sense of belonging in the world, that give us a centred place to begin from each day and return to at night.

This limited perception of choice, between cheapness and quality, is flawed. It’s absolutely possible to incorporate a more positive value system into the construction industry and still build affordable houses, as is being proved through the development of straw-bale and natural house building. Affordable houses can be built to very high standards of thermal efficiency, without compromising on quality and also providing a beautiful, organic ambience to the house that increases well-being.

It is possible for a contractor to build a thermally efficient three-bedroomed straw-bale house for £120,000. A self-builder could build the same house for £60-75,000 which will last upwards of 200 years – and these prices will continue to drop as more are built. Doing this yourself or with friends, organizing and managing the project and running courses to teach people the skills at the same time as building is how to make savings. This is on top of the long-term savings to be made on heating bills, as described in Chapter 1. In fact, with good design, it’s possible to build a straw-bale house that has no or almost no need for heating. In years to come this will be an increasingly desirable feature that will make your straw-bale home more attractive to buy than similar houses in the same area.

In UK mainstream construction, it’s possible to build a modern brick and block house to Code for Sustainable Homes (CSH) level 3 for between £1,000-£1,500/m² (£93-£139/ft²), giving a price of £97-£145,500 for a 3-bedroomed house. BUT it is not fair to compare such a house with the thermal efficiency, comfort and durability of the straw-bale houses currently being built, which achieve CSH level 4 or 5 – we have to compare like with like. In order to build a modern house that matched the insulation values and quality of a straw-bale house, the price would actually be about £180,000. This is why the construction industry is complaining that government targets for affordable but sustainable houses are not achievable, and indeed they are not achievable by using current mainstream materials. We need a radical change! To meet government targets, we have to stop thinking conventionally and build instead with straw.

In 2009 North Kesteven Council in Lincolnshire became the first local authority to build semi-detached council houses out of straw. Photograph © Rae Parkinson.

Designs for affordable houses have been developed by Straw Works over the last few years, leading the way in bringing the cost of desirable home ownership within the reach of the ordinary person again. In 2013 we completed a feasibility study for the Scottish Government via the Get Rugged Charity in which we gave designs and showed how (as mentioned in the previous chapter) a self-builder can construct a twobedroomed detached house of 82m²(883ft²) for £50,000. These designs are detailed at the back of this book (see Appendix 3). Large construction companies are never going to be able to build affordably unless they can build an enormous quantity at once, because their overheads are so huge. It can cost more than £1,000/week just to have these companies on site to build a couple of houses, before they do any work. You can see how managing a number of smaller companies working to build your house can save a lot, although you have to invest a huge amount of time in finding and managing these companies.

Prefabricated panels of straw, timber and lime render have been developed by the company Modcell. This system was used for the very successful grant-funded affordable homes project in Leeds for the LILAC co-housing project that provided a self-build house cost of £1,150-1,200/m2 (£107-111/ft²) (£1,350-1,400/m² (£125-130/ft²) without grant aid). These prices are sure to drop as more are built.

The LILAC co-housing project in Leeds, made from prefabricated panels, provided affordable homes for self-builders. Photograph © Modcell.

Affordability must take account of not just build costs, but also long term maintenance and running costs.

Simplicity is the key to affordability, together with use of easily obtained standard materials, both in terms of timber sizes and ‘off-the-peg’ doors and windows. Straw-bale buildings have proved themselves to be buildable by relatively unskilled people. They use materials that are predominantly of low embodied energy – not highly processed – and locally sourced, such as straw bales, recycled car tyres, pea shingle and clay. These materials generally require more labour time to source and use, but cost less.

Keeping costs down

If we are to accept the challenge of building affordable homes then we have to radically rethink the way we build, and using the UK cavity-wall masonry system is not the way to go: it’s a really bad design that has never worked well in practice, and the materials used to construct it are not sustainable. Tinkering about with this system can bring only minor changes in cost, and won’t give us the thermal efficiency that we need for the 21st century – and that’s without taking into consideration the interior feel of the homes. The preferred style of home in the US and Canada – a brick-skinned wood frame building has a huge embodied energy cost and is far more complex to build than a home of straw.

At the moment, the construction industry seems content to rely on outdated systems and add-ons, and still fails to meet government targets. We need a complete rethink of how we build houses. We have to be more creative!
An affordable house is achieved by a combination of factors as described below.

Simplicity of design

A simple design can keep costs down and still look traditional and beautiful. Photograph © Barbara Jones.

The more you add quirky little bits to your building – unusual shapes, different levels of roof pitch, even curves; in fact most things that make your house really quite an individual home – the more the price will go up, because all these things take extra time and extra materials. It’s a shame, but it’s true that a basic box is cheaper to build than a beautiful circular house. This is partly to do with the fact that modern building materials come in basic box-sized shapes, so when you try to make them into curves there is a lot of wastage. The standardization of materials has reduced the cost of labour (as a less-skilled work force is required) and made the construction process cheaper, but with this change we’ve also lost the ability to make our homes individual and beautiful and truly ours.

Often the simplest houses are the most beautiful. Look at a traditional cob (earth-walled) house: thick walls, rectangular structure nothing very fancy about that. However, these houses sell at a premium, they last upwards of 400 years, they have an organic feel to them, and they have an atmosphere that is hard to describe but feels safe, cosy and pleasing. They were built by lots of people coming together to help, with natural materials, and to designs that were tried and tested. In fact, they sound very similar to modern straw-bale houses, the main difference being lack of insulation! We can keep costs down by designing houses that are simple to build from a practical point of view, are made of readily available and common materials, use accessible construction, methods and have an internal design that’s well thought out and relevant to modern lifestyles – such as being open-plan so that heat can easily circulate around the house. A basic box made of straw is infinitely more pleasing than one of flat and modular materials, because of the organic nature of the curved corners and walls.

Local and natural materials with low embodied energy

Using local and natural materials will keep costs down and is more environmentally friendly.

Another way to keep building costs down is to use materials that don’t cost the earth. The more highly processed a material is and the further it has to travel to your building site, the more it will cost both environmentally and financially. The closer it remains to its natural state, the cheaper (and healthier) it will be. In one sense building a straw-bale house may only mean changing the wall system from a typical brick-and-block cavity to using straw, but why stop there? Eighty five per cent of the materials in a typical Straw Works-designed straw-bale house are natural, with low embodied energy. These buildings use no cement at all, not even in the foundations. They require no plastic damp-proof courses (DPCs), and use sheep’s wool, hemp and wood fibre, lime and clay as well as durable and small-dimension timber.

If you compare the embodied energy of 1m² of a standard Straw Works straw wall (load-bearing straw-bales rendered externally with lime and plastered internally with clay) with 1m² of a standard modern wall (brick, lightweight block, mineral-fibre insulation, gypsum plaster) of approximately equal super-insulation (to make the comparison of like with like), the results are as follows:

1m² of standard straw-bale wall: | 36.5 kWh/m²
1m² of cavity wall insulated with mineral-fibre insulation: | 172 kWh/m²

(contained in the MSc thesis of Carol Atkinson* a former student at University of East London, who has built her own mobile straw-bale holiday home with the help of amazonails on which she carried out extensive research).

More recently, in 2010, Maria Offin completed a PhD at Queens University, Canada which provided a comparative analysis of the embodied energy (EE) for various construction methods. A summary of her results can be found in the table below.

Extrapolating from this, a typical brick skin with insulated block and gypsum plaster interior finish common to most UK homes would contain even more EE than the wood-frame construction with brick siding.

It is absolutely clear that the difference in embodied energy between load-bearing straw-bale construction and typical construction in the UK and the US/Canada at the present time is at least 500 per cent. Clearly there is no comparison! These results are so vastly different that at the moment, most people working in the construction industry are not able to accept them (it’s a bit like talking a foreign language – it’s so different, it doesn’t make sense at first).

Summary of embodied energy (EE) analysis for various const ruct ion methods
Construction method | Average embodied energy for 1x2.4m section of the wall (MJ) | Increase in embodied energy compared with load-bearing straw bale (% difference)
Load-bearing straw bale | 393.3 | -
Post and beam straw bale | 505.34 | 28.5
Modcell panels | 550.7 | 40
Wood-frame construction with vinyl siding | 1181 | 200
Wood-frame construction with aluminium siding | 1545 | 293
Wood-frame construction with brick siding | 2370 | 503

* A copy of the thesis ‘Energy assessment of a straw-bale building’ can be found at www.homegrownhome.co.uk

Keep it small!

A small house can be very comfortable if designed well and small size helps preserve precious resources. Extra rooms can always be added later if you need them! Photographs © Rae Parkinson.

There is no justification for building huge houses with loads of bedrooms and bathrooms for two people to live in. As a rule, one person can live comfortably in 30-45m2 (320-480ft²), two people don’t need more than 60-70m2 (650-750ft²), and many people live happily in much less. It’s far better to design small, add extra bedrooms later if you need them, use sheds and share facilities. This will definitely make your house more affordable! Our affordable houses are designed so they can be extended easily and quickly when the need arises. Don’t be greedy with resources – create what is enough and no more.

Accessible building methods

Another way of keeping building costs down is to help with the building yourself, and get your friends and family to join in. This is possible with straw-bale building because the methods of construction are so simple and straightforward, and there are no hidden technologies. And the money saved on the labour costs will mean you can afford to be more creative with your design choices. Many self-builders have been able to afford ‘designer’-type houses because they have put in their own time and effort over a number of years, worked more slowly with volunteers and by running courses, and taken time to find reusable and recycled materials. Increasingly, we are following this process on mainstream construction sites as well, working with main contractors and running courses on site for the public, the builders and the intended users of the building. Working with your friends to put up large straw blocks into walls is fast, effective and fun! There are very few people who are not able to build using straw, and straw-bale building sites commonly include younger and older people, and people with a diversity of talent and experience. This alternative approach to building has the long-term benefit that people feel happier if they are involved in their own building; they feel a sense of ownership and identify with it.

Help with the building yourself, and get friends and family to join in: keep costs down, share and learn skills, feel involved and have fun! Photograph © unknown.

Working in partnership without fear

Whatever happened to trust? Being worried and fearful about money and the integrity of your colleagues can completely destroy trust. We live in a culture of fear rather than trust; we’re encouraged to expect the worst rather than the best, to insure against our fears and generally protect ourselves just in case, inhabiting a more and more frightened and insular world. We have seen shocking examples of major institutions mis-selling products to us and squandering our savings, yet managing to keep their top staff in prestigious jobs.

We need to learn how to rely on ourselves and our friends again, to build networks and support each other. A system built on fear and self-interest is bound to topple in the end because greed demands that there is always more to be had. The essence of sustainability is to use what is enough, and no more. We need to learn to be satisfied with what we have, to recognize worth when we find it, and to stop trying to get more and more and more! This is not life-enhancing.

We need to be able to trust our builders, and they need to convince us that they will work hard, uphold high standards of quality and work, and can be left alone in our homes without us worrying about safety. The industry needs to be able to trust the client, to know that we are willing to pay a fair price for a job well done, and that we will be honest and just in our dealings with those we contract. If this atmosphere of trust existed then we could also have a reasonable discussion about accounting; about how much things cost, what is a fair price for the job, and sharing the profit if all goes really well (or the loss if something goes wrong). Sadly this situation doesn’t yet exist, but increasingly we are creating it on self-build sites away from mainstream construction.

If we are able to communicate well, speak with clarity and listen attentively; if we can bring honesty and integrity to the discussion table; if we’re willing to trust each other, take risks, work together and learn from mistakes then we are opening up the boundaries of possibility. If we believe that we all want to create the best building possible, and that different perspectives can actually help achieve this, then working in partnership has got to be the way forward. The more you believe in somebody the more they excel. The feeling of all being in this together, working for a common aim, is a feature of many straw-bale builds. There are many ways in which we find we are interdependent and need to rely on each other. It focuses the mind and allows for innovation.

Maintenance

Some ‘savings’ are not cost-effective in the long term, when maintenance and longevity become part of the affordability debate. For instance, by investing in durable timber windows and doors made from good-quality larch or douglas fir, redecoration and later refurbishment recede into the distance. Using extra insulation, beyond what the Regulations/Codes require, means huge savings on heating costs far into the future. A straw-bale house requires only the same level of maintenance as other houses that is, regular clearing of gutters and downpipes, garden maintenance to keep edges away from the building, redecoration of timber parts and lime washing according to weather conditions. But obviously, a house built with good-quality materials and attention to detail will cost far less to maintain than a cheap house.

Long term savings on fuel bills

One of the major assets of a straw-bale house is the long-term savings that can be made because of their extreme thermal efficiency. Plastered straw-bale walls have a U-value of 0.11 and can reach an air exchange rate (airtightness) of 0.26 air changes/hour. These results were achieved by a straw-bale Passivhaus built in Suffolk in 2014 (but see more on airtightness in the next chapter). North Kesteven District Council has compared the energy efficiency of its straw-bale semi-detached houses with comparable houses retrofitted to CSH level 4 and found the straw-bale ones to be superior on all counts (see www.n-kesteven.gov.uk/straw-house/114698.article). This means that straw-bale self-builders can expect to make savings of at least 25 per cent compared with a typical CSH level 4 house, even more when compared with the usual (and lesser) standard of modern housing or older, less insulated properties. Building straw-bale houses to a Passivhaus level of airtightness will bring fuel costs down even more – depending on how much the windows are left open!

Is there enough straw?

If we were to build all the houses we are told we need each year, would there be enough straw, and would we have to use more land to produce it? The table on page 26 shows recent wheat-straw production in the UK, and its uses.

So in 2011 we produced 5.6 million tonnes of straw that had no other use than to be ploughed back in. If a standard 3-bedroomed house takes 350 bales of 25kg (55lb) each to build then this amount of straw can build 640,000 houses. So clearly the answer is yes, there’s more than enough straw, and no, we wouldn’t have to use more land to produce it; we’d just have to stop ploughing some of it in.

Available straw (information from www.ahdb.org.uk)
Of the estimated 12.2 million tonnes of straw below, more than half is from wheat. From our best estimates, about 30 per cent of wheat straw is used for animal bedding on farm, 30 per cent is sold off farm (mostly for bedding), and about 40 per cent is chopped up and returned to the soil.
Defra Statistics: UK Cereal Production Survey

Current uses of straw
2011 Straw production (thousand tonnes) | 12,244
Animal bedding (1) | 5,800
Carrots (over wintering) (2) | 405
Energy (electricity) (3) | 300
Mushroom compost (2) | 40
Export (4) | 80
Total | 6,625
Remainder (incorporated) | 5,619

(1) J Kilpatrick, ADAS report written for NNFCC, ‘Addressing the land use issues for non-food crops, in response to increasing fuel and energy generation opportunities.’
(2) HDC C Brewster 2012, pers. comm. 8 May.
(3) EPRL J Stannage 2011, pers. comm. Aug; and Drax R Wood 2012, pers. comm. 14 June.
(4) Defra statistics

The cost of commercial buildings

All the above ways to keep costs down for housing also apply to commercial buildings. In 2009 a 1,100m2 (12,00ft²) saleroom with offices was built for a fine art auctioneer company using a framework that compressed the straw. We ran training courses for the straw walls; everything else was built by a local construction company. This came in at a price of £950/m² (£88/ft²).

In 2011, we helped build an infill-straw-with-steel framework warehouse – two storeys with a kitchen and offices for Green Interiors near Banbury, Oxfordshire. This came in at a price of £847/m² (£79/ft²). And in 2014 we built a second one at a price of £527/m² (£49/ft²), the lower price a reflection of us being involved in the design as well as the build. A company called Modcell has developed a prefabricated straw panel system with many of the same environmental advantages as load-bearing and infill straw buildings. They have built about a dozen buildings so far, all with grant funding.

The design of affordable houses is an ongoing process. Straw-bale houses are being built by self-builders at affordable prices and they could also be built by contractors if they were willing… These prices will come down further as techniques are refined. Straw-bale building in the UK has reached this point in only 20 years, which is a remarkable achievement and a testament to its firm grounding in grassroots common sense, providing what ordinary people genuinely want.

One of the main barriers to us taking up these innovative ideas is a lack of awareness about them. However, positive media coverage has made a significant difference to this problem. The Channel 4 programme Grand Designs has brought the idea of natural building into homes across the UK and helped to inspire people to choose to build this way.

Another significant barrier to the self-building of straw-bale houses is the dire lack of land in England. The UK government is now recognizing the need to make self-build possible by exempting them from the Community Infrastructure levy (a saving of about £20,000/house) but this still doesn’t free up land for building. We need to see more help for cooperative ventures, making it possible to compete with developers, to buy land in bulk at lower prices, and divide it up for self-builder plots.

Planning policy also needs to change and adapt to new circumstances. It should be possible to be allowed to build modest houses in new areas that are made only from natural materials and that can be dismantled or moved easily, quickly and with no waste – because there will be no harmful environmental damage done by these either during construction, during their lifecycle, or at the end of their life. A straw-bale house has two seemingly opposing features: it is durable with a lifespan of at least 200 years, but at the same time it is built with simplicity and natural materials that mean it can be dismantled relatively easily and be reused or recycled. The smaller houses can even be removed by crane and transported elsewhere on the back of a lorry, due to the nature of their design. Surely these features help us argue the case for these houses to be built in more rural settings, where they will not harm the natural environment, and from where they can be moved if circumstances change.

Many unique individuals have built their own houses, keeping costs down by using their own and volunteer labour, running courses and reusing materials. Norita Clesham’s two storey, three-bedroomed spiral house in Ireland cost her £70,000. Ben Law’s house in the Prickly Nut Wood near Lodsworth, West Sussex cost him only £28,000. Rachel Shiamh’s house that won the Grand Designs Eco-home of the Year in 2008 cost her £70,000, and Harriet and Andrew Wishart built three 3-bedroomed terraced houses for £69,000 each in 2013.

Ben Law’s West Sussex woodland house has straw-bale walls and cost him only £28,000 to build. This amazing price was achieved by Ben living in a woodland and having access to free timber, and also due to enormous amounts of enthusiastic volunteer help, cutting down on labour costs.
Photograph © Barbara Jones.

The construction industry needs radical change if we are to bring the cost of homes back to a level that ordinary people feel they can afford without being in debt all their lives. This means a complete rethink of the materials we use to build houses – using less-processed and local materials; and of the process of building itself – making it possible for people to participate in building their own homes in some way, either as self-builders or through housing associations and partnerships with contractors.

CHAPTER 3

Low environmental impact

What does ‘low environmental impact’ mean? For me, it means causing the least damage possible to the natural world, either by our carbon footprint or our use of materials and energy; and also making it possible to live in a sustainable way, with a lifestyle that does not harm nature. This means using building materials that are as unprocessed as possible, and therefore have low embodied energy. It also means using natural materials and designing with plenty of insulation so that our need for heating (traditionally fueled by fossil fuels) is vastly reduced. Straw-bale buildings do exactly this.

Photograph © Rae Parkinson.

Straw bales are made of the dead stalks of grain crops, and therefore they store the CO₂ taken in by the plant and have a negative carbon footprint by their very nature.

Straw houses and carbon footprint

The Collins English dictionary defines a carbon footprint as “a measure of the amount of carbon dioxide released into the atmosphere by a single endeavour or by a company, household or individual through day-to-day activities over a given period”.

If we think of the endeavour as the building of a house, almost all types of houses have a positive carbon footprint (this is not good!) because of the materials they are built with. If you took into account all the materials used in their construction and analysed how much CO₂ was produced or stored by them inherently, or was produced during their manufacturing process by the use of energy that emitted CO₂, or was produced as a result of transportation associated with them, it would show that they are responsible for the release of far more CO₂ into the atmosphere than any that they might store. Governments are encouraging the construction industry to achieve a zero-carbon footprint, but we can achieve a resoundingly negative carbon footprint very easily by using a completely different approach to building walls.

Houses built of naturally grown materials (eg straw, wood, hemp, newspaper insulation, etc) are totally different. As they are made up of materials that have been grown in the earth, and as all plants take in CO₂ from the atmosphere and release oxygen, they all store CO₂ and have a negative carbon footprint by their very nature. Using these materials instead of unnatural ones will make a huge difference to our global carbon footprint.

The table on page 30 shows 25 common materials and their CO₂ pollution. You’ll see that the worst polluters are the metals, because they take an enormous amount of energy to extract from the earth. Fortunately, we use them in only small quantities, so their contribution to our carbon footprint is minimal. Cement, on the other hand, seems fairly average and on a par with plywood, until we realize just how much of this material we use. Next to water and sand it is the most-used commodity globally, so it becomes probably the single most damaging polluter in the world. If you look at where cement factories are situated – right next to electricity-generating plants – this gives you a clue as to just how much electricity is used to make it. They would put too much strain on our system if they were further away!

MacMath, R. (2000), ‘Carbon dioxide intensity ratios: A method of evaluating the upstream global warming impact of long-life building materials,’ published by The Center for Maximum Potential Building Systems, Inc., Austin, Texas, as Publication AD 4.5 [available from: www.cmpbs.org/publications/T1.2-AD4.5-Up_Gbl_wrm.pdf].

Most of the plant materials, on the other hand, are non-polluters except for the highly processed ones.

According to Carol Atkinson*, the average UK home produces 50 tonnes of CO₂ during its construction. In contrast, research by Dr. Behzad Sodogar of Lincoln University† shows that there is 1.35kg (3.0lb) of CO₂ per kg of baled straw. This means that a 16kg (35lb) straw bale stores about 21.6kg (48lb) CO₂. Since it takes about 350 bales to build a typical 3-bedroomed house, this type of building will store about 7.56 tonnes of CO₂ in its fabric.

This holiday cabin, which features in Carol Atkinson’s MSc thesis, used local materials and services, including wheat straw from a neighbouring farm. Photograph © Kuba Wihan.

In addition to using plant materials to build with, using materials with low embodied energy also reduces the building’s carbon footprint. The table on page 31 shows the embodied energy of all materials used in a typical straw-bale house, and the percentage volume of these materials in relation to the whole.

This shows very clearly that almost 85 per cent of a typical straw-bale house is made of materials with low embodied energy, in contrast to a masonry house, which uses roughly 85 per cent of high embodied energy materials.

Embodied energy | Materials | Percentage volume of all materials
Extremely high | Plastic straps, lead flashing, hinges, locks, handles, nails and screws, galvanized downpipe | 0.14%
Very high | Double glazing | 0.09%
High | OSB, external plywood, Tyvek™ membrane | 2.19%
Medium | Baler twine, floorboards, wooden gutters, cork underlay, Celenit fibreboard, door/window frames, lime plaster, hessian | 7.34%
Low | Local timber | 5.28%
Very low | Straw bales, sheep’s wool insulation, hazel, clay plaster, cedar shingles | 84.95%

Unpublished MSc thesis, Carol Atkinson 2008

Airtightness and a healthy lifestyle

Most old houses, and far too many modern ones, are pretty draughty. Air leaks into them through badly fitting windows, through electrical sockets punched into the cavity of the external wall, through gaps between the roof and walls, etc. If we don’t do anything about this, all the heat that we put into our houses to keep warm (or the heat we’d like to keep outside) will leak through these gaps eventually, meaning we have to use a lot of energy to produce more heat than that which is lost. It makes sense to design and build new houses, and to retrofit old ones, to stop all this heat loss, or at least most of it. This is the thinking behind airtightness – to build with good details and to a high standard of quality in order to reduce the fossil fuel we use for energy and the amount we have to spend on heating. However, it is generally accepted that indoor living spaces need an air exchange rate (the number of times all the air inside an enclosed space is replaced) of 3-6 per hour to have enough oxygen to breathe and maintain a healthy environment. Some would say we need more than this. The move towards thermal efficiency / airtightness is creating a potential health problem here. If we build airtight houses, so that the only air that enters is that which we invite in by opening windows or using other ventilation, then anyone can see that a house designed to have only 0.6 air changes per hour (the Passivhaus standard) is not going to be good for our health if we don’t open the windows, or the electrical/mechanical ventilation system breaks down or the filters get clogged. Personally, I always sleep with my window open, but not many people do. I suppose I don’t want to live in a house that’s so sophisticated and regulated. I prefer a simpler life, where I can close the window, put on a jumper, or place another log on the fire if it’s cold, and nothing much can go wrong. So, we have a conflict between airtightness and fresh air.

Modern houses are usually subjected to an airtightness test on completion. For this, all the windows and doors are shut, and a blower is fixed into one external door that sucks air out of the house (it can also blow it in). So if there are any gaps or holes in the fabric of the building, air will be sucked in through them. The blower has a measuring device on it that measures the rate at which air is drawn into the house and gives a figure in air changes per hour. The standard for comparison is measured at a pressure of 50Pa (Pascals – units of pressure). Houses built to a Passivhaus standard all use non-breathable tapes that are glued onto window and door frames, cover joins in tongue-and-groove (T&G) boards for roofs and floors, and are bedded into renders and plasters to stop air leakage at these junctures. They also use MVHR (mechanical ventilation with heat recovery) systems to take the heat out of expelled air and return it via fresh air into the house. All this means miles of tape, loads of ducting, and lots of expense. It is estimated that reaching Passivhaus standard adds at least 8 per cent onto the build cost.

Straw-bale houses generally have good air-tightness and can even reach Passivhaus standards. The council houses we designed and built for North Kesteven council in 2009 reached an air exchange rate of 2.62, achieved simply by following our own design details and paying attention to quality installation, without any airtightness layer or tape. Photograph © Rae Parkinson.

What is a sensible level of airtightness?

Current UK Building Regulations require only 10 air changes/hour, although this will soon be improved. One solution is not to get too hung up about it and make sure we reach an air change rate of 4, and combine this with a lifestyle of allowing fresh air in by choice. This will mean we have to spend a bit more on heating than if we had a Passivhaus, but we won’t have to have the noise and cost of the MVHR running all the time, and we won’t have to spend loads on airtightness tape and ducting. That seems like a good compromise. MVHR is not worth installing unless you reach 3 or less air changes/hour, because the amount of heat recovered from the building won’t justify the cost of running it. Also, it’s quite a big piece of kit, and even if it isn’t noisy, you can still hear it despite what the manufacturers say!

I would say we definitely need to reach standards of build quality that give us less than 10 air changes/hour. For the most extreme of us, reaching Passivhaus standard and living with it would be a good choice. This means a higher build cost, regular maintenance of the MVHR and accepting the noise of the heat exchanger/extractor, plus a back-up generator if the electricity fails. Most people will not be able to live with these standards, so something like 4-6 air changes/hour seems sensible, or 2-3 if we have a lifestyle of opening windows often – or going in and out to the outside.

How airtight are straw-bale houses?

The council houses we designed and built for North Kesteven District Council in 2009 reached an air exchange rate of 2.62 without even trying – that is, just by following our own design details and paying attention to quality installation. We did not use any airtightness layer or tape (and they don’t use MVHR). We did an air exchange test on the Passivhaus we helped build in Norfolk in 2013, after compression but deliberately before the airtightness layer was complete so we could see how the straw on its own was performing – no plaster or render. Using smoke to show air movement, this showed us that there was very little air leakage through the straw walls, and what there was occurred at joins, where some of the stuffing had come out (or hadn’t been done well enough) and in the gables where the straw could not be compressed to the same level as the walls. There was no leakage between the straw and the base plate (due to good compression), but there was some between the base plate and the plinth wall. This butt joint would be covered with a render stop before plastering, and with the floor/skirting internally. There was also some leakage around fixing posts where the notch had not been made very well – lime or clay/straw applied during plastering would deal with this. Another test was carried out after the external lime render was applied, but before the whole of the external airtight envelope had been totally completed, and this gave an air exchange rate of 0.9.

All this leads me to conclude that although we can reach Passivhaus standards with straw-bale designs, it’s not necessary, and I am happy to accept slightly more need for heating (generally via a wood burner) in return for a different way of getting my fresh air without mechanical ventilation or miles of tape.

Load-bearing straw-bale houses, like this Passivhaus built for Dave Howorth, are low in embodied energy, energy efficient in use, and by storing carbon have a carbon footprint of less than zero. Photograph © Rae Parkinson

Straw-bale building and sustainability: key points

alt Straw-bale houses are made from predominantly low-embodied-energy materials, many of which can be sourced locally. These are materials such as straw bales, clay, sheep’s wool, timber.

alt Straw-bale houses store carbon instead of producing it during their construction and lifetime – unlike most modern homes. They have a negative carbon footprint – less than zero. No other type of construction can match this.

alt They are structurally strong and durable, with a design life of 200-plus years.

alt They are thermally and acoustically efficient. Plastered walls have a U-value of 0.11, and airtightness values of less than 3 are easily achieved.

alt They can easily meet Code for Sustainable Homes level 5-6.

* MSc Architecture thesis written for the University of East London. Available at www.homegrownhome.co.uk

† Sodogar, B. (2011) The carbon-reduction potential of straw-bale housing. Available here: eprints.lincoln.ac.uk/3846/

CHAPTER 4

Building techniques explained: load-bearing, infill, hybrid and prefabrication

There are two main ways to build with straw bales. In a load-bearing structure, the straw bales themselves are structural, taking the weight of the roof. The infill technique uses a structural frame of timber, steel or wood, and straw bales ‘infill’ between the frame. Variations on these techniques, hybrids and prefabricated elements can all been used too.

Photograph © Rae Parkinson

A lot has changed in straw-bale building since the first version of this book was written in 2002! Self-builders are still out there pioneering new methods and having a go for themselves, but now there are also teams of straw-bale builders, working with mainstream contractors or for themselves to build straw-bale council houses, offices, retail units, environmental centres, private homes and more. The basic principles are still the same, but details and methods have been refined and developed to meet more rigorous standards for quality, ensuring thermal efficiency, airtightness and durability.

Load-bearing (also called Nebraska)

This is the original way of building with straw, pioneered by Nebraskan settlers in the USA in the late 1800s. I prefer this method above all others for building any type of construction, small or large, because of its simplicity, flexibility of design, enhanced thermal performance and cost-effectiveness. The potential for empowerment through working together on a shared project is one of the great advantages of this type of building, as it is such an accessible building method: pretty much anyone can do it, and it’s a lot of fun!

In the load-bearing technique the bales, which are structural blocks, are designed to take the weight of the roof; there is no need for any other structural framework. They are placed together like giant bricks, pinned to the base plate and to each other with coppiced hazel, and a continuous rigid timber ring beam (the ring beam(s) give a load-bearing building stability, see page 80) on top spreads the floor and/or roof loads across the width of the wall. For two-storey houses, the floor joists at first-floor level are attached to the ring beam before building up the straw walls again beneath the roof. The roof plate (a continuous ring beam that sits on top of the walls and under the roof) is fastened to the bales with coppiced hazel and may be strapped down to the foundations, depending on local weather conditions and the weight of the roof. The roof is constructed on top of the roof plate, following straw-bale design principles.

LOAD-BEARING DESIGN USING INTERNAL PINS

Windows and doors can be installed in various different ways. They can be placed inside structural box frames, which are built into the bales as the walls go up. This approach is usually used for smaller buildings and windows. It may require more timber than other methods, but it keeps the actual bale building aspect to its simplest. It requires little previous knowledge of wall construction and is very accessible.

A single-storey community building in Wigan, Lancashire, being built with the load-bearing (Nebraska) method on tyre foundations. Photograph © Rae Parkinson.

An alternative way of installing windows and doors, and one used predominantly on mainstream building sites, is to use doubled-up 100mm x 50mm (4'' x 2'') uprights either side of the opening, fastened securely to the base plate and projecting through slots in the roof plate. These are housed fully into the bale either side, ensuring airtightness, and the window or door is fixed to them. They are NOT structural posts, but do provide secure fixing points for windows and doors.

The world’s first semi-detached two-storey load-bearing straw houses, built for Ralegh’s Cross Inn on the edge of Exmoor, Devon, during construction and finished. Photographs © Barbara Jones.

The load-bearing style is a simple, straightforward building method, easy for non-professionals to design following readily comprehensible basic principles. Using a simple step-by-step approach, designs from with one-room to two or more storeys can be created. Curves and circles are easy to achieve, though at some extra cost due to more complicated timber work. And as the straw is very forgiving, total accuracy is not necessary, but wilder variations can be brought back into shape easily! Self-builders love it because it’s fast, fun and fulfilling! Most of the larger load-bearing buildings now being built in the UK and Ireland are actually pre-compressed load-bearing styles (see below).

Load-bearing in a wet climate

Over the years, the main disadvantage of the load-bearing technique – how to keep the straw dry throughout the whole building process despite sometimes prolonged wet weather – has been dealt with. Different ways we can address this issue are:

alt Prepare everything in advance (prefabrication) so the whole building goes together quickly once you start, like building with Lego. The vulnerable time of exposure to the weather, and the need to keep it covered with tarpaulins, can be reduced to a few weeks.

alt Design carefully so that the roof and/or floors can be built first, held on temporary posts. The straw can then go in underneath, with tarpaulins hung from the eaves, or external scaffolding can be sheeted. Once the straw is installed, the roof/floors can be lowered down on top. Works brilliantly well!

alt For small buildings, build underneath a marquee or large tent.

alt Increase the height of the scaffolding (needed to build the walls and roof), add a temporary roof over the top, and cover in the sides. This level of protection is required for the lime render anyway, so it can be cost-effective to protect the whole building, ensuring there will be no time lost due to the weather for any aspect of the building, not just the straw, at any stage. Many self-builders buy their own scaffolding and sell it on afterwards.

Load-bearing with jumbo bales

Warehouses and retail units

Large open spaces can be built very quickly and effectively using mini Hesston or quad bales – also called jumbo bales. At 2.4m long x 1.2m high (8' x 2') per bale you can appreciate how quickly a person with a forklift truck and telehandler machine can build a new DIY superstore. Obviously there’s a little bit more complexity to the design than this, but you get the idea!

Houses

Using jumbo bales, you can also build even more insulating houses, as Werner Schmidt has done in Italy and Austria, with walls more than 700mm (28'') thick. Obviously you need to have enough land to accommodate such thick walls, but these houses can be built extremely quickly, as most non-straw parts are prefabricated panels.

Acoustic walls

Jumbo bales are increasingly being used for making sound barriers for airports, major roads, and racetracks, etc. Their advantages are that they can be built relatively easily, with machinery, don’t require foundations, only levelling of the ground, and they work really well!

Basically there is no real limit to design or size when using load-bearing techniques. We have built circular and curvaceous buildings, as well as detached and semi-detached two-storey buildings using this method. At least four storeys could be built with load-bearing straw. This is how the ones in cities will be built. It’s all about understanding the material and having a creative approach to problem-solving.

Infill (also called post and beam or timber frame)

Next to load-bearing, infill is the other ‘traditional’ way of building with straw. It was developed in the USA in the late 1970s, as their construction industry was familiar with it as a practice, although there are a few earlier examples to be found in Europe and the USA. In this method, the weight of the roof is carried by a timber, steel or concrete framework; the bales are simply infill insulation blocks between the posts, a key for plaster, and also resist wind loads; they are not used structurally. You might make this choice because you like the look of timber and you want to use its beauty in your design, or because you are going for something really ambitious with large openings and spans, and lots of glazing. This method also has the advantage of giving peace of mind to clients and those professionals (architects, engineers and building inspectors) who aren’t quite ready to accept just what straw can do as a building material. It is often the preferred option for architects, as the structural concepts are not innovative and rely on an already established method of building, therefore the risk associated with an experimental technique is minimized. There is no need to satisfy oneself of the bales’ capacity to take the roof weight, since the framework does this. This method requires a high level of carpentry skill and uses substantially more timber than a load-bearing design, so has significant cost and environmental implications. I find that builders who are trained carpenters find it quite hard to accept the load-bearing method and prefer to stick with what they know.

Infill method with curved Glulam (glued laminated timber (see page 85)) roof and posts. Photograph © Barbara Jones.

This book does not cover timber framing – there are plenty of really good sources of information elsewhere for this. But it does deal with the all-important details of the junctions between timber and straw. When designing a timber framework to have straw as an infill material, it is really sensible to have somebody who understands how straw works to work with the framework designer; this means that all sorts of practical difficulties can be eliminated before you ever get on site. This type of building needs to be designed with the dimensions and properties of the bale as the first consideration, but what usually happens is that the building is designed as per standard framework techniques, and the bales have to somehow fit within that. Because of the nature of the straw, it is possible to solve most practical problems on site, but doing so can detract from the ease of build and the fun – if, for example, you’re having to make little parcels of straw, customize every bale, or sew packets on to the front of posts just to get around the fact that the frame wasn’t designed with the straw in mind. It can also give straw-bale building a bad name if it takes a long time or is frustrating to make the straw work with the frame; the designer should learn about this new material and how to work with it, or take advice before thinking that the straw doesn’t really matter!

There are two main issues regarding the design of infill buildings for straw bales: the location of the posts relative to the straw bales (and their size) and stabilizing the straw ready for plastering.

Location of posts

There are four options.

a) The posts can be completely separate from the wall of straw, and placed externally. This frees up space inside for living, the roof can be connected easily to the posts, and beams between the posts at floor and eaves level can be used to create compression on the straw to stabilize it. The external wall can be plastered continuously without interruptions from the posts, which is much better for weatherproofing. Details of junctions with posts are eliminated. It means point foundations under each post and possibly a strip foundation beneath the straw wall, although they could be held on the floor build up. External posts become prone to adverse weather and need to be of durable timber.

b) The posts can be completely separate from the wall of straw and placed internally. This can clutter up the inside space, and requires extra timber over the top of the bale walls at each floor level and the eaves that can be used to compress and stabilize the straw. The external walls can be plastered continuously, which is much better for weatherproofing, and details of junctions with posts are eliminated. It means point foundations under each post plus a strip foundation beneath the straw walls.

c) The posts can be partially housed into the straw internally. It doesn’t make any sense to house the posts into the straw externally, as this causes a lot of difficult details around each junction of post and straw that makes for poor weatherproofing. Posts partially housed internally can mitigate against the loss of space of design b) above while still having the benefit of beautiful timber to look at. In this location it also reduces the need for large double foundations, as the point foundations for the posts can be incorporated into those for the straw walls. Bales will need to be notched.

d) The posts can be placed in the centre of the bale walls. This is by far the best option as far as the straw goes, but the timber is no longer a feature. If the bales are notched to take a central timber, then lower cost and smaller dimension timber can be used. If laminated boards are used of similar width to the bales, these are more costly; but in each case posts can be placed according to bale lengths, making construction easier. There will be a beam central to the straw that can be used for compressing and stabilizing the bales as part of the structure, and the roof weight will be carried by the hidden posts. Wider posts between bales may create cold bridges*.

Options a) and b) are usually prefabricated off-site and take two to three days to erect. The other methods can be prepared off-site but are generally built in-situ. With all options, the roof should be built before the straw is installed, but remember – this does not mean the straw is protected from the weather! If the roof is two storeys high and you’ve laid one course of bales (or even part way up the first floor), it won’t stop rain hitting them.

In option a) and b), as in load-bearing, the bales are best laid flat. For the other options they can also be used on edge. In general, bales on edge are more unstable and work best if they are placed between posts one-bale length apart.

Size of posts

Any posts that are partially or fully housed within the straw bales should be kept to a minimum size, because the bales will need to be notched. The maximum dimensions for central posts are 100mm x 100mm (4" x 4") or 150mm x 50mm (6" x 2") if placed in parallel with the wall face, as these sizes can be accommodated between the strings (if placed centrally) without compromising the integrity of the bale. Laminated boards should be kept to minimum thickness to avoid cold bridging.

Posts that are partially housed into the straw can be any size as long as they penetrate no more than 75mm (3") into the side of the bale. If they penetrate more than this, they need to be placed carefully according to the bale plan, and located at junctions between bales. Otherwise, the notches into the bales cannot easily be made due to the location of the strings.

Posts that make no contact with the plastered bale walls can be of any size.

Post material

Finally, if the posts are not made of wood, you will need to think carefully about condensation issues. Steel posts, for instance, are much better on the inside of a building so that the straw insulation keeps them warm and makes sure condensation will not be a problem. If they are within the straw itself, you may need to design in some moisture protection or regulation around them. This can be done by spraying them with a thick coat of clay before inserting the straw, but adds extra time to the job.

Stabilizing the straw for plastering

Methods for compressing the straw within the frame are described above and in Chapter 13. The compressed bales give a solid surface on which to plaster. Alternatively, the straw can be compressed every four bales and compression held in place with permanent horizontal timbers fixed to the posts.

Traditionally, walls were stabilized using external pins and fastened to the base plate and beam above inside and outside the wall. These long pins would be sewn together with baling twine through the body of the bale using about two stitches per bale: as you can appreciate, this is an enormous amount of work. However, as long as the straw has been tightly packed beneath a central beam so that it is more stable than just a stack of straw, it can usually be plastered. In this case, it will benefit from using a jute mesh (8mm (¹/₃") holes) embedded into the wet plaster to give greater reinforcement against movement and therefore cracking.

Lightweight frame

This method uses the structural properties of the bales to enhance those of a lightweight timber framework. The frame is so minimal that it could not stand up alone, and requires temporary bracing and props to give it stability until the straw is in place. The straw is an essential part of the structural integrity of the building – more so than the timber – and it works together with the timber to carry the load of floors and roof. Small timber posts usually 100mm x 50mm (4" x 2"), are located at intervals and either side of window and door openings. They are designed such that the timber wall plate or roof plate, at first-floor and/or roof level, can be slotted down on to the posts once the straw is in place and the bracing and props are removed, allowing the bales to be pre-compressed. Compression of the straw-bale infill walls is essential to achieve enough stability of the walls for structural integrity and so that they can be plastered. In two-storey buildings, the first floor needs to be lowered before the first-floor walls are installed so that the posts have free movement through the wall plate. The posts do not become fully structural until the walls have been compressed and then the timber is permanently fixed to the wall and roof plates. A variation on this is the Greb method developed in France in 2007. It uses a double timber frame of small dimension, the same width as a bale and with one bale between each section. In this it is very similar to the way the Feuillette house at Montargis was built in 1921. Shuttering is fixed to the external frame, and a cement and sawdust-based render is poured in to coat the straw, which can be further rendered if required. It remains to be seen whether this is a durable method, since it is not a good idea to introduce cement into an otherwise natural building.

Pre-fabrication

Whatever size of building you are constructing, make as much of it as possible before you install the straw! This speeds up the actual time of the build and therefore its vulnerability to the weather. Some prefabricated building techniques are much easier for the novice or self-builder to achieve without too much skill and with enough time. Others are impossible for the self-builder, as they require major haulage and lifting equipment, and can only be carried out by contractors.

Prefabricated panels before and after plastering, as developed by Modcell. The panels were used on the LILAC co-housing project, in Leeds, seen on page 20. Photographs © Paul Chadderton and © Modcell.

The more a technique is taken out of the hands of a relatively unskilled person and confined only to that of the contractor, the more costly it will become. Not only that, but other aspects of the build – such as fun and beauty – will be sacrificed too, because you won’t be able to get involved – and prefabricated panels are flat and square! No organic shapes or curved corners! These are some of the costs of an industrialized process.

Prefabrication reduces the time spent actually putting something together on the building site. It doesn’t necessarily reduce the time it takes overall; it’s just that a lot of it is done somewhere else.

Load-bearing

We don’t think about load-bearing as a prefabrication technique, because that conjures up images of boxes and cranes. But in essence, prefabrication means only ‘made in advance’. And it’s sensible to do this! The base plate, ground floor, first-floor ring beam and joists, roof plate and roof can all be made in advance. Self-builders often spend the winter making all the timber parts so that they are ready to go as soon as the weather picks up in April, thus ensuring the whole structure of the build can be completed and rendered externally before September.

Prefabricated parts can either be made off-site and brought to the site later, or constructed on-site. Make the sections as large as you can manage, so if you will be running courses to help build, think about how much a group of 12 people can lift at one time (it’s a lot!). This reduces the number of joints you’ll have to make, which can be weak points. Make sure you have understood the logistics of what you propose to do. For instance, there’s no point in making really long sections if you can’t get them in because of the scaffolding! All these points are still relevant even if you have asked a local carpentry firm to do the work for you.

Commercially, a contractor can feasibly make whole floors and roofs, including the ring beams, in even larger sections for bringing in by crane, given enough space.

Infill

Traditional timber-frame buildings are always prefabricated and brought to site for assembly, which takes only a couple of days. With this method and a good design that takes the straw into account, the rest of the external structure (infill straw) can be completed in a couple of weeks, with the lime plastering following on in another couple of weeks. Other types of infill methods, using smaller timber studs, can either be built on-site or be made in prefabricated panels and brought to site for assembly. The same can be done for floors and roofs.

Prefabricated plastered straw panels

One type of these has been developed by Modcell in the UK with major government funding, and consists of large laminated timber boxes filled with bales of straw, cross-braced with steel and both sides plastered with lime. They are pre-fabricated at off-site ‘flying factories’ and brought to site by cran, e where they can be assembled very quickly. Other versions exist in Scotland, where the bales are cut in half to make thinner panels that can be manoeuvred by a couple of (strong!) people, and also in Belgium, Austria and Canada.

Hybrids

There are many types of straw buildings that use a combination of ideas from the above techniques, or use other methods. Being so simple, using straw allows for invention during practice. For instance, it’s possible to build well-insulated load-bearing walls to protect your house on the cold north side and combine this with a framework method on the south, allowing for lots of windows to maximize solar gain. This is what we’d call a hybrid – using the two main techniques in one building. The two-storey house that won the Grand Designs Eco-home 2008 award was designed like this. All it takes is a bit of ingenuity with the design to make the rigid parts work with the flexible parts. Many of our smaller buildings have been designed in this way.

Other methods

Other methods have been used at different times around the world too. For instance, in the early 1980s in Canada, Louis Gagné pioneered a bale-building method using cement mortar between the bales, called the Mortared Bale Matrix. The bales were used much more like bricks – stacked in vertical columns so the cement mortar, in effect, formed posts between each stack – and the whole building was cement-rendered inside and out. This approach is rarely used now because of the knowledge of simpler, more environmentally conscious and more enjoyable methods, but it is occasionally practised, particularly in France, through the French connection between Quebec and Europe.

In England there is at least one example of a barn made with peculiar cylindrical bales, about 400mm (16") diameter and 600mm (2') long.

Local lore says they were made with an American baler… What this tells us is that our ingenuity knows no bounds. We will always be thinking up new ways of doing stuff. Let hope that they are always simple, effective and affordable.

A builder at work on straw-bales.

This café was built with a hybrid of building materials, including an oak frame at the front with infill panels of straw and three sides of load-bearing straw. Photographs © Rae Parkinson.

* a cold bridge is a pathway through the building fabric through which cold can enter from the outside. Cold bridging can be caused by use of material that is not insulating, or poor design.

CHAPTER 5

Understanding your bale

As with any building method, it is important to understand the nature of the materials you are using. Choosing good bales is important, as is treating them properly during the build. Bales should be dry, well compacted with tight strings, be of a uniform size and shape, and contain long straws with virtually no seed heads.

Photograph © Rae Parkinson.

Bales come in different sizes, shapes and straw-orientations, which you will need to plan in to your building designs (see Chapter 6). As you get more familiar with straw bales, you can really begin to appreciate the simplicity and versatility of this natural material.

The nature of straw

If we leave a bale of straw out in the field to be rained on, it quickly becomes too heavy to lift because of water saturation and is of no use other than as mulch for trees. However, if we stack lots of bales carefully out in a field, raise them off the ground and put a good roof over the top, they will withstand the weather, and the outside edges simply get wet and dry out again. Talk to any older farmer and they will tell you that this is how straw (and hay) was traditionally stored – in the field for ease of access. The bales would be raised off the ground first, usually by using a sacrificial layer of bales (ie those that would go to waste later) laid on edge, and the rest would be stacked flat, with a roof of thatch over the top. The sides of the bales would be exposed to the rain and wind, but getting wet was not a problem. Straw does not ‘wick’ (suck) water into itself like concrete does. It simply gets wet as far as the force of the wind can drive the rain into it. When the rain stops, the natural movement of air or wind around the bales dries them out. This cycle of wetting and drying does not normally damage the bale.

There have been problems in recent years in some modern grain crops. If the straw contains an excess of nitrates (owing to the use of too much fertilizer) and then it gets wet, the combination of the two can create great compost! This fact is utilized when using straw for DIY urinals, as the added natural nitrate helps it decompose. Too much nitrate in straw from farming practices has been a particular problem with thatch, which obviously gets wet rather frequently, but shouldn’t be a worry with straw walls, as they will be designed so that they don’t get wet. The time to be aware of this issue is during transportation and storage.

Modern growing methods and the development of hybrids for grain crops has definitely changed some of the properties of straw, making grain for food more commercially viable but not necessarily improving the durability or length of straw for building.

Organic farmers often grow older varieties of straw and do not treat their crop with commercial fertilizers or pesticides, which gives their straw some advantages over non-organic straw in that it is longer and will not rot down quickly. Clay plasterers report that organic straw has a natural waxy surface that prevents uptake of water, which is not what they want for plastering but is what we’d like for wall building, as it is more durable. Allergy sufferers also find organic straw easier to live with, as it doesn’t contain chemical residues. Sometimes organic straw can contain a lot of weeds, which makes it more like hay than straw, so it needs to be selected carefully. However, it is usually quite hard to find organic straw bales, because it is mostly used by the farmers who grow it and is not sold on.

Bale and straw sizes

Most baling machines in the UK produce fairly dense 2-string bales that are 460mm (18") wide x 365mm (14") high and are of variable lengths from 900 to 1125mm (36"-45"), although a few machines are 500mm (20") wide and 390mm (15") high. In parts the Continent, bale quality can vary. For instance, German bales are on average less dense and shorter than those of the UK.

The USA and Canada also have 2-string bales plus a larger 3-string bale which is about the same length, although can be as much as 1.2m (47") long. 3-string can be between 365-435mm (14"-17") high and are 590 - 600mm (23"-24") wide.

Rectangular, 2-string bale typical in the UK, the USA and Canada. Photograph © Rae Parkinson.

Mini Hesston bales of 2.4m x 1.2m x 0.7m (7'10" x 3'10" x 2'3") or quad bales of 2.4m x 0.8m x 0.8m (7'10" x 2'7" x 2'7") can also be used. These are especially good for building extremely large spaces such as warehouses, or creating acoustic barriers. The Swiss architect Werner Schmidt has designed and built several houses using jumbo bales.

Modern farming practices are moving us towards shorter stemmed crops, and some types of combine harvesters also chop up the straw ready for ploughing back into the field. When choosing your bales, make sure the harvester is a ‘straw walker’ type. This has a conveyor belt that walks the straw up into the machine instead of chopping it – this will give you longer straws that make the bale stronger.

How to choose good building bales

Bales should be dry, well compacted with tight strings, be of a uniform size and shape, and contain virtually no seed heads. Straw stems should be quite long, at least 150mm (6"). They must not be damp, and must be protected from moisture during the building process. There should be no signs of vermin having nested during storage.

Bale standard

alt Bales must be dry, with a moisture content below 20 per cent.

alt Bales should be as dense and compact as possible. The baling machine should be set to maximum compression; in general this means bales contain about one-third more straw than usual. For 2-string bales, weight should be between 16kg and 25kg (35-55lb).

alt Density should be between 100 and 155kg/m³ (220-340lb/ft²).

alt Bales should be regular and uniform in shape. They should not have missing corners, nor be curved, nor be of different heights and widths.

alt Bales should be more than twice as long as they are wide and are usually measured in imperial units rather than metric: about 1.05m (41") long on average is good.

alt Strings must be very tight, so that it is difficult to get your fingers underneath. They should be about 100mm (4") in from the edges of the bale and not sliding off the corners. String should be sisal or hemp baling twine for a low environmental impact or polypropylene, but not wire, as metal is best avoided within a straw wall.

alt The type of straw is immaterial, as long as the above guidelines are followed. It can be wheat, barley, rye, oats, rice, etc. Rye would be a good choice, as it contains a natural fungicide so is very resistant to rot, but it’s almost impossible to get it in the right size of bales. Winter wheat would be the second choice, as it is more durable, having had to survive harsher weather conditions.

alt Straws should be long – 150mm (6") minimum, preferably 300-450mm (12-18"). Use bales from a straw-walker harvesting machine for longer straws.

alt Stored bales should be raised off the ground on pallets or similar, and weatherproofed but allowing for good ventilation.

Do not confuse straw with hay or grasses. Straw is the baled-up dead plant stems of a grain crop. It has had virtually all its seed heads removed, and contains no leaves or flowers. It is a fairly inert material, with a similar chemical make-up to wood. It does not decompose easily, usually requiring the addition of nitrates to do so. Hay, on the other hand, is grass baled up green, with lots of feedstuff (leaves, flowers, etc) deliberately left in there because it is fed to animals. It readily decomposes, unlike the dead plant stalks of straw.

The age of the straw does not matter, as long as the above conditions are fulfilled, and it has been stored correctly. The above conditions apply equally to all bales, whether they are being used for load-bearing or infill.

Bale size and length

It is vital to know the length of bales you will be using before finalizing the dimensions of foundations, wall plate, roof plate, etc. Bales can vary in length, from supplier to supplier and within each load, since whether or not the straw is picked up uniformly as it is baled depends on the skill of the tractor driver and the evenness of the field.

In practice, relying on the farmer to tell you the length of bales is not a good option, as you are unlikely to get an accurate picture. Besides which, you will need to satisfy yourself that the straw is baled dry, and kept dry while in transit and storage. It is far better to look at the bales once they’re harvested and determine the average length of bale at the same time. The best way of doing this is to pick 10 bales at random and lay them tightly end to end. Measure the whole and divide by 10 to find the average. Now you have to bear in mind that when you use these for building, you will dress them first (see Chapter 11) and this will normally reduce the average length by 30 -40mm (1-1½"). So a bale that measures on average 1.1m (43") is likely to end up as only 1.06m (41").

It is almost impossible to measure the length of one bale accurately; give three of your friends a tape measure and ask them to do it – you’ll find they all come up with different answers, and they’re all correct, in a way! However, if you find that your delivered bales are not the same length as you expected, this is not an insurmountable problem. It may mean a little more work in shaping the bales to fit, but this is straightforward and not too time-consuming. To accurately measure the width and height of the bale, lay one on a flat surface, place a spirit level on the top, make it level and measure down to the ground. You should only need to do this for a couple of bales, as they should be identical, having come out of the same machine. This gives you the height of the bale when laid flat. Turn the bale by 90º on to its side and repeat the process to find the width of the bale.

As straw-bale building becomes more widespread, good-quality construction bales are becoming more readily available, and the wholesalers that can supply them can be found listed under Agricultural Merchants or Straw Merchants in directories. It is possible to harvest and store straw in bales of uniform length and moisture content, ready for the building market each season, and wholesalers are starting to do this. As long as the bales are kept dry and well ventilated, they can be stored for a long time before using. As demand increases, so will the reliability and availability of supply.

In terms of design, measure the bales as above and use an average length as your standard block – usually 1m (3'). The width and height should be identical for all. If it isn’t – go to another supplier! When designing, you will need to make a good estimate of how much the bale will compress under load – for the very best bales this is only 10mm (³/₈") per bale – and make your finished design based on these. Your construction drawings should mention both values, compressed and uncompressed (for more on designing, see Chapter 6).

Straw is an agricultural by-product, the dead stems of grain crops such as wheat, oats and barley.
Photographs © Rae Parkinson.

Construction-grade bales

As more and more buildings are being constructed out of straw, this has given us the opportunity to really find out which are the best bales for building. It is possible to order ‘construction-grade’ bales from particular wholesalers that are familiar with the above requirements. These bales are often made using a modern baling machine which creates very dense, uniform bales that are delivered to site in packaged bundles with 21 on a pallet. Their dimensions are 1,050mm x 365mm x 460mm (42" x 14" x 18"). Knowing what your bale dimensions and properties are makes a big difference when you are designing and building. If using pre-compression methods, these bales are so dense that they will each compress by no more than 10mm (³/₈") per bale, so you can also predict with greater accuracy what the settlement of the walls will be and factor that in to the building process. Although it is still possible to use other types of bales, you must remember that there may be greater settlement on the walls, and allow for 25mm (1") or more per bale if using pre-compression methods or a reasonably heavy roof with unknown bales.

Orientation of the straw

In most bales, when laid flat with the strings on top, the straws are also laid flat, as this is the way that the majority of baling machines pick them up from the field and ram them into the end of the machine box. However, there are some balers, called bottom feeders, that ram the straw into the box from underneath rather than from the end, and the bales from these machines have vertical straws, not horizontal ones, when laid flat. These machines are generally made by Massey Ferguson, and have an extra contraption behind that can bundle 21 bales together with steel straps into a block that can be moved by a forklift truck. These modern baling machines are owned by large straw contractors that bale thousands of bales at a time. These tend to be the type of bales that are used for construction, because they are very dense, high-quality, uniform bales that are meant for transportation.

Bales made by a bottom-feeder machine (left) and an end-feeder machine (right). In the left hand pictures you can see the strings of the top and bottom of the bale, showing cut ends and the straw vertical. On the right you can see the sides of the bale, the lower one cut and the top folded.
Photographs © Rae Parkinson.

The orientation of the straw does not have any significance for the stability or the structure of a straw-bale building whatever its type, BUT at the moment, it does have significance for Passivhaus design. There has been a small amount of research that says bales are more insulating with the straws in a vertical orientation than if they are horizontal (personally, I’m not convinced). As much of Passivhaus design is based on the theoretical values of the properties of materials, a straw-bale house cannot meet the standard currently unless the straw is in the vertical orientation.

Moisture in bales

Bales should always be dry. This means the straw should be cut on a dry day, left to dry in the field for a couple of days, turned and then baled, all without any rain. Then when collected up they should be transported and stored dry, and kept dry as they are brought to your building site. On site they should be stored in a dry place, and protected from any rain during construction. It is the top and the bottom of the bales that are vulnerable. If rain gets into a bale, gravity will carry it down and into any bales below. However, bale walls will not usually be at risk as long as their tops are securely covered. Rain that hits the side of a wall will wet the bale as far as the wind can drive it (so less in a compressed wall/bale than an uncompressed one). But straw dries out quickly once the rain stops – air movement will carry moisture away.

During construction, any posts in the bale wall can create potential problems for waterproofing. Clearly, a tarpaulin cannot be placed over the top of a post, but has to be wrapped around it. In a severe rainstorm, water can run down the post and into the straw either side. If you’re using boards on the top of walls to protect them, the rain can penetrate the bales at the junction between the board and the post – so take great care in these positions!

Moisture content and relative humidity

If straw is going to rot, it requires two conditions to be present together: a) high humidity, and b) high temperature. In Europe, and many places in the USA and Canada, these rarely happen at the same time.

Safe moisture levels for the prevention of fungal and bacterial growth are as follows:

Moisture content should not exceed 25 per cent dry-weight basis. This means that the weight of water in the bale should not be more than 25 per cent of the weight of the same bale if it was thoroughly dried. Or, moisture content should not exceed 20 per cent wet-weight basis, meaning that the weight of water in the bale should not be more than 20 per cent of the weight of the same bale when wet.

Relative humidity should not exceed 84 per cent. This means that whatever the maximum amount of water vapour the air can hold at the current temperature, the bales must not contain more than 84 per cent of it. Air holds more vapour without the water condensing out if it is warmer (see Appendix 4).

In practice, the only time there will be a moisture problem in a straw-bale house that was built using good-quality dry bales is if there is an accident or mistake during construction, or if routine maintenance has not been carried out for several years. Most problems are caused by roofers not adequately covering the straw walls overnight when they are working, not covering the walls at all if it starts to rain, or not thinking about how rain can run down rafters and into bales despite waterproof covering over the walls themselves. Other problems may be caused by fixing the gutters to the roof of an unplastered house but not fixing downpipes – thus funnelling far too much water directly at the straw, or not understanding that the straw should not be sprayed with water before plastering. Other moisture problems may be caused in the long term by allowing the garden to build up against the wall of the building, or not repairing gutters and downpipes, etc. These sorts of problems affect all houses, not just straw ones.

Bales won’t rot in the long term

High moisture levels in straw, together with high temperatures, can cause mould growth (and bacteria and rot). The UK will rarely have both these conditions at the same time, nor will Canada, but some places may in the USA. However, you needn’t be too concerned about mould in straw if you have observed all of the above, making sure the bales are dry throughout the whole process. If you use lime renders and plasters, this will further guard against mould and rot. They have been used for thousands of years because their high alkalinity inhibits mould growth and therefore makes living and working spaces hygienic. This is another really good reason to use lime renders! In addition to all this, there is now evidence (from studies being done at Bath University, UK) that in straw houses that have been constructed for more than a year, even if the straw gets wet subsequently, microbial activity is far less than might be expected. We think this is because the dry conditions in the straw walls over the preceding year or more have actually killed off any residual mould that might have been there from harvesting.

The cost of bales

The cheapest way to buy bales is straight from the field after they’ve been made, and to buy locally so as not to pay large transport costs. This has the added benefit of minimizing the environmental impact of transportation. If you collect them yourself they can cost as little as 80p per bale plus your fuel costs.

When you consider that the average 100m² (1,100 ft²) 3-bedroomed house will use about 350 bales, this represents a material cost of only £280! However, most of us will have to buy bales and have them stored for us and then delivered to site, which will increase the price.

Construction-grade bales bought in bulk from a wholesaler, delivered to the site, can cost about £3.50 each, which would bring the price of 350 bales up to £1,225 (plus VAT). These bales generally have about a third more straw in them than ordinary bales. The same bales could cost £2 if you collect them yourself, with an extra £1.50/bale for the hire of transport and diesel costs, and your own time on top of that, so it is often not cost-effective to collect bales yourself.

Bales being delivered to site. Factoring in transport hire, fuel and time, it is often more cost-effective to have bales delivered by the wholesaler than to collect them yourself. Photograph © Rae Parkinson.

Although the walls represent only about 16 per cent of the total costs of a finished building, using straw instead of masonry or timber/masonry is a significant saving on the cost of materials within this, and becomes more so for construction firms building more than one house. Together with this, the labour time involved in straw-bale building is vastly reduced once the labour force is familiar with the material. Plus, you can run training courses to install your straw, learn how to do it yourself, get your walls built and have no labour costs! Can you afford not to build with straw?

CHAPTER 6

Bale plans: designing with the bale in mind

Think about what you want your straw-bale house to look like and how you want it to feel inside. Try to forget anything you’ve been told about building and imagine your ideal space, however wild that might seem! Then work within the practical limitations of the bales to come as close as possible to your dream. Follow good design principles, and keep in mind the dimensions of your bales so that your dream can be translated simply (and affordably) into a reality.

Photograph © Rae Parkinson.

The design of straw-bale houses is usually simple and elegant. Straw buildings are based on a block design and therefore different elements of the structure can be built up easily from the initial shape and dimensions of the foundations. Each section of the house has an obvious relationship to the other sections, and many different houses can be designed quickly and easily from the same basic plan.

For most small buildings, it should be possible for owner-builders to design their own houses with a little guidance, and certainly to work out the straw elements. The way a straw-bale house goes together is simple. It follows common-sense principles and is effective. By using this book, you should have no difficulty in working out the construction drawings and methods for any type of domestic dwelling.

Getting started

Once you’ve decided on what the building is for, what you want it to look like, and what you want it to feel like, think about the building’s orientation, design the rooms you live in most to face south, and have more or larger windows on this side to maximize solar gain.

Draw the shape of the building you require, as though you were looking at it from above. This is called the PLAN view. You will need to know what size your actual bales are (see Chapter 5). Draw in the shape of the bales, their width and length, planning where they lie on the first course of the wall, as in the drawing on page 54.

Now imagine you are looking at the finished building, standing on the ground looking north, south, east and west. Draw the face of the building you see from each direction, again showing where each bale is and how they turn corners or curve, etc. These drawings are called ELEVATIONS.

Bale plans ready for use on site.
Photograph © Jakub Wihan.

From accurate bale plans you can work out how many bales you need as well as how many hazel stubs (two per bale on the first course), pins (two per bale for the fourth and sixth courses), staples if needed (in every bale where it changes direction, eg corners) and other quantities of materials. Details of foundations, windows, first floor and roof can be worked out. You also now have the basis for drawing your own plans to use in an application for planning permission.

Principles of bale design and layout

alt Before you draw your final plan, and before you finalize the dimensions and lay out the foundations, you need to know the dimensions of the bales you will be using, as they can vary a lot!

alt Always draw a bale plan and elevations before doing anything else – the design comes from the bale dimensions, not the other way round!

alt The bale plan should be made up of a whole number of bales wherever possible. Any half bales should only be at an opening.

alt Do not have any places in the wall (eg beside a window) that are less than half a bale in length.

alt Spaces between openings should be no less than one bale length. If they are, consider using a hybrid system with a partial frame.

alt Window and door openings must be at least one bale length away from corners in load-bearing designs.

alt If at all possible, choose window and door sizes that, together with their framing, are multiples of bale dimensions.

alt Try to design frameworks so that the distance between posts is equal to a whole number of bales or half bales, thereby reducing the labour time involved in customizing.

alt You can calculate bale numbers and hazel pins/stubs that you’ll need from the bale plans and elevations.

alt Bales should always be at least 25mm (1") above finished floor level. A timber baseplate also provides a fixing point for skirting boards etc.

alt Remember ‘a good hat’ and ‘a good pair of boots’! Allow plenty of overhang on the roof (450mm (18") is ideal), use verandas and porches, and a plinth or upstanding foundation of at least 300mm (12"), preferably 450mm (18").

General design principles

alt Face your house to the south, have plenty of glazing here, and make sure it can be shaded in summer (but not winter!).

alt Place your utility rooms, and those you won’t spend much time in, eg toilet, boot room, on the north side so the rooms you do spend time in have the most light and sun.

alt Preferably design open-plan downstairs so you only need one heat source.

alt Make sure you have opening roof windows at the top of the stairs, or at least opening windows upstairs in case you need to let out the heat of your thermally efficient house!

alt If you have plumbing, eg bathrooms and toilets upstairs and down, try to keep them above one another to reduce the amount of pipework you’ll need.

alt A sloping site can be used for the chamber for a composting toilet, or storage underneath.

alt Make sure whatever you submit for planning approval can actually be built in straw – you’d be surprised how many beautiful designs are either un-buildable or unaffordable. For example, if you want a two-storey height window, have you allowed for the ring beam that will need to pass through it at floor level?

alt Make sure your planning design will also meet Building Regulations or Codes. For example, have you shown all the chimneys and vents that will need to go through the roof? Have you shown a ramp up to the front door for disabled access? Anything that changes the look of a building after planning consent may require a costly planning amendment.

In a load-bearing design, the walls will settle once the weight of the roof is on, or during pre-compression, so allow for this by leaving gaps above windows and doors. Always allow for a bit more settlement than you calculate, so that any gaps can be filled in later. Otherwise, you may increase the beauty of your building unintentionally by creating eyebrows above windows and doors! With construction-grade bales, settlement in a 7-bale-high wall should be about 70mm (2¾"); for less-dense bales you need to allow for more settlement than this, unless you have a lightweight roof. The amount of settlement depends on the density of the bales and the amount of loading applied to them (such as the weight of the roof, whether there is more than one floor, etc).

Interestingly, some of the laboratory tests aiming to find out how much load straw-bale walls can carry have appeared to show that only single-storey houses can be built this way. This is patently not true in practice! We need to be very careful when looking at research findings, and especially when talking to engineers, to make sure that everyone knows exactly what they are doing, and are in fact measuring the thing they claim to be measuring. Common sense and practical experience should be our guides. After all, it was engineers who proved that a bumble bee cannot possibly fly…

Sensible design

A successful self-build design must be relatively easy for a novice to understand and construct. It should involve simple, easy-to-follow techniques that do not require expensive machinery, specialist equipment or non-standard building products to achieve. As well as these considerations, it should be readily adaptable to changing personal/family circumstances, be built of materials that do no harm to our planet, and be possible to build between the beginning of spring and the onset of winter. If you follow all of the above, it will also be affordable.

The load-bearing straw-bale building technique (that requires no extra framework to support floors and roofs) has proved that it can meet these criteria, and also produce buildings with a lifespan of upwards of 200 years.

A successful self-build design should involve simple, easy-to-follow techniques.
Photograph © Jakub Wihan.

CHAPTER 7

Cement-free foundations: ‘a good pair of boots’

Straw-bale buildings need good, self-draining foundations: ‘a good pair of boots’ as old cob-builders used to say. Foundation designs for straw-bale buildings have been developed by going back to basics and thinking through how and why traditional designs worked. What’s great about this approach is that it keeps things as simple and cost-effective as possible, and provides foundation solutions that are applicable to all buildings, not just straw-bale ones.

Photograph © Rae Parkinson.

All buildings need to have a foundation on which to build, but this may very well be provided by nature herself. We seem to have forgotten how builders used to make foundations in the past, blinded by modern materials, but luckily we still have thousands of examples still existing around us to look at and be inspired by.

An introduction

The natural foundation of the earth may be bedrock, gravelly soil, firm clay, etc, but in today’s world we are more familiar with artificial foundations such as poured concrete strips and slabs. As the foundation has to transfer the weight of the walls above it (and other loadings such as floors, furniture, roofs and even snow in winter), onto the earth beneath, it is important to know what type of earth (or subsoil) is found on your building site.

Different types of earth will be able to carry different weights. Gravelly soil, for instance, will carry much greater weight than soft clay. On the other hand, if you increase the surface area that bears the weight on soft clay – much like wearing snow shoes in the snow – even this can take the weight of a house. For a small building constructed of lightweight materials, there is obviously no need to build massive artificial foundations on any type of soil. Equally, for a heavy building built on what we call good bearing soil, there is also no need to use deep foundations. Almost all the buildings in the world that are more than 200 years old have natural foundations with little or no artificial ones. They often use larger stones at the base of the wall, making it slightly wider than the wall itself. In all cases, the builders removed the topsoil (organic growing layer) and dug down to something solid, and because they chose their building sites well, this was usually just below the topsoil.

There are hundreds of thousands of houses still lived in today that can be excavated by only 150mm (6") or so to find they are sitting on the ground itself, and yet they are completely safe and sound. Unfortunately, there are many misconceptions about foundations that are partly caused by the rising popularity of cement and concrete. In some building colleges, students are taught that buildings can only have foundations made of concrete, despite the evidence to the contrary that surrounds us. While learning about straw-bale building, you will be encouraged to look at what’s around you, to keep things as simple and straightforward as they can be.

There is no need to overcomplicate anything, only to understand what it is that we want to achieve and make choices based on the different ways that it is possible to do so.

Well designed, non-cement foundations use a capillary break and breathable materials to prevent damp problems by allowing any moisture to escape harmlessly. Photograph © Bee Rowan.

We use foundations to achieve a stable base that distributes the weight of whatever is built upon it over the ground beneath; to be sure that there is no unequal settlement throughout the building, to allow flexibility for the building to move over long periods of time as the ground itself moves; and to keep the inside of the building dry. In the past, natural materials were used to protect buildings; nowadays, manufactured ones are used, and they do not perform in the same way. So we also need to know the essential properties of materials in order to use them wisely.

It is necessary to discuss the causes of damp in buildings because there are so many misconceptions, about rising damp in particular. If we can understand the causes then we are more equipped to design well and prevent it happening. This applies to all buildings, not just straw ones, and the principles discussed below can be applied universally.

Causes of damp in buildings and their solutions

Damp problems in any building, straw or otherwise, are caused by poor design, lack of maintenance and/or inappropriate use of materials. They are usually slow to show themselves, sometimes taking 20 or 30 years, which is why it can be hard to understand why some of our buildings have become very damp, because cause and effect are not so obvious. But very few buildings in Europe actually suffer from rising damp, contrary to popular belief. Damp is almost always caused by something else instead.

Whatever a wall is made from, it can be divided into three main areas that need attention: the base, because of contact with the wet earth or from splashback as rain bounces off the ground on to the walls; the top, owing to rain running off the roof; and the face of the walls themselves, in driving rain.

Poor design includes:

alt poor detailing between two different roof pitches, so that rain running off one is not prevented from running into the wall of the adjoining building;

alt poor placement or complete lack of drip moulds, eg above windows, below render;

alt flat roofs in most cases.

Common causes of damp due to lack of maintenance include:

alt slipped slates or tiles;

alt lack of maintenance of gutters and downpipes;

alt vegetation growing up around the foundation, holding permanent damp against the wall and preventing moisture from leaving.

Inappropriate use of materials may include:

alt use of cement pointing and rendering on older lime-mortared houses

alt use of cement mortar and gypsum plasterboard (drywall) for renovations in older houses that were originally lime-plastered;

alt use of cement renders on breathable wall systems (eg straw).

At the base of the wall, foundations need to be designed so that:

alt moisture from the earth cannot travel upwards through them;

alt rain bouncing from the ground is not able to pass through to the interior of the house;

alt any moisture that does find itself in the foundations can escape harmlessly to the earth or the exterior.

In the past, moisture was prevented from rising up from the earth by using non-porous building materials such as stone, eg slate, granite and some sandstones; or bricks, eg engineering bricks. These would be laid with vapour-permeable lime or clay mortar and coupled with good design above ground, eg double-skin walls with rubble infill. A natural slate damp-proof course (DPC) would be used if the building material was porous or was expected to become so over time, eg some sandstones and brick.

Such materials and design were also used to prevent most driving rain from entering the face of the walls. Moisture that did enter through small cracks or damage would be caught by the central rubble, filter down inside the wall and exit safely through the vapour-permeable joints once it had stopped raining.

Problems can occur with these designs for all the reasons described above, and also if:

alt modern materials have been used for renovation or repair that are not vapour-permeable, most commonly cement pointing, cement rendering and modern gypsum interior plaster;

alt alterations have been made to the walls using poor workmanship and inappropriate choice of materials, eg a new window has been added using cement mortar, which has been allowed to fall down into the rubble infill, thus creating a bridge for damp to travel across into the internal skin of the walls.

Cement is used in buildings as a rigid, non-vapour-permeable and strong material. Unfortunately, all these properties can contribute to damp problems. Firstly, its rigidity means that as the ground (and therefore the building) moves imperceptibly over time, the cement will crack because it isn’t flexible. Secondly, rain gets into a building through these cracks, filters down behind the stone or render finish, and builds up at the base of the wall and can’t escape because the mortar joints are not vapour permeable. Thirdly, its strength means that it’s often stronger than the brick or stone it’s holding together and erosion of the weaker material happens at the interface between the two, causing the brick or stone to flake away, leaving gaps, and the cement pointing to remain intact.

One other characteristic of cement that has not been fully addressed in construction is that it sucks water into itself. It is porous, but not breathable. This makes it a liability over many decades, which is only just coming to be understood, as damp problems in buildings take a long time to show themselves. We therefore need to think of cement as a ‘wet’ material. A cement render will wick water through itself and into the background behind; it drinks in water faster than it can evaporate out.

So we always need to protect our buildings from cement. In modern designs this is usually with a plastic and waterproof membrane, and with a cavity wall. This is where lime renders and mortars can be shown to be eminently superior (see Chapter 15). They absorb moisture and become saturated, and this prevents any more moisture from passing through, thus protecting a building. Cedar shingles work in a similar way – by absorbing moisture, they prevent the passage of further moisture. Aren’t natural materials wonderful!

The problems caused by inappropriate use of materials and design take years to become apparent, so it can be hard to accept that the damp at the bottom of your 150-year-old stone-and-lime kitchen wall is caused by the cement pointing the previous owner did 30 years ago coupled with the new gypsum you plastered the walls with two years ago.

Traditional building design accepted that rain and moisture could not be totally kept out of walls, but made sure that any that got in could escape safely to the outside, and also used flexible materials to prevent cracking. In the 20th century we decided to build rigid buildings that moisture could not penetrate, but in practice this theory has never worked well because either the materials crack, which they inevitably do because the ground is always moving, or they wick moisture through the mortar joints and render, which then filters downwards and can’t get out again. These are some of the main causes of damp in modern houses.

Using porous materials with a DPC on top shouldn’t be a problem, as long as there are no holes in the DPC – which is easier to ensure with slate than plastic. But if the DPC extends the whole width of the wall – eg on top of poured concrete foundations – then we have created a waterproof barrier that allows moisture to collect at the base of the wall, just where we don’t want it, and prevents it from filtering downwards and safely out of the building. Modern buildings use a tray DPC that directs any moisture at the base towards the outside wall, not the inside, as part of a cavity-wall system.

The use of metal wall ties to hold the two wall skins together in cavity-wall construction also hold any cement mortar that is dropped into the cavity, a common occurrence when bricklayers are working fast and not caring too much about quality. This mortar then becomes a bridge for moisture to pass along to the inside of a building, and causes problems years later.

Designing foundations

When thinking about designing foundations, the first thing to decide is whether the ground itself can provide us with a natural foundation. If it can, then there’s absolutely no need to dig it away and create an artificial one. It has become the norm not to think about what the actual ground conditions are on a given building site, but to go for the lowest common denominator – a solution that will work in pretty much every situation, regardless of what the earth is doing for us – and dig 450mm- (18"-) deep trenches and fill them full of wet concrete. In many cases, this is not necessary and thus creates an unjustified environmental impact that can also be expensive! But it does mean you don’t have to think or have interesting discussions with your engineer or building inspector.

For stability, find out what your subsoil is

There are several ways you can find out what your subsoil is and if it’s good bearing soil:

alt Look at houses in the area (as long as they’re pre-1900 and made without cement) and see what their foundations are made of. If they’re quite shallow, and the houses are not showing any structural problems, you’ve almost certainly got good bearing soil. Not only that, but you can use the same foundation designs for your own house.

alt Ask older people in the area, who might remember houses being built in their youth, what foundations were like then.

alt Consult your building inspector / Code official, who should have a good local knowledge of older houses and subsoil in the area.

alt Dig two or three pits about 800mm (31") deep in the place where your foundations will be, have a look at the profile of the soil, and take photographs of it to show your building inspector. Does it seem solid? Does any of it fall away? What’s it made of – is it gravelly soil, stony or clayey?

An average 2-storey straw-bale house with a foundation width of 450mm (18") has a bearing pressure of about 43kN/m² (898lb/ft2). This means that every square metre of foundation has to be able to hold up 43kN of weight. This compares with an average masonry house with a foundation width of 600mm (24") that has a bearing pressure of 52kN/m² (1,090lbs/ft²). Masonry buildings usually have foundations much wider than the actual walls in order to spread the bearing weight over a wider surface area. If a masonry building of wall width 300mm (12") only had a foundation this size as well, the bearing pressure would double from 52 to 104kN/m² (1,090 to 2,170lbs/ft²). None of the figures above include the weight of the foundation plinth itself for simplicity.

COMPARATIVE WEIGHTS AND FOUNDATION LOADS FOR A TWO-STOREY HOUSE

Straw bale
Timber frame rendered
Timber frame brick-clad
Traditional cavity brick-and-block

TOTAL WEIGHT (kg)
Excludes foundations
41,300
31,100
53,400
76,800

TOTAL IMPOSED LOADS (kg)
Roof, ceiling and floors
21,000
20,500
20,500
20,500

TOTAL (kg)
62,300
51,600
73,900
97,300

AVERAGE FOUNDATION LOAD (kg/m)
Excludes foundations
1,950
1,680
2,370
3,120

ASSUMPTIONS:
Two-storey house with 50m² internal floor area at each level (based on 5m x 10m internal footprint).
Tiled dual-pitch roof with gable ends and flat ceilings.
Suspended timber floors at first floor and ground floor.
Average 17.5% openings in external walls.
Imposed loads are the maximum allowable live loads in accordance with BS 6399.
Ceiling heights 2.8m at ground floor and 2.4m at first floor.

Source: Peter Beresford of Structural Solutions (personal communication, August 2009).

The majority of building sites are on good bearing soil, and this type of soil will easily take the weight of a straw-bale house (and most other houses in fact, including stone ones). The types of soil where we need to think a bit more carefully about foundations are:

alt heavy clay soils;

alt made-up ground (ie you’re not sure what it’s made of because it isn’t undisturbed subsoil, eg landfill, brownfield sites);

alt wet soils;

alt moving sand soils.

All other types of subsoil should be good bearing soil.

For flexibility, use appropriate materials

There was a major change in foundation design that occurred in around the 1920s and 1930s, when cement started being used in mainstream construction, and this was a shift from flexible foundations to rigid ones. It might seem strange to think that foundations and whole houses made of stone or brick could be flexible, but because of the mortars they definitely are! Lime and clay mortars have a very high degree of flexibility, whereas cement ones do not. We know that our old houses were, and still are, flexible, because they have changed shape over time. In old houses you can see that floors are no longer quite level and doors are no longer quite square, but in fact we seem to really like this and will pay more for houses that have these quaint features. Despite having moved as the ground has shifted over long periods of time, these houses have not become unstable.

Foundations using lime or clay mortar are flexible, meaning that they move with the ground and remain stable, rather than cracking.

However, many houses built since the 1920s have experienced structural cracking, and have had to be repaired or demolished as a direct result of using the rigid material cement as a mortar. Instead of working with the natural movement of the ground, we started trying to design houses that would somehow float on top of it, and this hasn’t been very successful. Long expanses of wall laid with cement mortars need to have joints filled with a flexible material built into them to control cracking, which is why you see those ugly vertical joins every few metres in modern buildings – unlike in Victorian walls, which were able to run for miles around gardens and estates if necessary without any such joint because the lime mortar they were built with did this job itself. Modern houses are far less durable and won’t last several hundred years, as their predecessors did, because of this lack of flexibility.

This is not to mention the other undesirable effects to do with damp and condensation that are attributable to the use of cement in modern housing stock.

So it’s really important to build flexibility into the design of your foundations, and you can do this by using flexible mortar made of either lime or clay.

For moisture control, use capillary breaks and vapour-permeable materials

All walls, whatever they’re made of, will contain moisture because of normal human habitation – breathing, taking showers, etc – plus foundations are in contact with the earth, which is often wet. So to keep the inside of our houses healthy and free from damp, we need to design a foundation and wall system that take into account the ways moisture can get into the wall and provide pathways for it to naturally escape back out again.

If we make foundations of non-porous materials such as stone (though be careful, as not all stone is non-porous), engineering brick or recycled Foamglas™, then any moisture in the earth cannot travel upwards through them. There cannot be any rising damp, which is caused by capillary action wicking water through porous materials. And if we use vapour-permeable mortars such as lime or clay then any moisture that does appear in there will migrate out through the mortar joints.

Traditionally, a capillary break was used to prevent the movement of moisture from the earth into the foundation. This is a layer of stones all the same size (usually about 40mm (1½")) and at least 75mm (3") thick, such that water cannot pass up through them by capillary action, because the spaces between the stones are too great for this to happen. In many Continental countries, this is still the preferred method of providing a DPC, and it is becoming more popular again in the UK and Ireland as we are thinking through some of the causes of damp and trying to find sustainable solutions for them. Capillary breaks can be used instead of a plastic DPC, particularly underneath solid floors.

A layer of small stones underneath the foundations acts as a ‘capillary break’, allowing any water to drain down to the ground below, but preventing water moving upwards into the walls. Photographs © Barbara Jones.

Moisture in the walls will migrate downwards very, very slowly, because of gravity, so we don’t want a sheet of plastic catching this moisture and creating a problem in 20 or 30 years’ time. Instead, we want walls to sit on foundations that allow any moisture to travel down through them and evaporate harmlessly out through the mortar joints. One of the main problems with plastic damp-proof barriers is that they are waterproof! The design we want is not a damp-proof course but one with a capillary break, which will let water pass down through it but won’t let it travel upwards.

For thermal efficiency, use insulating materials

To meet the challenges of the 21st century – to build houses that are energy-efficient and require very little fuel to heat them – we have to make sure that the foundations do not allow cold into the house via what’s known as a ‘cold bridge’. The space we live in needs to be wrapped in a complete thermal envelope, with no gaps.

So, while taking into account all of the above points, we also need to choose materials that will act as a heat/cold barrier, and use enough of them to actually work! Ideally these materials need to be non-porous, load-bearing and environmentally sustainable as well. It has been a hard challenge to find sustainable materials that meet these requirements. At the moment the only real choice for the internal skin of a plinth foundation is recycled Foamglas™ block, with the infill between the outer and inner skin of load-bearing recycled Foamglas™ chunks. Insulating materials need to have low enough U-values to at least meet Building Regulations / Code, (the lower the U-value, the more insulating a material is) but these regulations do not as yet go far enough to provide truly thermally efficient houses. Although we know that we need to reduce our dependence on fossil fuels and our energy needs, which can be partially achieved by building houses that require very little heat or cooling, governments are reluctant to insist that the construction industry actually provides us with houses that do this. The cavity-wall system will never provide a thermally efficient solution to house building, and no amount of tinkering with it will alter this. Whatever we build with, walls are becoming increasingly wide to accommodate the amounts of insulation necessary to make an appreciable difference to heat loss. No other insulation material can match the low cost of straw bales.

Foundations with thermally-efficient recycled Foamglas™ chunks infill between the outer and inner skin. Photograph © Barbara Jones.

Low-impact foundations

All foundations for new buildings can be and should be low impact, not just those for specialist designs. The most low-impact foundation you can possibly design is one that requires no intervention at all but simply uses the earth itself. For all foundations the topsoil must be removed because this will always compress and compact, causing movement. Some parts of the world have no topsoil, particularly the hotter parts. Others have very deep topsoil, which might mean that the foundation needs to be quite tall, to bring your building clear above ground level. Very cold places may need to have deep foundations to counteract the effects of frost. But wherever you have good bearing soil, there is never any need to dig trenches into it. The most you may need to do is widen the width of your foundation as it touches it.

You may have some trouble arguing this with your building inspector, because these methods are not detailed in most building regulations / Codes in the UK (mind you, neither are quite a few other common building methods). However, building inspectors have, in the most part, proved themselves to be sensible, forward-thinking individuals who understand building physics. In parts of the world where you are unlucky enough to have prescriptive regulations or Codes, such as Germany or the USA, no amount of sensible discussion will allow you to build foundations that have not been described in the Code (see Chapter 19).

It’s a common misconception that all houses need to have cement-filled trenches as their foundations. Fight against it!

Foundations for good bearing soil

Below are options for foundations for good bearing soil.

Strip foundations

Strip foundations are those that follow the shape and dimensions of the building, holding up the walls fully along all their length.

If you’ve got good bearing soil, why dig it away when that’s not necessary? All you need to do is take away the topsoil in the place where you want your foundations to sit, and begin building from there. If your site is sloping, you may want to make sure that this strip is level all the way round, which might mean that part of it is deeper into the ground than the rest. Sometimes this can make building easier, but it also increases the cost because you’ll need more materials. If the slope is very pronounced, it might be better to make a step in the foundations instead of digging them all level. If there is not much of a slope, it can be dealt with by increasing the thickness of the limecrete or lime mortar to suit below the bricks/stone.

Insulated strip foundations at Elmfield Farm, Kent. Photograph © The Wisharts.

A straw-bale house needs good boots – the straw must be raised off the ground by at least 300mm (12") and preferably by 450mm (18") to protect it from the rain, so if your topsoil is deep, this means you must build your foundations from the subsoil up to 300-450mm (12-18") higher than the ground surface.

For insulation purposes, only the part of the foundation that encloses the floor needs to be insulated; anything below this can be made of other materials. The diagram on page 67 shows well insulated strip foundations – this design can be used with a suspended timber floor, as shown, or with an insulated limecrete slab. You can see that the insulating Foamglas™ block begins at the same level as the insulated floor joists, but that it is sitting on a stone or brick plinth in order to raise it up to the right height.

In the same way, the centre of the wall (this is NOT a cavity!) is filled with chunks of recycled Foamglas™, but again, only to the depth of the floor joists. Below this the space is filled with drainage gravel.

Other things to note about this design are that there is no need for any plastic. The gravel and recycled Foamglas™ are a capillary break, as well as being load-bearing. The outer skin comprises engineering brick laid with a hydraulic lime mortar – this ensures that no moisture from the earth can wick upwards through it, and any moisture in the wall that gravitates downwards can escape harmlessly through the mortar joints. Finally, the inner skin is of load-bearing Foamglas™, and no water can pass upwards through this.

The external render is held by an oak render stop incorporating a drip groove underneath, and this stop also covers the join between the brick and timber, thus ensuring airtightness. This design of foundation has a U-value of 0.17 – far better than Building Regulations / Codes ask us to achieve, but absolutely necessary for a thermally efficient house. We have started using a layer of wool carpet underlay between the baseplate and foundation, to create a squashable layer that improves airtightness.

By using TECHNOpor™ Foamglas™ chunks as a capillary break, load-bearing fill and insulation, the thickness of a limecrete slab can be reduced to 65mm or 75mm (2½-3").

Pillars or pier foundations

This method disturbs far less of the ground than a strip and may cost less because of using less material, but you must remember that the weight of the building has to be carried by something across the gaps between pillars. For a straw-bale house this is usually done with a structural timber box beam (see page 82), which has to be designed so that it is strong enough to do the job we are asking it to do.

DESIGN FOR THERMALLY INSULATED PLINTH WITH SUSPENDED TIMBER FLOOR. ACHIEVES U-VALUE OF 0.17

Car tyres filled with pea shingle make excellent pillar foundations. Photograph © Rae Parkinson.

Another consideration is that this means that the weight of the building is concentrated on several points rather than spread across a whole strip, so the ground below must be strong enough to take these point loads. Pillars can be built out of many things, including car tyres, bricks, stone, gabions (gravel in wire cages) and timber, and are one of the most cost effective ways of building foundations, especially if you use car tyres. In the example on page 68 it is car tyres filled with pea shingle that do the job. On top of the tyre pillars is a well-insulated structural box beam made of timber, designed so that it will support the weight of the building above (for more on tyre foundations, see Appendix 5).

Ground bearing pressure
When using car-tyre pillars for foundations for straw-bale buildings, the bearing pressure on each stack with a span of 2.5m (8') increases to about 150kN/m² (3,100lb/ft²) instead of the usual 40–60kN/m² (840-1,300lb/ft²) for a continuous foundation.

DESIGN FOR CAR-TYRE PILLAR FOUNDATIONS WITH STRUCTURAL TIMBER BOX ABOVE

Gravel bed foundations

This is a brilliantly simple idea, and one that you’ll find all around you but may not have noticed (thanks to my friend Stefano Soldati for bringing it to my attention). Look at how railway tracks are laid – a thick bed of draining gravel with timber sleepers embedded into them, the tracks and trains run on top. The gravel provides a capillary break between the earth and the timber, which is also solid and permanent. We can build houses like this too!

In Denmark they use a mound of mussel shells (because they have lots) to take the place of gravel, which are also insulating. All you have to do is take away the topsoil, sometimes level the ground and place a layer of gravel over the top, about 500mm (20") wider than the footprint of your building. The depth of gravel should be at least 150mm (6") to provide a capillary break, and can be deeper to keep you above the level of the surrounding topsoil. Then build your strip foundation on top. The foundation can also be made of durable timber, because the capillary break of the gravel will keep it rot-free.

To insulate solid floors

Simply replace the gravel inside the footprint of the building with a load-bearing insulation material that is also a capillary break – this would be recycles Foamglas™. Add insulated strip foundations and a limecrete floor. Perfect!

To stabilize poor bearing soil instead of using concrete

A gravel bed can be used instead of a concrete raft to spread the load of a building across a wider surface area and thus build on poor bearing soil. In this case, layers of gravel are built up using a geogrid to stabilise it (in the same way that roads are built). Each layer must be well compacted before laying the next. We built foundations on a silty marsh for The Look Out building at RSPB Conwy in Wales, when the original idea for piles failed as they could not find anything solid!

SELF-COMPACTING DRAINING GRAVEL FOR MADE-UP GROUND

Gravel trench foundations

This is another old idea, pioneered particularly by Frank Lloyd Wright as a low-impact alternative to using cement. It would be used in two circumstances:

1) If the ground is not quite strong enough for the building you want, gravel trenches enable you to make wider foundations to spread the load over a larger area.

2) If you need to dig into the subsoil, perhaps because it may be affected by frost, you will require deeper trenches, which you can then fill up with gravel instead of cement.

As long as the sides of the trench are strong enough to contain the gravel – the topsoil alone would not do this – then compact it every 150mm (6") until you reach the beginning of the topsoil, at which point, add strip foundations as above.

Even better than using gravel is to use pea shingle, because it doesn’t have to be compacted. You have to be careful with trenches because if you make them too deep they become unsafe, as the sides might fall in. Deep trenches must be shored up with timber posts and bracing. This means that a person can’t stand in a deep trench to work after a digger has excavated it, so gravel can’t be compacted. However, by pouring in 10mm (½") pea shingle you avoid this problem, because it just needs to be jiggled around with a poker rather than compacted, as it flows like water and will find its own level if shaken.

Raft foundations

Another well-established method of spreading loads across an area of ground is to use a raft. Roman builders were familiar with this type of construction, as they were able to create timber rafts across marsh land for roads.

Winchester Cathedral in southern England is partly built on a raft of timber. This was made from tree trunks laid side by side, with the chinks between them filled with chalk and then another layer placed on top at 90º to the first, again with the gaps between them filled with chalk, and then the walls of stone built on top. An extremely heavy building was able to be built in this way on very wet ground, and the timber didn’t rot because it was completely immersed in water. Problems only started to occur a few hundred years later, when the surrounding marsh was drained and the water table was reduced, leaving the timber exposed to wetting and drying, which made it subject to rot. Many modern buildings are built on a concrete raft, to spread the load over the top of any weaknesses in the ground, or to float on the earth instead of breaking when the earth moved, but concrete is not a sustainable material – there are other ways, such as using a gravel raft (above) that are better.

Piles

Timber post foundations can sit on stone or concrete slab sitting directly on good bearing soil, as here; or timber piles can be driven deep into the ground until they reaches the solid subsoil. Photographs © Barbara Jones.

Piling is a common way to make foundations where solid subsoil is far from the surface. A pile is a post that can be driven down through softer material until it reaches solidity. Timber was used traditionally, but it is more common to use steel, concrete or stone these days. In terms of sustainability, it is best to use timber or stone. Timber does not rot if it is either totally wet or totally dry, so piles driven below the surface of the ground and capped with stone will last a very long time. As an example, the houses in Venice and Amsterdam are built on timber piles. A machine pile drives the post into the ground, spliced together if greater depth is needed; many piles are placed for each building and then the tops of the piles are linked together with something solid. This can be done with low environmental impact using timber beams, or by building stone plinths on top and linking the plinths together with a structural timber box beam (see page 82), or with greater environmental impact by using concrete piles and a concrete bond beam. Rammed stone piles are a good solution for commercial buildings on heavy clay soils, as the action of ramming compacts the surrounding clay and makes it stronger.

TIMBER PILE FOUNDATION

Foundations for other types of ground conditions

Not all soils are strong enough on their own to hold up the weight of a building, or parts of a building if it is designed with point loads. Some other types of soil are:

alt wet ground – marsh or bog, land that has been reclaimed from the sea, flood plains, or poorly drained land. Suitable foundations might be raft, gravel bed, or piles;

alt made up ground – this describes soil that is of uncertain composition because it isn’t just undisturbed subsoil. It may be a brownfield site – one where buildings have stood before. Or it may have been an allotment, bomb site or landfill site. Suitable foundations might be pillars, piles or gravel trench;

alt moving sand – a lot of sandy soils are very solid to build on, but others contain sections of sand that are not enclosed and can move. Suitable foundations might be raft, gravel bed, pillars or piles;

alt sloping sites – not strictly an issue in terms of bearing soil, but foundation options need to be thought through carefully to accommodate different levels. Suitable options might be pillars, stepped strip, timber post or gabion.

Frost heave

Many people mistakenly believe that heavy clay soils are susceptible to frost heave (I did!). In fact, it is silty and sandy clay soils which are, but only in certain circumstances. This is because for frost to affect soils it first has to be the sort of soil that promotes capillary action (or flow), which is soil that contains fine particles, like silt and fine sands. As well as this, there has to be a water table below – the ground must be wet underneath to feed the icy lenses that form beneath the soil, causing it to push up, and then there must be freezing weather conditions. If you have this type of soil, you will either need to start your foundations at a depth below the frost line to avoid the possibility of heave, or replace the susceptible soil with soil that is not affected by frost and/or insulate the foundations below ground. In the UK this is never more than 700mm (28") but it can be 2m (78")or more in colder places.

Soil heave

This is the name given to movement in a soil and mostly occurs in clay soils when the water content changes quite quickly. There are many different types of clay, and not all are affected by water content changes. The ones that are we call shrinkable clays. In the UK these are found only in the south and south-east.

Changes in water content can cause structural damage in some cases, so an understanding of clay soils is needed when designing foundations on this type of subsoil. Structural damage to buildings on clay soils can be caused by damage to underground drainage or pipes (eg water leaks); cutting down or pruning trees – this changes the amount of water in the soil; or the drying out of the soil either during a drought or due to the growth of trees and plants.

Do straw-bale houses need to worry about heave?

In general, the answer to this question is no. If you have designed your building with flexible foundations, and use flexible renders and plasters, then the sort of soil movement that might occur will not affect the structural stability of your building. There might be some visible signs of movement, such as a slight tilt to the floor, but these are usually considered quaint. These movements will not be frequent; such occurrences happen maybe once or twice in a lifetime, and your building should easily be able to cope with it.

Engineering

In many cases, simple buildings do not need engineering input to design the foundations. They do need knowledge of ground conditions and foundation types. Very often the information required about the soil can be found by asking the local building inspector, or local people. It can also be found by consulting geological maps, and seeing what type of plants grow there. Most of all it can be found by digging trial pits in the location of your foundations, and looking at what you find. There has been a trend in the last decade or so to stop thinking for ourselves and simply hand the job over to an engineer, and this has led us into over-engineered solutions in some cases, plus a lot of extra cost. First of all, we know that engineers rarely learn about alternative foundation solutions in college, but only about cement. This means we have to spend time educating them. Fear of a new (to them) solution may make it impossible for them to really hear or understand what we are asking of them. Then, if we ask them to design for us, they fear that we might sue them if something goes wrong. All these things may lead to an expensive and over-engineered solution.

On the other side, local authorities may prefer to stay with solutions they’ve already heard of (ie cement), may be unfamiliar with alternative techniques, and therefore demand an engineer’s stamp to validate something they’re ignorant or fearful of.

Sometimes, trying to quantify things mathematically is counterproductive, if not impossible. In order to quantify a product or material, it has to be tested to certain standards, and verified. The sample tested must be the same as every other sample that might be used. This is an extremely expensive process, so only big product manufacturers can do it, and it can only be done with standard manufactured materials. How can we possibly say that one straw bale is identical to the next? We can’t. But experience tells us how straw bales will perform, and how to know the good from the less good. We can write a standard that describes this, but it can’t be expressed mathematically.

Very often, natural materials are tested using methods that have been devised for other materials, or are tested on a small scale that actually wouldn’t ever occur in practice, and these tests can give very distorted results. We need to be careful of scientific methods and be aware that there may be flaws in the methodology. Unfortunately, once a scientific paper has been written and published it tends to be taken as fact, especially by our engineers who desperately need these facts in order to do what we are asking them to do – provide calculations for something that perhaps cannot be calculated.

In the same vein, all new buildings now require energy calculations, based on theoretical modelling of the design and the materials, to show their thermal and energy efficiency. In general these models are not able to include natural materials such as straw bales because these are not ‘products’ that have been tested, or that have producers who have lobbied the model makers to include them. They also do not take into account the fact that all tests are carried out in laboratories under ideal conditions – the quality (or lack of it) of installation is a major factor in the failure of materials to meet the tested standards in practice.

What all the above serves to tell us is that there must be room for experience and common sense as well as mathematical / theoretical modelling / engineering solutions. In the past, it was experience that dictated design. No one could become a designer or architect without first learning a practical skill and gaining many years of experience on building sites, working alongside a master / mistress craftsman. I’m not against the theoretical process – far from it – but I do want experience and common sense to be given equal value. If we could just have sensible discussion and agreement, how wonderful this process could be.

And don’t forget these details when designing your foundations…

Timber base plate

Strip foundations will always have a timber base plate on top, and pier foundations will have an insulated structural box beam linking the piers. These provide fixing points inside and out for render stop, skirtings, tie-downs, etc, and also for short hazel stubs on which the first course of bales is imbaled (!), preventing it from sliding off the foundation. The base plate should also ensure that the straw itself is raised above the finished floor level by at least 25mm (1"), so that if you accidentally drop a bucket of water on the floor the flood won’t touch the straw.

Tie-downs and ratchet straps

The foundation design will usually need to incorporate some method that allows for the walls to be compressed during the build process using ratchet straps; for the straw to be held temporarily in compression with polyester straps; and sometimes the wall plate and roof will need to be fastened down permanently to prevent the roof from being lifted off by strong winds. If the roof is heavy (eg slate or planted) or the site is sheltered, this may not be necessary. (For more details about carpentry and compression, see Chapters 8 and 13.)

Fixings for door frames, etc

Anything that fixes directly to the foundation, such as door frames, posts, etc must have provision made for it. Fixing posts for windows and doors will need to be fastened securely into the base plate. Posts as part of a timber frame will need solid support underneath them.

‘A good pair of boots’: good foundations use local, natural materials that allow the foundations to be flexible and that provide a capillary break (instead of using a plastic damp-proof course). Photograph © Barbara Jones.

Foundations checklist

The above examples of foundation types have all been used successfully with straw-bale buildings (and other buildings too – they’re not exclusive!). It is also possible to use these ideas in combination, as long as you follow the following basic principles, and remember ‘a good hat and a good pair of boots’.

alt Raise the bales off the ground by a minimum of 300mm (9”) but preferably 450mm (18”), and by at least 25mm (1”) higher than the finished floor level.

alt Secure the bales to the foundations, usually with hazel stubs attached to a timber base plate.

alt Protect the bales from moisture from above, below and outside by good design and choice of material.

alt Make sure the foundations are as insulated as the straw walls above.

alt Make provision for tie-down and compression straps.

alt Make sure all point loads eg posts are fully supported

And for durable foundations with the lowest environmental impact:

alt Use local and natural materials.

alt Use materials and design that make the foundations flexible.

alt Use a capillary break instead of a plastic damp-proof course.

* Leca or Optiroc is lightweight blown clay particles, sometimes used in agriculture for aerating soil

† a rigid, lightweight and highly insulating material made from recycled car windscreens www.foamglas.co.uk

‡ recycled Foamglas™ available from www.womersleys.co.uk

CHAPTER 8

Carpentry for load-bearing buildings

If you are using the infill method with a timber frame, you will probably need to work with an experienced carpenter. However, for load-bearing buildings the carpentry is simpler and may be something you want to learn and do for yourself. I trained as a carpenter and joiner in 1980 and absolutely loved it. My own experience, and love of practical work, has always been behind my drive to make building accessible to everyone, especially women. Why not try it yourself?

Photograph © Rae Parkinson.

Exposed timber frames inside a house can be superbly beautiful, especially when they are made with traditional joints, but these types of houses will rarely come into the ‘affordable’ category for most people, unless you are a skilled carpenter and have access to free wood.

Load-bearing designs are much more suitable for the competent beginner, because most of the carpentry is simple and easy to manage for anyone with time to put it together. Timbers are usually standard dimensions and lengths, which keeps the cost down too. This chapter talks about carpentry for load-bearing designs – you’ll need to look at other books if you want to know how to make timber frames. It’s the ring beams at interfaces between the walls and floors and roof that make the load-bearing building strong and stable, and this chapter will tell you how to make these to a high standard.

A note on cost

alt If you’re a self-builder, you can save money by putting in your own time for free. If you are paying someone, it may be cheaper to pay more for timber that will take less time to assemble.

alt Standard timber sizes are cheaper than unusual sizes. For instance, 300mm (12") deep timber is harder to get, so if you can use a combination of 225mm (9") and 75mm (3") instead, this will save money. Alternatively, move the pillars of your foundation closer together so you can use smaller dimension timber.

alt Sometimes it may be cheaper to use two timbers nailed together but not as deep, making a beam that is wider but shallower, eg two pieces of 38 x 170mm (1½ x 6¾") = 72 x 170mm (3 x 6¾") – check timber span tables to check costs.

alt Using two smaller timbers together will be more cost effective and often stronger than buying one larger one, eg fixing posts are generally 100 x 100mm (4 x 4") so make them from two pieces of 100 x 50mm (4 x 2") nailed together.

alt Any sort of laminated beam (Glulam, Kerto™ etc) will be expensive compared to ordinary timber, so try to design without these too.

alt Timber that looks good (planed up) is more expensive than that which is not (sawn timber).

alt Manufactured beams may save you a lot of time because they are lightweight and regular, even though they cost more. And they have good environmental credentials, as they are made from small dimension timber.

alt Nails are much cheaper than screws, much stronger in shear (if you try to break them), and can be hammered in faster once you’re practised. And it’s a lot more fun than using a PowerDriver!

alt Buying in bulk and shopping around brings the price down.

alt Second-hand timber, especially in Europe where it’s likely to come from an old building, is often of better quality than new timber.

alt Use structural timber span tables to work out optimum timber sizes. You can often use timber of 38mm (1½") rather than 47mm (1¾") thickness.

alt Where durable timber is specified or required, do not skimp on this. It will cost a bit more, but will make the difference between your building lasting a long time or not.

alt Don’t make life hard for yourself – buy off-the-peg timber (or steel – a lot of it is recycled) frame buildings if you want a frame, and infill them. Agricultural buildings are very cost-effective, with no frills.

Durable timber

Any timber that is exposed to the weather should be durable. Oak, Douglas fir and larch are the most common timbers in this category, but you could also use chestnut or cedar. It is a common misconception that all hardwoods are durable – they are not. When making ring beams, you can use durable timber for the outer skin, and cheaper softwood for the inner, but you might find the saving isn’t that great. Outer timber for the ground-floor base plate and the structural box should be durable. Further up the building it may not be so necessary, as the timber will be less exposed, but any cladding – at gables for instance – should also be of durable timber.

Ring beams

Load-bearing buildings may have a structural box beam (also called floor plate, see page 82) at the base on top of pillar foundations, and/or a base plate on top of strip foundations. Then there will be a wall plate at first-floor level, which might also be the roof plate if this is single storey, or there will be a roof plate at second-floor height. If more than two storeys, there will be a wall plate at each floor level. All these plates can be called ring beams, because they form a continuous ring around the building, and they are what gives a load-bearing building stability. They are prefabricated before any straw building takes place, often made in sections and then assembled in situ with strong joints.

Make the ring beams in as large a section as can be physically managed. Remember that many people can lift surprisingly heavy weights, with good coordination and team work. The sections need to be planned carefully so that long timbers will slot into each other, and can be fixed securely, making sure the plate does not bend at these joins. Use SmartPly as a board material, also known as OSB3. This is exterior-grade oriented strand board that contains no formaldehydes.

In this load-bearing design you can see ring beams at various stages. The base plate sits on top of insulated strip foundations; sticking up vertically are fixing posts; and resting on the scaffolding at the top are pre-fabricated sections of the wall plate waiting to be assembled in situ. Photograph © Rae Parkinson.

18mm (¾") is usually strong enough for the lid and base of the boxes; anything less will bend.

The following are general principles for all plates. Most of these points are also illustrated in the drawings on pages 83 and 88:

alt All joints must be square cut – use a chop saw if you are not too good at this.

alt All joints in timber and SmartPly must be staggered so one is not opposite the other, and be a minimum of 600mm (24") apart.

alt Where two timbers come together in a length, the joints must be braced with another timber at least 600mm (24”) long.

alt No timber joints can be over spans (eg over a window or door) – they must all be supported on a pillar or straw wall below.

alt Stagger joints at corners to make them strong.

alt Full-depth noggins should be placed at minimum 1.2m (4') spacings.

alt Place noggins both sides of any post into the box.

alt Close any ends with full-depth noggins.

alt Strengthen corners with full-depth noggins.

alt Use flat noggins at the top of the box to take hazel stubs for the straw above, where there is no full-depth noggin. These should correspond with the bale plan.

alt Use 100mm (4") large head nails to fix timbers and noggins together generally.

alt Use 75mm (3") large head nails when skew-nailing timber (using a nail at an angle so it cuts across the grain of end-grain timber).

alt Ring shank nails are preferable to screws for fastening the SmartPly. They are stronger and cheaper.

alt The lid and base must be glued and nailed firmly on to the side timbers using exterior grade wood glue and ring shank nails.

alt Nails should be at least 2.5 times the thickness of whatever they go through – ie for 18mm (¾") smartply they should be 45mm (1¾") long.

alt Nails into the lid should be at maximum 150mm (6") spacings.

When making any ring beam, first make sure you know the exact dimensions you are working to. Remember that you may be using wood fibreboard on the outside or inside edge, to achieve airtightness and better insulation – does the thickness of this affect your dimensions? Is your timber really 100mm x 50mm (4" x 2")? Or is it actually 97mm x 47mm (3¹³/₁₆" x 1⁷/₈")? Make sure you understand if there are different dimensions needed for the structural box, the wall plate and the roof plate. If you are marking up for posts, are you looking at the top of the plate or the underside?

Make provision for tie-downs

Often it’s necessary to use polyester straps to hold the compression of the straw either until the rest of the building weight has been added or permanently to tie down the roof. You will need to allow for this in your plates. It’s common for the straps to be fed underneath the base plate. This way, the plate holds them down, but you must leave slots in the underside of the plate so you can thread them through; or be sure to lay the straps before you place the baseplate.

Sometimes you will also need to make provision for hooking ratchet straps on to the structure so that these can be used for compression. (See Chapter 13 for more information.)

Here you can see a wall plate on top of a straw wall with blue ratchet straps to create compression and white polyester straps to hold it once the ratchets are removed. Photograph © Rae Parkinson.

Structural box beam or floor plate

A structural box beam is a box with two sides, a top and bottom, firmly glued and nailed together so that the whole is greater than the sum of its parts. It’s a way of using relatively small timber and sheet materials to make something very strong. However, it must be made correctly or it doesn’t work at all! It is designed to take the weight of the building above across the spaces between pillars.

The size of the timbers is determined by two things: the distance between the pillars below (the span) and the amount of depth required for adequate insulation. Usually, the latter means that the structural strength of the timber is more than necessary.

A well-insulated timber floor suspended above the ground ought to have at least 250mm (10") of insulation within it, with a wind barrier. If you can attach wood fibreboard to the underside or top of the joists (often physically impossible) then this depth can be reduced. This is more than building regulations / Code require, but their standards are too low for good thermal efficiency. Spans between pillars are usually 2-3m (6'6"-10") and use timbers that are 250 x 47mm (10" x 1 ⁷/₈") deep. Sometimes the box needs to be stronger than this, either because the spans or the weight of the building are greater. In this case, the timbers can be doubled up and nailed together to make wider sides.

The floor can be insulated with straw, if you’re pretty sure it will never get wet, eg from a sink overflowing. We did this with the RSPB in Wales.

A box beam has two sides, a top and bottom, glued and nailed together to form a strong structure. Here you can see a box beam with a base plate laid on top. Photograph © Rae Parkinson.

Base plate

This is a timber ladder laid flat, either directly on top of strip foundations or on top of a ring beam. It is used to make a division between masonry and straw, to raise the straw above finished floor level, and to provide a fixing point for skirting boards, render stop, etc. The outer timber must be of durable timber – 100mm x 50mm (4" x 2") timber – with noggins placed according to the bale plan, drilled to take hazel stubs. The drawing on page 83 shows a base plate that will have wood fibreboard attached to the outside edge; it also shows the location and orientation of fixing posts, with noggins either side. It shows how the base plate is chamfered at the corners to follow the natural curve of the bales, and where the polyester straps will go for the tie-downs.

A base plate is a timber ladder laid flat, either directly on top of the foundations, as in these photographs, or on top of a ring beam. Photographs © Barbara Jones.

Fixing posts

These are timbers placed either side of openings, central to the bale, which are used to attach other framing timbers so that the windows and doors can be fixed closer to the outside edge of the building. They are made from two timbers measuring 100mm x 50mm (4" x 2"), nailed together, and fixed securely into the base or wall plate below. They go into holes in the plate above, and are fixed in place only after all compression has finished. When making the ring beams, the exact dimensions and locations of these must be known so that they can be allowed for with noggins either side to hold them in position in both the top and bottom plates. In practice, only the noggin on the side of the opening is fixed, and the other one is fixed after the post is in position to ensure a tight fit. This also means that the width of the opening can be made exact. Holes need to be cut through the SmartPly to allow for these posts, and a section of SmartPly must be removeable above each post so that the posts can be firmly fixed to the noggins after compression, and possibly also after the roof has been constructed.

Fixing posts are placed vertically either side of window or door openings, and fixed in to the plate above and below. Shown here before and after bales have been laid. Photographs © Barbara Jones.

Engineered timbers

Modern carpentry may make use of some engineered timbers such as I beams, Glulam or Kerto beams™, or truss rafters. These are ways of using small-dimension timber to do the job that would normally require large pieces of wood. In many ways these can be more sustainable because they are made from off-cuts of another process, or from small trees instead of the larger ones. Larger timbers come from older trees, which may be from forests that cannot be or are not being replaced. We design with I beams a lot because they make it possible to carry weight across long spans (distances) without using whole trees to do it. They are like a timber version of a rolled steel joist (RSJ) and are made in the shape of a capital letter I with a piece of SmartPly or hardboard as the vertical part of the ‘I’. They cost more than using regular timber, but are easier to use because they are lightweight.

However, we try to avoid designing with Glulams or Kertos™ – large-dimension timbers made up of small pieces of timber laminated together using glues to make a bigger one – because these really are expensive and there are usually more cost-effective alternatives. Sometimes we design with roof trusses, but only if they don’t mean we lose useable space in the roof apex.

Wall plate or roof plate

The size of timbers used for this ring beam will depend on the loading it will carry from above and the span of any openings below. It is a structural part of the building, and may also act as a lintel (see page 87) if there are windows or doors below it. Timbers would never be less than 100mm x 50mm (4" x 2") laid vertically, not flat, and can be as much as 225mm x 50mm (9" x 2") or even be made out of ‘I’ beams. Often the outer section is timber and the inner piece is an ‘I’ beam, to match the size of the floor joists, also made from ‘I’ beams. To give this plate structural strength and make it into a box beam, it will need to have 18mm (1") SmartPly glued and nailed to the bottom and top.

Wall plate on top of the first storey of this building at Clow Beck Eco Centre in Darlington, County Durham. Photograph © Barbara Jones.

The top lid is left loose until after the plate has been assembled on the wall, the fixing posts have been aligned, and hazel pins have been fixed down into the bales. Usually there will be a removable section of lid above each fixing post below, because the lid will need to be fixed before the posts can be (because of compression). After compression, the fixing posts are fastened securely to the noggins either side – this has to be done from above. As soon as the lid is fixed down, even in the removeable sections, all joints must be covered with duct tape as a precaution against rain, unless you are working under a roof. I’ve seen many buildings suffer problems after a rain storm when people were unable to get tarpaulins over the top in time.

Sometimes, in terms of engineering the building, the top lid is made from tongue and grooved (t&g) smartply that slots into the t&g floor covering. This provides a continuous sheet attached to the floor joists, which then act as a structural insulated panel (SIP) that provides racking strength to the building, preventing it from distorting in shape.

A wall plate with hazel pins fixed down into the bales. This wallplate fractured during compression and has been fixed with a splint repair, nailed on in a W pattern.
Photograph © Rae Parkinson.

The wall or roof plate performs various functions:

alt It evenly distributes the load of the roof or floor across the width of the wall, and around the perimeter of the building.

alt It provides a rigid perimeter plate that affords compression of the straw walls at an even rate around the whole building.

alt It provides a fixing point for strapping or anchors to the foundation in order to hold the roof structure down against wind uplift, and the fixing point for the rafters themselves.

The ring beam and base plate provide fixing points at the top and bottom of the wall for external cladding, or internal timbers for cupboards etc.

The foundations or base plate can provide a good template for the wall plate, and one can be constructed on top of the other then set to one side for use later.

Fitting the ring beams

After building the walls, and once the wall plate is in position, any distortion in shape that the walls have suffered as a result of their flexibility, or bale frenzy (overexcitement when working with straw!), can be adjusted. Don’t worry if your straw walls don’t seem to be quite level once you’ve finished building – the wall plate will sit on the high points first, compress those, and then compress the rest of the walls evenly. The weight of the plate immediately gives the walls greater stability. It’s hard to describe the transformation that takes place from working on very flexible walls that sway like a ship when you’re walking on top of them to finding that most of that movement has disappeared when the plate is placed on top, even before compression.

Lifting a section of the ring beam into position may require teamwork!

To fit the plate, begin at the place which seems the best fit, persuade the walls back into correct alignment if they need it, and pin the plate down with hazel pins 25-38mm (1-1½") in diameter, 1m (3') long or longer; two pins per bale. Once all the hazel pins are in place, the box should be well insulated with tightly stuffed straw or other natural insulation, and the top plate should be glued and screwed down firmly. This is essential, as you must cover the tops of the pins so that any rain cannot possibly hit them and be taken down to the centre of the bales.

Pin the ring beam down by hammering hazel pins into the bales below. Photographs © Barbara Jones.

Roof plate

This is a ring beam constructed like the others above. It will have hazel pinned down through it and a closed lid over the top. The noggins for the hazel are laid flat in the bottom of the box, and are placed according to the bale plan for the final course of straw. For more on roofs, see Chapter 9.

Lintels

A lintel is a structural piece that carries load over the top of an opening. In masonry lintels would be made of stone, timber, steel or concrete and sit on about 100mm (4") of masonry either side of the opening. In a straw-bale house it would be a ring beam that spreads the load over a much wider distance. If you did use a timber lintel, it would need to bear on at least half a bale either side to avoid point loading the walls and crushing them.

The ring beam is in effect a huge lintel that goes around the whole building, and usually the strength of this is enough to carry weight across openings, except when they are quite wide. In general, a ring beam made with 150mm x 50mm (6" x 2") vertical timbers will be strong enough to span an opening of up to 900mm (3') wide. Anything wider will need greater strength. This can be achieved in a few different ways:

alt by increasing the size of the ring beam timbers to something like 225mm (9") for up to about 1.5m (5') openings;

alt by filling part of the space inside the box with more of the same size timbers. This needs to be well thought through, as a box entirely filled with timber is not insulating enough;

alt by using flitch beams – a fairly thin piece of steel encased both sides with timber. The extra strength of the steel means the size of the timber can be reduced;

alt by making the door or window below into a framework that will take the loading itself. In this case, you need to be sure that the foundations will carry the point loads of the posts.

Door or window openings wider than 900mm (3') will require the wall plate to be strengthened, spreading the load more evenly around the building, as was done here on the Martin council houses in Lincolnshire. Photograph © Rae Parkinson.

CHAPTER 9

The roof: ‘a good hat’

A straw-bale building needs ‘a good hat’: a roof with a large overhang to protect the walls in wet climates. There are many different types of roof design and coverings, none of which are specific to straw-bale buildings, but which can beautifully complement the aesthetic and environmental ambitions of your design. However, straw-bale walls do not carry the weight of the roof in the same way as masonry walls, meaning that you and your roofer will need to do things a little differently than on a conventional build. You also need to keep in mind insulation (the roof is the most important part of the structure in controlling heat loss and gain) and the need for adequate thickness of the rafters. Roof design can be complex so you may want to get a professional to ensure it will work.

Photograph © Rae Parkinson.

The design of the roof for a straw-bale house is not unusual nor particularly different from that for any other building, so I won’t go into detail about roof design here. The main consideration for load-bearing designs is that the load is spread as evenly as possible around the perimeter walls, across their width, and the load is directed down the centre of the wall. Unlike masonry design, where the roof weight is usually carried on timber placed on top of the inside wall, with straw it is vital that the roof weight is transferred down the centre of the straw-bale wall, and not on one edge or another.

All loading through straw walls should be vertical as much as possible, otherwise it will tend to knock the walls over. These principles must be remembered during all aspects of construction, and you will need to make sure that your roofers (or you) know this. When delivered and moved to the roof, truss rafters should be spread along the walls evenly, not stored at one end of the building as is usual. As the roof is loaded up with slate, tiles, etc, these should also be distributed evenly and not loaded in one spot; nor should half the roof be slated before the other half.

ROOF AND PLATE CONSTRUCTION

Distribute tiles uniformly across the roof to keep weight spread evenly on load-bearing walls. Photograph © The Wisharts.

Types of roof

The earliest straw houses, built over a hundred years ago, had hipped roofs. This is a particular design with no gable ends where the roof weight is carried on all four walls. It’s a very stable design, very well suited to straw-bale houses, because the roof load is evenly spread around all the walls and it means that you don’t have unprotected gables exposed to bad weather. This type of roof is far less common now because it’s more expensive to build and we live in a society that values lowcost more than quality.

Hipped roofs are a stable design suitable for straw-bale houses because they spread the weight across all four walls and protect the gables from the weather. Photograph © Barbara Jones.

More common today is a duo-pitch roof, usually with a pitch of 30º for grey (stone) slate, 45º for Welsh slate or tiles and 52º for thatch. Planted (‘green’) roofs are at a pitch between 10º and 25º, and shingle at least 14º. Flat roofs are notoriously prone to failure and should be avoided if at all possible (this is the roofer in me speaking!), although developments in single-ply membranes have made flat roofs a little more secure, if installed by someone experienced. I would not recommend any self-builder to make a flat roof. You will also see mono-pitch roofs working very well, especially for smaller buildings and extensions. Circular or curved roofs of various pitches are also quite popular designs on many straw buildings.

Examples of various roof designs: circular;
Photographs: © Barbara Jones

Curved covered with a single-ply membrane;
© Laura Kudelska

A garden studio that combines circular at the ends and duo-pitch in the middle.

Mono-pitch.
Photographs © Rae Parkinson.

Roof construction

Traditional roofs

Originally (that is, several hundred years ago), all roofs were what is known as ‘cut roofs’, hand-cut from sawn or in-the-round timber. They were pretty heavy and often used whole trees for purlins, king posts and A-frames, with rafters being in lighter, smaller-dimension timber. As the years went by, timber grew scarcer and smaller houses were built. We still had cut roofs but they were now a simple ridge board, often a floorboard, with pairs of rafters 100 x 75mm (4" x 3") pushing against each other, held together by a crosspiece of 100 x 25mm (4" x 1") placed one-third of the way down the slope, with the rafter ends sitting on a 100mm x 75mm (4" x 3") timber wall plate. Sometimes these roofs had a couple of purlins under the rafters of up to 200 x 100mm (8" x 4") on either slope, particularly for the heavier roofs, which carried the roof weight horizontally on to side walls of stone or brick. Rafters were fastened over the wall plate using a bird’s mouth joint, giving a very secure fixing that also helped to prevent the rafter foot from pushing outwards.

A traditional-style ‘cut roof’ showing rafters, purlins and A-frames. Photograph © Rae Parkinson.

How this type of roof works is that the rafters hold each other up by pushing against each other, creating compressive forces. But the tendency of the weight above them is to push the rafter feet outwards, hence the crosspiece making the collar of the A shape, which physically prevents this from happening. It has been worked out (first by experience, then by calculation) that the optimum place for this rail is a third the way down the rafters from the ridge. On top of this, the particular type of joint we call a ‘bird’s mouth’, together with skew nails angled back towards the roof, gives a very strong fixing at the foot. Finally, most houses would also have lightweight ceiling joists that held the rafter feet together across the width of the building.

In load-bearing buildings, the rafter is bird’s-mouthed over a timber fixed centrally to the roof plate – see below.

In this cut roof at Elmfield Farm in Kent you can see the rafters bird’s-mouthed over a piece of wood central to the ring beam (carrying the weight down the centre of the wall). Note the rafters are cut short to allow for a veranda to be added at a different pitch to the main roof. Photograph © The Wisharts.

Truss roofs

Trusses have been used for a long time too, with names like ‘king-post truss’ and ‘queen posts’, which had complicated but wonderful timber joints. Nowadays it is far more common to see truss rafters made of small-dimension timber in a factory and held together with metal hanger plates. It can be much more cost-effective for a self-builder to buy trusses than to build your own cut roof, but this often means you also lose out on being able to use the loft space, as it becomes filled with parts of the trusses. Careful design can get around this, and create trusses that will give you a room in the roof too, but this will add to the expense.

Truss roof. Photograph © Barbara Jones.

Building your own

Unless you use a very simple roof, or have experience, you will need some help in designing your roof to make sure it works. All sorts of different designs can be used, depending on such things as the span (distance between walls that the rafters sit on), internal walls (can they take weight?), open-plan ideas, choice of roof covering, etc. Building Regulations will require calculations to prove the roof structure works, and truss manufacturers will provide these for you as part of the service, as long as you tell them: the location of the house (to calculate wind and snow loadings), the dimensions of the building, the size of overhang, where the bearing weight needs to be (crucial for straw buildings) and any other particular features of the design. In a straw-bale building a crucial design factor for roofs is also the depth of insulation required. NEVER skimp on this. It will usually be between 225mm and 300mm (9"-12"), and often means that the rafters are deeper than is required simply for the strength. The extra depth of the rafter may mean that they can be placed further apart, depending on the strength of what is covering them. From this can be calculated the size of trusses required and a quote for supply and delivery. Alternatively, you can ask an engineer to design the trusses for you, once you have a good idea of what you want them to look like and what you want them to do (eg create an attic, hold water tanks, etc).

A simple hipped roof design. Photograph © Aggie Jakubska.

Roof insulation

In the past, roof design did not take into account the need for adequate insulation, and most of our older roofs are difficult to insulate because the rafters don’t have enough depth. Roof insulation is the most important aspect of controlling heat loss or gain. Heat rises and is mostly lost through the roof; the sun shines down, and an insulated roof is the first protection against it. In order to get enough insulation into the roof, rafters usually need to be about 225mm (9") deep, and/or the insulation has to be placed above or below the joists. Rafters of this depth are so much stronger than needed to hold the weight of the roof covering that their width can often be reduced to 38mm (1½") and the spacing between them increased to 600mm (2').

Straw can be a cost-effective solution for roof insulation but it is heavy compared with other choices, and ceiling joists or rafters need to be strong enough to take this weight. However, it is easy to design the roof so that partial or full bales of straw can either fit between the rafters or sit on the ceiling joists, and placing the straw is not a difficult job. Straw as part of a compressed and plastered wall is not a fire risk, but extra protection needs to be given to it when used in single bales, as it would be for roof insulation. This is because if laid flat over a ceiling, the strings are vulnerable in the event of a fire and the ordinarily dense bale could become a hazard with no strings, as loose straw is very flammable. Therefore it should have a fireproof coating on both sides – a plasterboard ceiling on one side with 30mm (1¼") of clay on the other would do this.

The curved roof of The Look Out at RSPB Conwy, Wales, was insulated with straw packed between the rafters and covered with a clay mix which forms a fireproof, breathable layer of protection. Photograph © Barbara Jones.

Wood fibreboard is an excellent, breathable and sustainable insulator that can also create an airtight roof. It comes in many thicknesses and can be fixed above and/or below the rafters with T&G joints. This would also totally enclose the space between the rafters, therefore making it possible to use raw sheep’s wool for insulation, which keeps cost down. Unprocessed (once washed) sheep’s wool is very cheap but needs to be protected from moths – so enclosing it with breathable wood fibre in the roof is a great idea.

Roof coverings

While any type of roof covering can be used, as long as basic design principles are followed, there are some choices that particularly complement a straw-bale house because they’re more environmentally sustainable.

Using cedar or oak shingles has to be one of the best choices, for environmental and aesthetic reasons. They are natural and breathable materials, and rely on good-quality installation and ventilation for their extremely long life. It is advisable to use a vapour-permeable membrane to protect the rest of the house while it is under construction, as shingling is slow (albeit satisfying) work, and also to act as a wind barrier. When it rains the shingles swell with water, which seals up any possible cracks between them. They hold this water – it doesn’t drip through – so the rain itself in the shingles makes it watertight! It’s a bit scary to watch underneath your shingles as they get wet for the first time, until you find out that they really don’t drip. They need to be laid with the correct spaces between them, so they can expand and contract with the rain, with a three layer coverage, (unlike slates which only have two), and good ventilation so that any moisture can evaporate safely away. Shingles are very compatible with the equally breathable straw-bale walls.

There are a variety of environmentally sustainable, and beautiful, roof coverings available including shingles of oak (top photograph of Aurial Hill’s House, Exmoor, Devon, UK © Rae Parkinson) or cedar (bottom photograph of Norita Clesham’s spiral house in Co. Mayo, Ireland, © Barbara Jones).

Wheat straw (or reed) thatch is also a great choice as a roof covering. Again, it is a totally natural, renewable and beautiful material that should last a long time. However, it can be an expensive option and can’t be done by novices.

Fran Bradshaw’s Passivhaus in Norfolk, UK, has a thatch roof. Photograph © Anna Maria.

Planted (‘green’ or ‘natural’) roofs also complement straw houses, particularly the more modern versions of such roofs, which use only about 25mm (1") of a gravelly soil, with shallow-rooted plants such as sedum, sea thrift or strawberries growing on them. There are many companies selling different versions of roof build-up for natural roofs, but the design illustrated top-right is a perfectly good one for self-build – the main differences being that it’s cheaper, simpler and there are no manufacturers’ guarantees; its long life depends on the quality and care with which it is installed.

Green roof planted with sedum on a straw-bale building in London. Photograph © Rae Parkinson.

SEDUM ROOF

Overhang at the eaves

Straw houses need a good ‘hat’ to protect them from the weather. A large overhang is a feature of straw-bale buildings, especially in wetter climates. Just as traditional thatched houses have a roof overhang of about 450mm (18"), so too do straw ones. This gives the top of the walls really good protection against the rain.

For two-storey and gabled houses, a roof overhang won’t protect the first floor or the gable. This may not be a problem if you are in a sheltered spot, but otherwise it would be sensible to design first with a hipped roof; if not then durable timber cladding over the gable end, and perhaps a veranda on the south-west side (usually the most exposed to weather) that will protect the ground floor.

A large overhang protects the top of the walls against the rain.

This small building at Fasbourn Hall, Suffolk, is in an exposed spot so has timber cladding on the gable end to protect the walls from the weather. Photographs © Rae Parkinson.

CHAPTER 10

Keeping the straw dry: ‘dropping the roof’ methods

Gone are the days when we risked the weather and relied on the Straw-bale Goddess alone (well, and some tarpaulins) to keep our buildings dry. Now we have methods for keeping straw-bale buildings dry during construction, whatever the climate!

It is also a bit of a myth that a framework building will protect the straw during construction better than a load-bearing one. First, a roof gives no protection to bales on the lower courses, because it is too high up to prevent rain blowing in through the sides. Second, having posts in the wall makes it very difficult to protect the straw either side, as rain runs down the posts straight into it!

Nevertheless, keeping the straw dry while building is essential. There are a few ways to do this, regardless of whether the design is load-bearing, infill or hybrid:

How to keep the straw dry during construction

1. Build the roof and floors first on temporary posts and beams, and use these to protect the straw from above, with tarpaulins attached to the eaves for protection at the sides, or sheeted scaffolding all around instead of tarpaulins.

2. Build the framework and roof first then hold it out of the way a little higher than its finished position and use it to protect the straw from above, with tarpaulins attached to the eaves for protection at the sides.

3. Build the scaffolding up above the eaves and create a temporary roof over everything, so the roof and straw can be built underneath, with the sides of the scaffolding sheeted in for weather protection.

4. Use the ring-beam / wall-plate sections (in load-bearing designs) on top of the bales at the end of every day, or when it rains, to protect the tops. Alternatively, use sheets of timber weighted down.

5. Cover everything with tarpaulins at the end of the day, or when it rains.

There are five main ways to keep the straw dry during construction. If it is a large building, it may be worth building the scaffolding up above the eaves and creating a temporary roof, as in this picture. Photograph © Barbara Jones.

Another option is to hold the frame out of the way using temporary propping (scaffolding is used here) and attaching tarpaulins to the eaves to protect the sides. Photograph © Rae Parkinson.

A note on tarpaulins

You will need some of these, whatever design you are using. Make sure that the tarpaulins you buy are of very good quality. There may be times when these will save you from considerable damage, so do not skimp on the budget here – good ones can be sold second-hand, bad ones won’t last the first big storm. Many that are described as ‘heavy-duty’ but certainly don’t come into my definition of this phrase! Never buy any that are less than 270g/m² (10oz/ft²), and make sure you have plenty of rope to attach to the eyelets. When you tie down a tarp, never attach weights to it; always tie it securely to something that can’t move. A high wind is capable of lifting heavy weights (like concrete blocks or lumps of timber) and flinging them about like conkers on a string. And make sure that the wind can’t get underneath it to raise it like a sail, or that the rain won’t pool inside it – the weight of water can rip the eyelets out from the ropes. Having said all that, a tarpaulin is your friend – keep one handy at all times!

For buildings of any size, the first three options are the best. The third option can be quite costly for a self-builder, so many choose to buy their own scaffolding second-hand, and then sell it on once they’ve finished with it. Some companies in the UK that are now offering a buy-back service. Scaffolding may not be such an expense for a contractor if they already have their own scaffolding.

The first two options are very popular, the first because it can be more straightforward and economical. With an infill building, since the framework holds up the roof and floors anyway, this is obviously how it has to be built. But for a load-bearing building, how much more economical it can be to use temporary posts and beams to hold up the roof, and then remove them and use them for something else, and not have the expense of all that extra timber!

Using temporary posts and beams to hold up the roof and floors

This method works really well because of the simplicity of the load-bearing design, which has a structural ring beam at the junction of each floor and at the roof. This makes it possible to place temporary beams underneath the ring beam and hold them up with removable posts. Your design must be created for this construction method from the outset, and then usually no more than four posts are required for the ground floor of a conventional size house, eg approximately 50m² (540ft²). The same method can be used to build the first floor above the ground floor, and the roof above that. In all cases, posts must have direct contact with the ground, so posts on the first floor must have similar ones underneath them below the floor to make contact with the ground. The timber used for the posts and beams can have a second use somewhere else in the building later (eg for floor joists).

You can also make use of the fixing posts beside each window and door, and use them as temporary props to hold up the wall plate above, maybe with some extra temporary posts as well. You can fix timber restraints to the scaffolding to make sure that none of your temporary props move during construction.

When using these methods, make sure you have left yourself at least 100mm (4") above the height of the uncompressed straw so you can get the last bale in before lowering the wall plate / roof!

Timber posts can be used as temporary props to hold up the roof plate, as shown here. Photograph © Barbara Jones.

Once the floor or roof is in place and the sides have been protected from the weather, it’s time to install the straw. All the bales are inserted except for the half bales that have to be left out because the beams are in the way. When all is complete, remove the temporary posts and beams by replacing the posts with Acrow props (like giant metal jacks) and carefully lowering the beams until the ring beam is sitting on the straw walls.

The walls may be compressed enough just by the weight of the floor/roof itself. This was the case when we built the Look Out building at RSPB Conwy, as the roof already contained straw-bales and clay as insulation, adding to its weight.

Or the walls may require some extra compression beyond the weight of the ring beam – we used ratchet straps as well for the Outback Centre.

When we built the council houses in North Kesteven, Lincolnshire, we made use of the crane we already had on site, and placed 1-tonne sacks of sand on each corner and at the junction with the party walls. This worked a treat, and compression happened in about two seconds!

Making use of the crane to place prefabricated timber structures, such as the wall plate and roof. The crane also came in handy for pre-compression using 1-tonne sand sacks. Photograph © Rae Parkinson.

When building houses for North Kesteven District Council, the floors and roof were held up on scaffolding trusses as the straw walls were being built, as shown here from below and above. Photographs © Rae Parkinson.

Acrow props holding up a temporary beam at Woore Fruit Farm in Crewe. Photograph © Barbara Jones.

Other methods of holding up the roof and floors

When we built the council houses for North Kesteven in 2009, the floors and roofs were built on the ground beside the building, and then a crane lifted them into position, where they were held on the scaffolding rather than on temporary posts and beams. The scaffolding was built up to just above ground-floor height and scaffold trusses were used across from one side to the other to take the floor. When all the straw was in place, the floor was lifted up, the trusses were removed, and the floor was lowered on to the straw. The trusses were then moved up the scaffolding to their next position, just above the height of the first floor walls, where they held the roof until it was ready to be lowered. This worked well but it’s an expensive method, as there’s not only several days’ hire cost of the crane to factor in, but also the cost of the scaffolding team. They have to visit site to put the scaffolding in place and then remove it at each level.

Also, if you are using the first floor as a temporary roof, it will need a temporary stem wall (a timber stud wall) to be built centrally along its length. This wall should have ‘rafters’ to the edge of the wall plate so that a heavy-duty tarpaulin can be placed over it at a pitch (not just lying flat) to allow any rainwater to run off. With this method, the tarpaulins should extend to the outside edge of the scaffolding, which should be sheeted on the outside for the same reason and which will protect the sides of the walls from driving rain.

Alternatively, if the building is a hybrid design, like the cafe we built at Manor Park, Sutton, London, a different method can be used. At Manor Park we built the glazed frame first, and used the scaffolding to hold up the ring beam for the other three sides of the building. Then we built the roof before inserting the straw. Scaffolding poles are strong enough to hold this type of weight – we had the structure checked by our engineer to prove it – and we left gaps in the last course of bales where the poles were until we had lowered the ring beam. After lowering it on to the straw, we compressed the walls using ratchet straps. The frame was designed with long tenons so that the timber joints could be raised during the installation of the straw, and lowered at the same time as the ring beam. Thus, with good understanding of how the straw would settle, we were able to make sure that the ring beam was level with the frame after the straw beneath it had been compressed.

Holding the framework out of the way and then lowering it (compressive frame)

The two main drawbacks to the framework method of straw-bale building, described earlier, are the difficulty of stabilizing the straw enough for plastering and the extra cost associated with using a secondary frame of timber as well as the structural straw. The compressive frame method addresses both these problems by making a framework building as much like a load-bearing one as possible – although for this reason it’s often more sensible just to stick with true load-bearing! The basic idea of this compressive frame method is the same as for the compressive load-bearing method: that is, using various means to hold the floors and roof out of the way while the straw is installed, pre-compressing the walls beneath them and then dropping these elements down on to the walls. In framework techniques, the floors and roof are held up using posts that remain a permanent part of the building, rather than temporary removable props.

The basic principle is to design a frame with the floors and roof about 100mm (4") higher than their finished level, hold it out of the way and then, once the straw has been installed, lower the floor and/or roof on to the walls, often using their weight to stabilize the straw without the need for pre-compression straps. This gives a covered space inside the building in which to store straw, but still means the sides of the building need to be protected from rain.

In a compressive-frame or hybrid building, the upper-storey floor (or the roof) can be held up on top of the timber frame, with folding wedges to hold them higher than the finished level while the straw walls are being installed. Once the straw is in and ready for compression, the wedges can be knocked out, allowing the beam to drop down on to the posts.

There are many different ways this can be done, but it should be kept as simple as possible, using folding wedges of timber on the shoulders of mortice and tenon joints, or threaded rod of some description, or metal shoes for the posts, etc.

If you do end up with lots of metal in the walls (which is expensive), the straw will need to be protected from its coldness by wrapping the metal in hessian or something similar, because cold metal could cause warm, moist air to condense on to it, resulting in potential damp problems.

Loadbearing designs involve simple carpentry that most people can manage, especially with some teamwork. Photographs © Barbara Jones.

Adapting mortise-and-tenon joints

Traditional timber frames use mortise-and-tenon joints to connect posts with beams. Often these are stub-tenon joints, but if the tenons are made long, with through-tenons, you can support the beam on the shoulders of the post using folding wedges. Once the straw is installed and ready for compression, the wedges can be knocked out, allowing the beam to drop down onto the posts. This was the method we used for the RSPB Conwy Look Out building, and it worked very well.

Using metal connectors between posts and beams

In this method, the timber is connected together as though it were steel, rather than timber, using metal connections. This means that the natural advantages of the timber are lost, but it may be an easier method for those who are not familiar or comfortable working with wood. However, it’s a lot more costly, both on the pocket and the environment. We used this method for the house at La Boa Permaculture Centre near Venice, Italy, where the engineers, in their anxiety at being the first to use this method in Italy, over-engineered everything to be on the (extremely) safe side. It worked really well but unfortunately cost the client an extra £20,000 – the price of being a pioneer.

Metal connectors can be used in compressive frame buildings to connect the vertical posts in the walls with the horizontal beams in the floor/roof.
Photograph © Barbara Jones.

Using metal shoes, allowing the post to be dropped later

Here each post is housed in a specially made shoe with a slot in two opposite sides. A metal pin is placed through the slot that holds the post 100mm (4") above finished level, and the posts and framework are assembled as normal. Once the straw is in place, temporary screws are fixed into the posts through the slots above the metal pins, the pins are removed, and then the screws are also removed, allowing the posts to slide down into the shoe. This is the method we used for the saleroom we built for Sworders in Essex in 2009. On dropping the frame, we were not able to force the posts completely down into the shoe, and we had to fix them where they were. It wasn’t until some years later that we found out this was because the framework had been exposed to the weather during construction and the shoes had filled up with rain! The water pressure below the posts prevented them from dropping.

There are many variations on the above methods, and most of you will be able to come up with your own way of doing this job. Just try to make sure you keep it as simple as possible – remember, just because it works doesn’t mean it’s necessary!

CHAPTER 11

Wall raising

Whether you’re building a true load-bearing structure, a hybrid or using some sort of framework, you’ll need to prepare your bales first (unless you’re just building something quickly to see how it’s done, or don’t mind about the quality too much). Many bales will need dressing before use, some will need customizing to make them fit in different spaces in your walls, and all will need choosing for the spot where you want to lay them, like stones in a drystone wall, as building with bales can be a bit like making a jigsaw. Bales will also need pinning, stapling and ‘persuading’ into shape – it is physical work and lots of fun.

Photograph © Barbara Jones.

You’ll already know your bales, their length, width and height. You should also know how much they will compress, and will have done your bale plans, all excited to be at the start of the actual building process.

Preparation

Never skimp on planning and preparation – it will save you hours of headaches and problems further down the line. Even if it means delaying your build for a few months, it is better than risking everything getting wet because your windows were late so you couldn’t plaster the walls, or having to re-design the roof trusses had to be re-designed, leaving you with tarpaulins over everything. So:

alt Prefabricate as much of the timber work as possible.

alt Get everything stored on site, or close by, before you start.

alt Order windows, doors and roof trusses well in advance.

alt Do any site works, levelling, drainage, build foundations and add services.

alt Build the roof and floors first.

alt Put up scaffolding or access equipment.

alt Set up a tea room and toilet!

Brace, brace, brace!

A really important part of the preparation process is to brace all your timber work. You will be putting it under a lot of pressure as you work, so it must be secure. Also, it must not get in the way of the straw you are installing AND it must not make waterproofing your building more difficult.

It is important to brace timber work, especially as shown here at the corners and between fixing posts. Left photograph © Rae Parkinson, right photogarph © Barbara Jones.

Corner braces

It’s really sensible to use temporary corner braces to keep your building nice and square, and make building a lot easier. It won’t matter too much if you’re careful and your building is small, but it makes a big difference to larger buildings and those with long walls of straw. Corner braces can be made from a couple of scaffold boards, fixed to themselves, the base plate and the wall plate (if it’s already in position). This means you can also keep everything lined up while you’re building. If the wall plate isn’t in place, you can use the scaffolding to fasten the braces back. And if you don’t have scaffolding, then use temporary props that hold the corner braces firmly in position. Make sure their inside faces correspond to the outside edges of your straw. Sometimes the wood fibreboard on the outside face of wall plates will not be added till later, and this can throw your wall out if you forget to allow for it.

Brace along length of wall

When you have a long length of straw wall (more than 3m or 10') it tends to snake unless you hold it in line with some vertical guides. Again, these can be scaffold boards attached to the base plate, and braced to keep them vertical.

Brace fixing posts

When you start building with straw, it will exert some horizontal pressure on to the fixing posts. For this reason, they must be braced to stop them bending or moving out of true. This can be done by temporarily fixing a horizontal timber between the posts, the exact size of the opening, or a timber holding the tops of the posts together at the correct distance. You can use the scaffolding to attach temporary bracing to the posts and the boards. If all these things are impossible, use temporary timbers at an angle to brace the posts.

Dressing (trimming) bales

The earliest straw buildings did not use dressed bales very much, but it has become more common now that houses need to meet more stringent standards for airtightness and insulation. If you’re not too concerned about these things then it might not be quite so important to follow the guidelines below.

Basically, when you take a normal bale, it may have ‘fat ends’. By this we mean that on the ends there’s more straw in the centre, between the strings, just where it is most difficult to measure how long the bale is. If you place two of these bales together, obviously the fattest bits will hit each other, leaving a space around the rest of the end of the bale that could allow air to pass through and so reduce the insulation of the wall. By dressing, we mean flattening off the end of the bale so that as much surface area as possible from one bale makes contact with that of the next. Construction grade bales should not require much dressing, but others will do. First, relocate the straw on the bale end underneath the strings from a high point to the low point by hand, if you feel strong enough, or the prongs of a claw hammer placed either side of the string.

Sometimes a bale has one corner that’s lower than the other three, and this method can be very effective in sorting that out. Because you’re repositioning the straw, not taking it out of the bale, this will not reduce its density. Then pull out any excess straw from the remaining high points, making sure you don’t catch bale frenzy and pull too much out, leaving the strings loose!

Usually the bulging part of the straw is caused by the strings being very tight, so if you’re careful, you can take out the excess without taking out any from underneath the string.

Dressing bales to flatten the ends can be done by hand, with a claw hammer or with an alligator saw. Photograph © Bee Rowan.

Dressing a bale generally shortens its length slightly, so when you’re working out what length your bales are, don’t forget to dress them first. You can generally allow for losing 30-40mm (1³/₁₆"-1⁹/₁₆") off the length of each bale after dressing.

Dressing using an alligator saw

If you’ve got hundreds of bales to dress, then an alligator saw can really speed things up! You must be careful not to cut the strings of course, but after you’ve realigned any straw under the strings by hand or with an claw hammer, you can gently stroke the saw blade over the top of the bale and quickly trim all the high points off.

Customizing bales

This describes the method of altering the length or the shape of a bale. Half bales are always needed, because of the running bond from one course to the next used in bale building (as in brickwork). So if you start on the first course with a whole bale by a doorway, then on the second course you will need a half bale to begin again in the same place. Sometimes, if you’re working to designs that didn’t take the bale length into account, you’ll need to make different lengths of bale rather than only halves. And at window openings you may want to angle the end of the bale so that the splay throws light into the room. In more complicated work, particularly where you have to compensate for poor design, you may need to make all sorts of different shapes and sizes of bale parcels!

A handy tool for customizing is the custom-made baling needle. There are various different types, but one is a circular length of steel about 5mm (³/₁₆") in diameter and about 600mm (2') long, with a 100mm (4") handle bent on the end; the other end is flattened and sharpened with two holes drilled through it. It can be made relatively cheaply by a metal workshop.

Baling needles can be used to cut bales in half or change their length by threading strings through the bales with the needles.

If you want to cut a bale in half, don’t just cut the strings or the whole bale will fall apart! Instead, you need to retie the bale into two separate halves first, and then cut the strings. To do this, it’s easier to work with a partner. Take the bale you want to split and lay it on edge on top of another bale you’ll use as a table. Now measure out four lengths of baling twine, each one long enough to go completely around all four sides of the smaller half bale you will be making, and then add a bit extra for tying knots. (Your two new half bales will have two strings each.) Take two of these strings and thread them through the holes in the end of your baling needle. It helps to thread them from opposite sides so that a short end of twine sticks out in either direction (see page 115).

Ask your partner to steady the bale from the opposite side, making sure they are not too close so you don’t unexpectedly perform body piercing! Measure halfway along the bale, place your threaded needle by the original string at this point and push it into the bale about 25mm (1") to steady it while you sort your strings out. Here comes the tricky part – you need to push the needle all the way through the bale, at right angles to the face, to come out on the opposite side where the string is and exactly halfway along the bale length without twisting the strings together! So keep your fingers between the strings as you push the needle through the bale and don’t let go until your partner has carefully pulled the ends out of the needle on the other side. Because you threaded the strings through in opposite directions, it should be easy to tell which string belongs to which side of the bale. Take the two ends of string that go round one of the halves, and tie a loop in the end of one of them (not a slip knot). Thread the other end of the twine through this and pull tight, but only to measure about a hand’s width (225mm or 9") back from where the twine passes through the loop. Make another loop at this point, in the twine that doesn’t already have a loop. Now you can tie a sort of trucker’s hitch by threading the end through the first loop and back through the loop in itself and pulling tight.

When splitting a bale in half, first feed the new strings through the bale using a baling needle. Then tie a trucker’s hitch to enable you to pull the string tight. Photographs © Rae Parkinson.

By pulling on the string between the two loops, you can get the string even tighter. It needs to be at least as tight as the original string; and if you can get it tighter than that then it means the original strings could’ve been put on tighter and the bale would’ve been more dense. Your partner will have been tying an identical knot on the other half of the bale. Now flip the bale over on to its other side and repeat the whole process where the second original string lies, ending up with two tied parcels and (hopefully) slacker original strings ready to be removed. Don’t just cut the strings at random, but look for where the original knots are. These are always close to the corners, one above the other. Cut by the knot and pull off the original strings, then you have two more strings to reuse next time. All being well, you should be able to gently pull the half bales apart from each other, and dress the ends as necessary. If the worst happens and you do get the strings twisted, all is not lost, as you can cut one of them and retie a new string, as long as you are careful about how you handle the bale when it only has one string on it. Once you’re familiar with this method, it only takes a few minutes to make a half bale.

Always customize bales to be slightly smaller than you expect. This allows for an attack of bale frenzy and the desire to force your new bale into the gap allowed for it because you’ve just spent time making it. And, because of the flexibility of straw, this is possible. However, it will almost always result in a distortion of the wall somewhere else, usually at the nearest corner or in the buckling of a framing post for a window. Don’t give in to the temptation to go for speed rather than a snug fit. Watch out for your work partners and encourage them to adopt a calm and measured approach too!

Flakes or wedges of straw

If you cut the strings on a bale, it will naturally fall apart into discrete sections, called flakes or wedges. These are the parcels of straw that were picked up by the baling machine and rammed into the box to make the bale, followed by the next one, etc. Sometimes you will need to use these to make up any discrepancy between the bale sizes and the heights of frames, or the desired finished heights you want. It’s always better to design without the need to use flakes, because this will add extra time and an unnecessary ‘faff factor’ onto the build, as well as being slightly less stable. However, having built hundreds of buildings, I know you need to understand how to use these!

Where you might need to use flakes

1. The bales you’ve had delivered are a bit shorter after dressing than you expected. You’ve altered the bale plan but you’re still short of some longer-than-average bales to fill in one wall length. Here you can use a flake between two bales to make up the length. Never use more than one flake, and always make sure it is very tightly packed into the wall. If you have to use them on more than one course of bales, make sure they are nowhere near each other.

2. You want your windows at a specific height, which is not an exact number of bales. Make up the difference using flakes laid flat along the bale. If you need more than one (compressed) flake then tie the flakes together to simulate compression before placing them on the bale. If you need two-and-a-bit bales, place the flakes between the two bales rather than on top.

3. Your bales have compressed more than you expected and you are working with fixed heights of beams in a framework. Use the flakes as in the latter example above, and place them between bales rather than at the top.

Notching bales

If you’re using fixing posts at openings, or have an internal framework, you’ll need to notch bales around these to make your building stronger – so that the straw can’t slide against the post if there are strong winds pushing against the walls. It also ensures that you don’t get a draught between the straw and frame; in other words, it makes your building airtight. The best size of post to use is 100mm x 100mm (4" x 4"), but made from two pieces nailed together, as this is stronger and cheaper. These are located centrally to the bale and in the middle of the bale end because this fits nicely between the string, and a notch can be cut out without affecting the integrity of the bale.

Bales need to have notches cut in them so that they fit snugly around fixing posts at window and door openings to ensure an airtight fit.

When the post is beside a window, the bales below the window will only be notched to half-post depth, so that bales either side of the post are notched for airtightness. But the bales that are beside the window will need to be notched to full depth. Choose your best bales for this job, because the ends of the bales by window and door openings need to be as firm and well made as possible, as they will be forming a base for the plaster to come later.

Use a timber template of the same dimensions as your post, stand a bale on end after dressing, and place the template on the bale in the same location as the post will be once finished. Using a sharp saw, cut down beside the template each side to the depth of the timber, then carefully pull out the straw from the centre of the cuts. You’ll find that the saw tends to slide off the straw into the centre and you’ll need to compensate for this by angling it away from the notch. Also, because of the orientation of the straw, just cutting down like this won’t release all the straw; some of it will need to be pulled out from the centre as well. The alligator saw is a good tool to use for this job, as long as you make sure you don’t cut the notch too wide. Because this is a powerful tool, you can be very accurate with it, and will need to compensate for the thickness of the saw blade. It should be a snug fit! Otherwise you are creating gaps instead of making your walls airtight.

To cut a notch, cut around a timber template using an alligator saw and pull out the cut straw. The template should fit snugly in the notch. Photographs © Rae Parkinson.

When it comes to fitting, it always takes two people. The bale needs to be handled carefully and should be held in place horizontally above its final position, and the notch should be held open as it is fitted around the post so the straw from the sides doesn’t get in the way of a snug fit. Make sure that the bale pushes tight up against the post, and that the end of the bale is lined up with the edge of the post. When you’ve fitted all the bales around the window opening, they should line up nicely.

It takes two people to fit the bale tight up against the post, holding the notch open, and then to push it down into position. Here you can can see the bales being placed onto hazel subs in the base plate.
Photographs © Barbara Jones except bottom left photograph © Rae Parkinson.

Sometimes you have to tie up the corners of a bale around window and door openings to stop the strings from slipping into the notch. For this technique, use the baling needle with one length of twine and push through the side of the bale at 90º to the strings, close to the end that is notched, near the corner of the bale and about 150mm (6") away from the strings. Tie one end of the twine to the string and pull tight enough to bring the string back from the notch but not so tight as to pull it off the bale! Pull through the other end of the twine and fasten it to the other string, again pulling tight enough but not too tight. Now do the same with a length of twine near the other corner. You should now have a notch between two strings that are securely fastened back from the notch.

Tying the corners of the bale on either side of the fixing post. This stops the strings from slipping into the notch, which would loosen the bale. Photograph © Gemma.

Laying bales

The first course of bales must be placed slowly and carefully, as these provide the template from which the walls will emerge. It is important to ensure that the bale is placed centrally on the base plate, lining up with the outside edge of the timber, and securely pinned by hazel stubs. You must follow the bale plan accurately or you might end up with only a half-bale space left in the middle of the wall (which is structurally unsound) as different teams set off from different corners!

Bales go together like giant bricks, a second-course bale straddling equally the joint between two lower bales. Work from fixed points into the centre of each wall; place the corner bales first, and those beside any fixing posts. Make sure that corners are vertical and don’t move as more central bales are placed. Use a spirit level! Bales need to be hand-picked to ensure a snug, not overly tight fit.

Bales should be placed like bricks: lay the first course and then the second on top, with each bale straddling equally the joint between the two bales below. Photograph © Rae Parkinson.

Stuffing and jumping

There is no substitute for climbing on to the bales and jumping hard between the strings! This helps with compression (bales) and depression (humans). Do this only after each course of bales has been completed. Take care if the walls are high (it may not be safe to jump) or if there is a beam overhead. It’s important to keep your weight between the strings, and your jump vertical, so you don’t push the walls out of true. This can be lots of fun and very satisfying! There are usually some small gaps at the joints between bales, even after the most careful dressing and customizing, and these must be stuffed (but not over-stuffed) with straw after every course of bales before beginning the next. For this, take a handful of long straws and arrange them so they’re all aligned with each other. Now twist the bundle as tight as you can, and then bend it so you have a very tight folded end of twisted straw, and some straggly loose ends. Push the loose ends into the gap between bales, and keep adding twists like this till the last folded end is just below the surface of the wall. The stuffing must be pushed back like this or it will pull out when you trim the walls down later.

After each course has been laid, climbing up on the bales and jumping, or pounding them with a ‘persuader’ (giant mallet), is important to compress the bales. And it’s fun!
Photograph © Barbara Jones.

Tips for building

alt Always complete each course of bales before moving on to the next – if you build up a stack between fixing posts without building either side, the posts will bend and the wall will be pushed out of shape.

alt The bales are very flexible. It is possible to force them into gaps, but if they are too tight they will cause the wall to snake or the corners to go out of true. Don’t be too macho or try to work too fast! We want a snug fit, not an over-tight one.

alt Never jump on bales unless the whole section has been completed – this will only push the bales into the gap.

alt Only stuff at joints between bales, never under a bale – any gaps here will disappear after compression.

alt Remember to stay calm, work together and be aware of what other teams are doing.

Curving bales

Making bales fit the shape of a curvaceous design is a highly technical and difficult part of the job. Care must be taken not to laugh too much. The procedure is to turn a bale on its side, lift one end up on to a log, and jump on it! The middle straws in the bale can be moved fairly easily in relation to the strings – just make sure you keep the end straws in the same place while you move the middle ones, and don’t curve the bale so much that the string slips off. That’s all!

In order to make very well insulated walls, you may need to shorten the inside string of each curved bale and trim the ends to form a wedge shape instead of a right angle. This makes a curved bale that fits snugly next to its neighbour without lessening the density of straw at each joint.

Bales may need to be curved in order to fit a circular design. To do this, turn the bale on its side over a log and jump! Photograph © Rae Parkinson.

Pinning

There are various ways we pin the bales together as we build, for various different reasons. We usually use hazel for this job because it grows in long straight sticks that can be coppiced from the stump every five to seven years without killing the tree. Sweet chestnut can also be used: the choice depends on which part of the country you live in and which tree is most common. What we’re after is a tree that has some flexibility but not too much (willow is generally too flexible), grows straight, can be harvested regularly, is durable and relatively cheap; so other trees could be OK too. Bamboo is sometimes used, but its smoothness may not create enough friction to hold itself into the wall. Hazel has been used since the Stone Age for building, so we’re pretty sure of its qualities! Think about wattle-and-daub panels – they are mostly made with hazel or sweet chestnut wattles and have lasted rather a long time.

Stubs

These are short lengths of hazel that are fixed into holes in cross noggins of the base plate, to secure the first course of bales – so that’s needed for each storey. The holes are usually 32mm (1¼") in diameter and the hazel is 350mm (14") long by 32-38mm (1¼-1½") diameter. It’s important that the hazel is long enough to only impale the first bale and not stick out above it, for obvious rather important health-and-safety reasons. It should be whittled at the fatter end so that the natural but irregular hazel fits tightly by friction into the regular drilled hole, and sharpened at the narrower end. Two stubs are used per bale, placed centrally, a third of the way along each bale. It should be possible to see the bale plan by the location of noggins and hazel stubs in the base plate. Once made, the stubs should not be placed until directly before placing the bale onto them, to avoid vampire-type injuries. Stubs are also used like giant Rawlplugs before plastering, knocked into the centre of a bale wherever you want fixings for sockets, radiators, kitchen cabinets, etc (see Chapter 14).

Hazel stubs fixed into the base plate to secure the first course of bales. Photograph © Barbara Jones.

Staples

At one time we used to use hazel staples at every radical change of direction, such as at corners, or at delicate points – such as where the bales go above windows and need attaching to their neighbours. Nowadays this is done only if the building requires it, for instance when the bales are a little less than perfect, or the design needs a bit more robustness – because the lumpiness of staples can prevent the straw from compressing enough at corners. Staples are made from 1m (3' 3") lengths of hazel, 25-32mm (1-1¼") in diameter, by splitting the fibres apart with a heavy hammer at the point where you want them to bend – without breaking – and then twisting and bending. It’s a method used by basket makers and is very effective. You do need to make sure that you have split the fibres enough and that you twist and bend, and don’t just belt it then snap it!

Hazel staples, made by hammering and twisting a hazel length into shape, can be hammered in to the bales at corners or above windows for extra stability. Photograph © Gemma.

Internal pinning

Once the walls are four bales high, they need to be pinned with long lengths of hazel. Again, sometimes we don’t use these pins at all, but they do help a lot for self-builds when people are not too familiar with bale building. Also, they give the wall integrity, so that each bale acts together with the others instead of independently, and this can be very important when building in areas with lots of wind (engineers call this ‘high wind loading’). The pins are as long as the height of four bales, less 50mm (2"), which is 1.35m (4' 6"), and they should be 38-50mm (1½-2") in diameter, straight, sharpened at the narrow end, and without excessive knobbles. There are two pins per bale (dividing the bales into thirds), driven down through the centre of the bales to overlap with the hazel stubs that stand up from the foundations. The same length pins are used on the 6th course (unless there is no 7th, in which case they go through the wall plate instead – see below), building up a series of overlapping pins throughout the wall system. The walls of a single-storey ground floor are usually either 6 or 7 bales high, depending on the design of the foundations and the type of floor installed. First floors are generally from 3 to 5 bales high, but can be higher. The same pins, but slightly less in diameter, are used to pin the wall plate down to the straw. Use a template hole beforehand to make sure that each pin will actually fit through the hole drilled in the wall plate.

Once the walls are four bales high, long hazel pins should be driven into the bales by twisting and then hammering with a persuader. Photographs © Rae Parkinson.

Windows and doors

Underneath or above windows you may need to use shorter pins, as there are usually less than four bales at this point. Window and door frames can be pinned in sideways to the bales, but only after all settlement has stopped (or the settlement might snap the pins). If you are using fixing posts instead of box frames, this won’t be necessary.

Pinning for frame designs

By far the better method is not to use pins at all, but to compress the straw (see Chapter 13). However, all of the above techniques could be used for frame methods, as the bales must be stabilized somehow before plastering. Often the frame prevents the use of pins, as there is a beam or a roof over the straw such that pins cannot be inserted. In this case, other methods need to be used. These include fastening some sort of mesh to the posts and pushing the bales tight against it. In the USA this may be chicken wire but we strongly advise against using metal on the straw and in the render or plaster. The mesh can also be fibreglass but by far the best choice, because it’s natural, is jute or hessian.

External pinning

Here the pins run from base plate to the wall plate in one continuous piece. (Remember to trim the straw ready for plastering before the pins are put in position – it makes the task of trimming quicker and easier.) The pins are placed externally to the straw, again two per bale, inside and outside the wall, four per bale in total, eg about 350mm (13¾") apart and opposite each other. Grooves are cut into the straw with a tool such as the claw on a hammer, so that the pins are flush with the straw. Pairs of pins on either side of the wall are tied together through the straw at each course of bales with baling twine, and are fixed to the base and wall plates with screws or nails. The pins are covered with hessian to provide a key for the plaster, either before or after placement, and can be hazel, chestnut, etc, or sawn softwood. This is also a good method if you have really poor-quality bales but want to use them anyway.

Straightening the walls

All through the build process you will have been trying to keep your walls straight. You’ve used corner braces, you’ve checked from time to time that they haven’t gone out of plumb, you’ve braced your fixing posts and guarded against bale frenzy, you’ve used your persuader wisely and in fact done everything you can to keep your flexible straw-bale walls straight. Just before you pinned your wall plate into position, you did a final straightening of the walls with a team of you on either side.

Then you permanently fixed your wall plate into position, and now, with some weight on the walls, when you bash the straw with the persuader it finally seems to want to stay in the same place!

As you build, Be sure to check as you build that your walls are in the correct shape and not leaning in or out. Photograph © Barbara Jones.

It’s when you’ve got some loading on that you really can straighten the walls, before you compress them fully. At this point, take a team of energetic people and a long plank such as a scaffold board either side of the wall, and working together, beat the plank against the wall, occasionally using a persuader, until both sides of the wall are as straight as you can get them. Once you’ve done this, compress the wall and use the persuader again, if necessary, during this process. Make sure the force you apply for compression isn’t off-centre, as this may start to make your wall lean.

Preparing for plastering

This sounds like a fairly innocuous job but can be one of the hardest and most frustrating, depending on the aesthetic finish you require and the quality of your bales. You’ve done your best to keep the walls straight, you’ve used the best-quality bales you can find, you’ve stuffed all the joints and you’ve compressed the walls and put on the floors and roof. Now you need to trim the walls so that you get as flat a surface as possible without all that hairy straw sticking out. This reduces surface area so that you use far less plaster than you would otherwise; it also reduces the risk of flame spread across the surface of the wall while the building is more vulnerable before plastering. Good tools to use for trimming are a garden strimmer, sharp hedge cutters or clippers, a chainsaw (be careful!) or an alligator saw. The corners and around windows are usually the only places where strings are exposed, so take extra care here not to break them. (Although, as with everything in straw-bale building, there’s always a way round it – just carefully tie string extensions to each end and then retie the string.)

It’s amazing how transformed the building looks once you’ve given it a really good haircut! After trimming, it’s much easier to see if the walls need more straightening, and if there are any joints that still need stuffing. Often some of the stuffing is pulled out by the trimming, but don’t skimp on this part of the process. In all holes and in fact anywhere that you can stuff some tightly packed straw, do it! This may feel a bit laborious, but attention to detail at this point will make a big difference to the heat loss / insulation and airtightness of your building. Of course, you could give up and let the plaster fill all those gaps – many do – but plaster is nowhere near as insulating as straw, and you’ll use a lot of plaster!

This part of the process can take quite a long time, depending on how well you have done all the other parts up until now. Just remember that this is all about long-term durability and very low heating costs in the future. It’s worth it.

Strengthening up around windows and doors

Pay good attention to the shapes you’ve created around doors and windows. If you had good tight bales then you should be able to shape these with your tools and also provide a sound surface on which to put your first coat of plaster. If not, you’ll need to add something to the bales in order to make a good key for the plaster, such as a long straw mix, or vertical reed matting, fastened tightly into the straw by sewing through from the inside to the outside, or pinning into the straw using barbed hazel pins. In all cases, use jute mesh into the wet plaster as reinforcement. Variations of these methods can be used above windows and doors, around alcoves and niches, and so on. (See Chapters 15 and 16.)

Pay special attention to the shapes around windows and doors when preparing to plaster. Use a long piece of straight timber as well if your spirit level is too short. Photograph © Rae Parkinson.

Tie-downs

Tie-downs, also called ‘strapping’, are external bands that go around the wall plate and under the base plate. They are usually made from strong polyester with a metal buckle, or sometimes metal rope with an adjustable fastener. Either can be tightened as the walls compress.

A tie-down has two functions:

alt a means whereby the roof structure can be fastened down securely to the foundations, to prevent uplift when subjected to strong winds;

alt a device to keep the walls in compression while the weight of walls and roof above are added.

Tie-downs are usually fixed as the walls are pre-compressed, after the wall plate has been pinned down, but this can wait until the roof has been constructed, depending on the method of compression. Remember to trim the walls before the tie-downs are attached. The tie-down generally goes underneath the base plate or under the foundations, up and over the wall plate, and is fastened on both sides of the wall with a tensioning buckle.

Permanent tie-downs aren’t always necessary. In two-storey houses, for instance, the ground floor doesn’t need to be tied down, because it isn’t going anywhere with all that weight on top of it, and the roof plate can be tied down to the first floor. If the roof is sufficiently heavy, there is no need to tie that down at all either, unless you are in an area that experiences high winds. Often temporary tie-downs are used to hold the walls in compression until the full weight of the rest of the building has been added, and these are removed just before plastering. If the tie-downs are permanent, they must be covered really well, either with a wide strip of hessian/jute scrim about 300mm (1') set into the body coat, or by using long-straw-rich plaster over the tie-down.

Tie-downs go over the wall plate and are fastened with a tensioning buckle.
Photograph © Rae Parkinson.

Buildings with a frame

You’ll need all of the above skills and knowledge to work successfully either to infill your frame with straw, or to wrap your straw around the frame and stabilize it. Ideally, you will have designed your frame to fit the bales, rather than the other way round.

Depending on the type of frame construction, frames can be built off-site and then assembled once the foundation is finished, or built in situ. All framing, including temporary bracing and propping, is done before the straw is placed. The roof is also constructed, with vapour-permeable membrane and battens to provide waterproof shelter, leaving the final roof covering until the straw is in position.

Applying the methods described above, bales are laid on to a timber base plate using hazel stubs. They are also pinned with hazel where necessary. Often the bales are wrapped around the outside of a larger frame, and in this case they will still need to be compressed to create airtightness and stability. Bales may be notched around smaller posts, especially fixing posts for windows and doors for the same reasons.

Some problems that can occur in frame buildings that haven’t been designed with the straw in mind are:

1) Posts are set in the line of bales but are too large to be notched in. This can create cold bridges where the posts are, as timber is not a very good insulator, and the bales can slide against the smooth surfaces of the post. To remedy this, hessian about 200mm (8") wide by 600mm (2') long can be attached to the posts and laid into every course of bales to provide friction that will prevent the bales from sliding beside the posts under wind loads. Then parcels can be made from flakes of straw and physically attached to the front and back faces of the post, sewn into the bales either side using the baling needle and twine. As you can imagine, this takes a lot of the fun out of the simple process of bale building!

2) Posts are too far apart to prevent the straw from snaking over a long distance. In a load-bearing building, long walls are stabilized when the ring beam is placed on top, and the wall is persuaded back underneath. This can’t be done with a framework where there is no ring beam, as compression has to be done below the beam rather than dropping the beam down. The remedy is to add extra timbers of 100mm x 100mm (4" x 4") into the length of the wall to reduce the distance between sections in which compression and straightening can happen.

3) There is no central beam above the straw. This is an essential in order to be able to compress the straw, and has to be added in later if missed out from the design.

4) The straw goes up for two storeys without a break, maybe 10 or 12 courses high. Straw needs to be stabilized approximately every 7 bales high (or less), so in this case horizontal timbers will have to be introduced that can be attached to the frame to keep the bales below stabilized.

Bales laid on edge.

All the descriptions above are about bales laid flat, but bales can also be laid on edge. They must still be dressed, and can be customised in the same way. They are harder to stack up vertically as they are less stable in this orientation, and so work best if used as part of an infill design. They also can’t be notched because the strings are in the way, but it does reduce wall width by 100mm (4”). Research has suggested that the loss of 100mm (4”) does not affect the insulation value, due to the changed orientation of the straw, (vertical instead of horizontal) which I find hard to believe.

CHAPTER 12

Windows and doors

We need to consider the design of window and door frames and openings for several reasons. We need to be sure that they are structurally sound, both in terms of being strong enough to withstand the pressure of bales being wedged in beside, above and below them, and to help take the loads above them if necessary. We also need to make sure they are durable and will match the long life of the rest of the building. We need to ensure that they are airtight and don’t let in draughts; and that they are the right size and in the right place to maximize solar gain without overheating, and allow enough daylight into our homes and offices.

Photograph © Rae Parkinson.

There are some details below that relate particularly to load-bearing designs, but the basic details of fixing and weatherproofing windows is appropriate for most windows and doors in straw walls, whether load-bearing or infill.

Load-bearing methods

All window and door openings in load-bearing houses must have some way of supporting the weight of the bales, floors and roof above them. Owing to the flexibility of straw, the use of concrete or steel lintels is inappropriate and in fact their weight would create problems – as they sit on only a small bearing surface, they would compress the straw too much in a small area. Weight (load) needs to be spread over as wide a surface area as possible, and in general must be spread over at least half a bale either side of any opening. Usually this weight is carried by ring beams – continuous rigid timber plates that encircle the perimeter of the building. The dimensions and quantity of timber in these ring beams can be matched to the strength required to span an opening. The ring beam can also be designed as a spreader plate, so that point loads from roof trusses above can still be accommodated by the load-bearing design.

Fixing posts

We generally use fixing posts instead of the older method involving structural boxes (see below) for the attachment of windows and doors. These use less timber and are easier to work with on larger buildings, as well as providing location points for the wall plate above, once it is placed. Many designs now allow for the building of wall and roof plates first, and propping them temporarily while the straw is installed below. The fixing posts can be used very effectively as temporary posts by locating them in the holes in the underside of the wall plate, and fixing them temporarily until you are ready to drop the plate.

Nowadays, fixing posts are the most common way to attach windows and doors.
Photographs © Rae Parkinson.

Fixing posts are made from two pieces of 100mm x 50mm timber (4" x 2") nailed together, which is stronger and cheaper than using a 100mm x 100mm (4" x 4") post. These are firmly attached to the base plate, with the two smaller dimensions facing the centre of the bale – see diagram on page 129. This is because when subjected to sideways pressure, they are stronger both as two pieces instead of one, and in this orientation – so don’t change them around! They are placed one either side of each opening, central to the bale wall, bounded on all sides by cross noggins in the plate for stability and strength, and left long enough to just project through holes in the wall plate. It’s important to make sure that the length of the posts after compression will be accommodated within the depth of the wall plate, and not push off the lid! It’s very hard to cut the tops off, once the wall plate is on. These fixing posts must be well braced to prevent any distortion of them during building (see Chapter 10), as bales will be pressed up against them with quite a bit of force. They need to remain vertical and central. The distance between posts is the width of the actual window or door plus 5-10mm (³/₁₆"-³/₈") to allow for fitting, plus the thickness of the lining timbers, usually two of 50mm (2").

Subframes for windows and doors

Window and door openings are framed out with rough-sawn lining timber (the subframe) so that instead of placing the window in the centre of the bale, it can be moved towards the outside edge. This is much better for weatherproofing – the optimum place for them is 100mm (4") back from the face of the outside wall. If they are closer to the outside, they are not very well protected from the rain; and if they are deeper into the wall, you create a poorly ventilated space around the window that encourages damp. This timber is usually 200mm x 50mm (8" x 2"), although the header needs to be wider –300mm (12") to hold up the bales above. The timber sill is fixed horizontally into the posts, usually at two or three bales high but this varies according to design. Bales are laid first, and either pre-compressed manually with straps, or the whole frame of the window is used to compress the bales above and below (see Chapter 13). You calculate the height of the sill based on the number of bales below and the amount of compression to be achieved. In practice it is very easy to ensure that all window heights are identical.

The sides of the opening are lined with similar timbers and, together with the header, are also fixed into the posts after compression. Remember the header is wider than the sill if there are bales above, and must be of sufficient size to take the weight of bales above it without bending. For openings larger than 900mm (3'), therefore, you may need increase the depth of the header and/or physically attach it to the above wall plate after compression – use small-dimension timber (eg 50mm x 50mm / 2" x 2") jointed on to the header and grooved into the face of the bale.

HEADER TIMBER SLOTTED ON TO FIXING POSTS

Window frames should be set towards the outside edge of the bale, about 100mm (4") back from the outside face of the wall.
Photograph © Barbara Jones.

Using the wall plate as a header

If the doors or windows are designed to go right up to the wall plate above, there is no need to add an extra header as well. In this case, the side lining timbers are attached to the plate, usually by projecting through slots in the plate so they can be fixed really well, and hang from it, with the settlement gap below instead of above.

The settlement gap

If window and door subframes (not the glazed part, only the rough framing) are built in during the installation of the straw, then you need to leave a gap above them (or below if they are hanging from the wall plate) to allow for the amount that the bales will settle or compress after all the weight of the building above has been added, or you have pre-compressed them. How this is worked out is:

Say your window opening (this includes the subframe, don’t forget!) is 3 bales high, and the whole wall is 7 bales high to the wall plate, if you know the bales will compress by 10mm (³/₈") per bale, then the 7 bales will compress by 7 x 10mm = 70mm (2¾"). But where the window is, there will only be 4 bales to compress; the other 3 are missing because of the window. So these 4 will compress by 4 x 10mm = 40mm (1³/₅"). This means that your settlement gap needs to be 40mm (1³/₅"). You must also make sure that your window frame external dimensions are the same as 3 compressed bales, not 3 uncompressed bales!

If your bales are not as dense as construction-grade bales, they will settle/compress more than 10mm (³/₈”) per bale. Settlement could be 15mm (³/₅") per bale or possibly more. In this case, in the example above, 7 bales would settle by 7 x 15 = 105mm (4¹/₈") and the settlement gap would need to be 4 x 15mm = 60mm (2³/₈"). Always err on the side of caution, and leave a larger rather than smaller gap – it can always be stuffed with straw, whereas if there’s not enough space, you’ll cause too much pressure on the header of the window, which could bow it and eventually break the glass below.

As the window has fixing posts either side of it, you could use any size of window, and make allowances for its different height by using flakes or parcels of straw above it instead of a whole bale. This is a little more tricky to work out, but very possible. It means you can use off-the-peg windows, and/or you can order your windows well in advance so their late delivery doesn’t hold up your work.

MORTISE-AND-TENON JOINT SHOWING USE OF FOLDING WEDGES ON SHOULDER

Structural box frames / bucks

This is a simple way of dealing with openings that is used mainly for small buildings or small windows. The actual window or door is fixed into a structural box frame. In this case, you will definitely have to leave a settlement gap above the box. The box must be strong enough to hold the weight of the bales above, and the forces of compression as they are applied.

It is more important that structural frames should be multiples of bale dimensions than when using the fixing post method, so external dimensions of the frame could be anything from half a bale to three bales in width and any number of (compressed) bale heights.

Frames are pinned into the bales with hazel, through the base during construction, and through the side once settlement has finished. Door frames would not have the base box as for a window. Instead, the sides of the frame stand directly on the foundation and are fixed in position with bolts into the foundation or attached securely to the timber base plate.

The actual sizes of timber used, particularly for the top of the box, will depend on what weight the timber has to carry and over what span. This will be affected by the design of the wall plate above it, which may be able to partially act as a lintel for the window or door. In general, an opening of over 900mm (3') wide will require the box above it to include timber of at least 200mm high x 50mm wide (8" x 2") to make it strong enough to prevent the box from bending as it carries weight from above across the gap. It is most common to use the wall plate / ring beam above as the main lintel for wider doors.

During settlement, the settlement gap is maintained by folding wedges of timber with which you can gradually reduce the gap as the building compresses – see diagram below. Imagine a piece of 100mm x 50mm (4" x 2") timber cut along a diagonal line on the wider face from corner to corner. If you now stand these on edge, you can slide one along the other. This has the effect of reducing the height of the timber while keeping the edges parallel. This is a pair of folding wedges! These wedges would be used in all places where settlement needed to occur.

Sometimes the box frame can be attached directly to the wall plate, using that as a lintel instead of the box having its own top. In this case the settlement gap would be left at the bottom of the frame, not the top.

STRUCTURAL BOX FRAME

These used to be the norm for load-bearing buildings but now they are mostly used only on small buildings or for small windows, having been replaced with fixing posts (see page 128).

Weather protection

The detailing around windows and doors is extremely important, as the junction between the window, sill and render in particular gets a lot of rain, and so you need to be especially careful to make sure this does not damage the wall or window in the long term. The places where there are openings in the bale walls are always where moisture problems occur, if they are going to.

Pay particular attention to the quality of the design and the quality of the installation at these points. Never take shortcuts!

A very common problem during the construction, for self-builders and contractors alike, is that the windows and doors are late to arrive on site and the pressure of the schedule pushes you to start the plastering before they are fixed in place. It is NEVER sensible to apply more than the first (key) coat of plaster before the windows and doors are fixed in place. The detailing works to keep out the wind and rain by a careful build-up of layers of protection, and failing to carry this out exactly will result in moisture problems and a building that will not be durable. None of the most experienced straw-bale practitioners worldwide recommend the use of vapour-permeable barriers anywhere in your straw-bale building, but if there was to be an exception, this would be it. If you are not sure whether your details are robust and durable, use a waterproof barrier (flashing) around the windows and doors that extends no more than 100mm (4") into the surrounding plaster.

There are plenty of studies that show the use of vapour-impermeable membranes on straw-bale walls is not only unnecessary, but they can actually cause the moisture problems you are trying to eliminate. So beware of using them! See this paper from the Canadian Mortgage and Housing Company (CMHC), a study of 34 straw-bale houses built over a 13 year period: Fugler, Don (2007). ‘Straw-bale Construction in Atlantic Canada’. www.cmhc-schl.gc.ca/odpub/pdf/65828.pdf

The join between the subframe and straw, and between the glazed window frame and subframe, must be more than adequately covered with render and timber to prevent draughts and the possibility of rain penetration. Pay attention! Under no circumstances should you alter or skimp on these details.

Weather protection is very important for windows and doors. Here a secondary sill made of roofing slates has been used to prevent rain penetrating the walls below; and on these French windows, the frame is wider at the bottom to protect the walls to the side. Photographs © Rae Parkinson.

In the drawings on pages 135 and 136 you can see two methods for protecting the straw below the window. One uses a secondary timber sill that sticks out beyond the finished render by at least 20mm (¾") and the other uses a secondary layer made from roofing slates, fixed back to the rough frame. Notice that the timber sills are tongue-and-grooved into the window and the frame, which is a much better detail than a simple butt joint. Wherever you have two pieces of timber butted together, the join must always be covered over first with a durable timber strip (oak is best), glued and pinned, or with wood fibreboard with jute mesh in the render over the top.

The design opposite adds extra protection at the junction between the window and the sill. At the head of the window, the join between the glazed window frame and the rough frame is covered with a grooved drip mould, and the render comes down to sit on this. Windows and doors should always have a drip mould over the top.

Best practice for good detailing is to make sure that any wind or rain trying to get into the building has to go round as many corners as possible, and any joins are covered over at least twice. This makes it almost impossible for the elements to get in. All materials used should be of the best possible quality, so that they won’t deteriorate, rot or need a lot of maintenance. Finally, keep them maintained! Ultimately, with good design and top-quality materials, maintenance will make your building durable.

Do not be tempted to ‘simplify’ details around windows and doors. Unless you really understand about weatherproofing and durability, you risk water coming in from the outside, timber rotting too soon, and rain getting into your straw bales.

It’s not worth it. If you don’t understand how and why these details work, then talk to someone who does, or go on a course. And it really is not sensible to try and save a small percentage of your build cost by buying cheap (poor-quality) windows – unless you haven’t got your sights set on a house that will last 200 years.

Framework methods

In framework methods, windows and doors are designed into the framing. Sometimes this means a complete frame all along one elevation of the building, with perhaps no straw at all, or infill straw between the posts and beams – see the photos below. In this case, the detailing for windows is the same as for any type of timber-frame building. Make sure it is robust and durable! If the frame is hidden inside the straw, there will be an upright post either side of the window/door that runs from the base plate to the beam above. These posts can be of various designs, but the details for the window would be the same as shown on page 136. If the frame is inside the building, not within the straw, then you will need to bring the windows and doors towards the outside of the building, and allow for fixing posts either side of the opening, designing the window for a load-bearing building with fixing posts.

Straw-bale buildings can have large windows – sometimes even whole elevations with no straw at all – if built with a timber framework. Left photograph © Laura Kudelska and right © Rae Parkinson.

With a framework, the dimensions of windows and doors do not need to be multiples of bale lengths, but the design should ensure that the gap between one fixed post and the next does relate to full or half-bale lengths, to make the bale installation easier. If there is a bale between the top of the window and the beam above, framing must be designed to carry the full width of the bale, and allowance should be made for compression of the straw beneath the top beam or wall plate on to the straw.

Windows can be designed in all shapes and sizes to suit your building. Photographs © Barbara Jones.

Window and door design

alt It is not usually sensible to design windows that go up to the wall plate without any bales above on single-storey buildings, as the generally large overhang on straw-bale buildings will obscure any light that might come in from the top of the window.

alt Tall, thin windows let in more light than short ones with the same area of glazing.

alt It makes sense to splay the internal reveals (sides) of windows and doors to make them wider than the external reveals. This way more light can enter the room, and anyway it’s more beautiful than just having a square hole through the wall.

alt Place windows 100mm (4") back from the surface of the outside render.

alt Look at designs for Passivhaus and stormproof-type windows and doors. These are made to a high specification and are meant to last.

alt You may prefer to use two windows, one located as described here, and the other on the inside edge of the wall (called secondary glazing). This is common in France, Germany and other Continental countries. The large air gap between the two single-glazed windows gives very good thermal efficiency, and usually costs less than double glazing.

alt Off-the-peg windows are not necessarily less expensive than those made to your own sizes by a good local joiner.

alt Always use durable timber, eg oak, Douglas fir or larch.

alt Windows and doors should always have a drip mould above them.

alt Ideally do not use waterproof membranes. If you do, keep them to a minimum so as not to compromise the breathability of your walls.

alt Sills and any horizontal surfaces must always have a fall (slope) to the outside.

CHAPTER 13

Compression

Straw is a plant material; stalks of the crop are baled up and bound by strings into a dense, rectangular block that we use like giant bricks to make a wall. These blocks or bales are incredibly strong, but when you put weight on them, they will compress slightly. The straws in the bale can scrunch down into each other a bit more if you apply extra force, but they quickly reach a point where this stops. There is still plenty of air in the bale for it to be an extremely good insulator, but not enough for it to be a fire risk, and it can hold up your floors and roof. How good is that! Compression can either be allowed to happen over time, with the weight of the building compressing itself, or it can be speeded up by ‘pre-compression’ using ratchet straps or jacks.

Photograph © Rae Parkinson.

If you’re a bit nervous about compression, and think (as some do) that it might go on and on and never stop, imagine unwrapping a sheaf of photocopy paper, putting it on the floor, and standing on it – it compresses a small amount. Now ask six of your friends to stand on it with you – it doesn’t really compress further, does it? And if you imagined standing there for 100 years, it still wouldn’t compress further would it, if nothing else changed. This is what happens with straw too.

So, all bales will compress, or settle, when you put weight on them. They will also settle a bit even if you don’t put weight on them (this is called ‘creep’) just by their own weight over time – which is why we need to compress the bales in infill designs as well as load-bearing designs. However, this can be done either by letting the weight of the floors and roof do it for you, or you can pre-compress the straw yourself. It’s still perfectly feasible to allow the floor and roof weight to compress the walls without using pre-compression, as all early straw-bale houses did. Every load-bearing design is compressive; it’s just a matter of whether you let the weight of the building structure itself compress the bales, or whether you help it along so you can build faster. Traditionally, in load-bearing buildings the roof weight would compress the bales over a period of about six weeks, with some minor settlement happening over the next year. Commercially, it’s important that all the settlement happens before plastering, otherwise you may get small cracks in the plaster. Although these cracks can easily be dealt with (see chapters 15 and 16) trying to avoid them, and to speed up the building process, has led to the development of pre-compression techniques.

With these methods, settlement caused by the roof and floor loads can be controlled and encouraged to happen quickly, plus extra loading can be applied to make sure no further creep will happen in the future due to snow loads, etc.

The effect of render and plaster on the strength of a wall

Without doubt, adding renders and plasters to a wall increases its strength – a lot. They stiffen up the wall and make it act rather like a structural insulated panel system (SIPS), except of course there is also strength in the straw itself. To my mind, this is an added benefit rather than a necessity, as straw bales are structurally strong and able to carry tremendous weight. However, in practice, any durable building will always be either rendered or timber-clad (another strengthening feature) and plastered internally.

Pre-compression should be of such a force that the straw will not compress any further even if subjected to more force (weight), eg by a heavy snow fall. Forces must be vertical, and not excessive, otherwise they may distort or buckle the wall!

Commercially, there is a need for pre-compression, but for smaller or self-build types of building you may find it unnecessary. Whichever method you choose, the building still needs to be carefully designed to allow for settlement.

Why pre-compress?

The following reasons for compression apply equally to load-bearing, infill, hybrid and pre-fabrication methods:

1. To ensure the straw walls are airtight. As we are now far more conscious about energy-saving and thermal efficiency, it’s important to build to the highest standards in order to achieve this. Compression means that the bales are pushed more tightly together at joints, at junctions with timbers, and between courses, ensuring no air leakage.

2. To stabilize the straw ready for plastering, otherwise the render/plaster may crack due to small movements (This is only a problem for infill designs, as load-bearing is itself a compressive technique.)

3. To speed up and improve building techniques to make them more appropriate for the demands of modern construction and the Building Regulation (Code) standards.

4. To balance the force (weight) of the floors, roof, people and things inside, snow in winters etc. with the amount of force applied to the walls to compress them, or make them settle. Then there will not be any more compression of the walls at a later date.

Here the bales towards the back are being compressed with straps, and form a more solid, sturdy wall than at the front, where compression hasn’t been started yet. Photograph © Barbara Jones.

How much does a straw bale compress or settle?

This is a really good question, and one that doesn’t have an exact answer, but don’t let that put you off! When building, we often choose ‘construction-grade’ bales, as these are baled as dense as possible by the machine, so that the amount of compression or settlement of the bales is reduced. We could use any other type of bale, but the less-dense bales will settle more under the compressive forces of the floors and roof, and this needs to be allowed for in the design.

A good-quality construction-grade bale can be compressed by about 10mm (½") per bale. A less-dense – but no less usable – bale may compress by as much as 25mm (1") per bale. Within this you have your parameters, and you can design more easily with this knowledge. Ideally you will already know where your bales are coming from, and can therefore make an educated guess as to their willingness to compress (see Chapter 5).

In a load-bearing building, it generally won’t matter if the walls compress a little more or a little less than expected, as long as you have allowed enough space above doors and windows – the ‘settlement gap’ – to make sure the door frame doesn’t hit the ring beam (see Chapter 12).

For infill and other buildings it may be more crucial. If the framework is already fixed, then it may be necessary to add flakes or parcels of straw to make up any difference. It’s sensible to design the rigid components so that there is some leeway within them to allow for extra settlement. However, sensible design includes knowing your building materials so as not to create unnecessary building time!

How do I know if it’s compressed enough, or too much?

If you bang your fist on to an uncompressed straw wall, not only does it move but you also hear the sound of your fist hitting something soft. So the first sign that the wall is compressed enough is that it doesn’t move when you apply quite a big force to it – eg kicking it, several people running in to it at the same time. The second sign is that when you thump it with your fist you hear a deep, short sound, quite different from that heard before compression. The purpose of compression is to stabilize the wall and ensure it won’t move at a later date. It is possible to overdo it if you’re too macho, so beware of this. If you subject the wall to too much force, it will bulge or buckle. So always check for this as well. The finished wall should be straight and true, solid and with a satisfying dull sound to it when thumped.

Methods of pre-compression

Various methods of pre-compressing straw walls have been used around the world since at least 1994. The idea is to force settlement of the bales and maintain the pressure by the use of tie-down straps, fastened over the top of the wall and down to the foundations, until the straw is stabilized. When you see this happen, it transforms a very flexible wall into a solid, strong, extremely impressive structure in a few moments! (No-nonsense builders, previously sceptical of this new-fangled building material, have become instant advocates at this point in the build and have been found showing their friends round and boasting about their building’s credentials.)

Methods include rubber tubes placed on top of the walls, fastened down with chains and then filled with air (first used in Canada); industrial-strength ratchet straps, placed over the top of the walls and fastened to the foundations, and then ratcheted down tight (as used in California); or pairs of threaded rod attached to the top and base plates and screwed down externally (used in Australia).

These days most pre-compression is created by using one of two methods, either ratchet straps or hydraulic car (bottle) jacks.

Ratchet-strap method

In this method, heavy-duty ratchet (or lorry) straps are fastened over the wall and tightened to put the straw under pressure and compress it. For load-bearing buildings, the straps go over the wall or roof plate, and this is pulled down with the straw. Where there is no plate, in some framework buildings or when the wall is more than seven bales high before the wall plate, then a plate has to be introduced so that the straps do not crush into the straw. Sometimes this could be a temporary thin but rigid metal plate, with holes at the outer edges where ropes are attached, which can be removed once the straps are released. In other designs, it might be a horizontal timber that is permanently fixed between existing upright posts once compression is achieved.

Heavy-duty ratchet staps fastened over the wall / roof plate and tightened to compress the straw.

As these straps are never permanent, it’s usually necessary to use polyester straps as well – these can hold the compression until further weight has been applied to the walls. These straps are not strong enough to create the compression on their own, but can hold it once applied, allowing you to remove the ratchets and carry on building.

Ratchet straps are not left permanently on the walls so you may need to use polyester straps to hold the compression until the roof is added later.

Photographs © Rae Parkinson.

If there is no wall / roof plate, a thin metal plate can be used with hydraulic jacks. After compression, and once the last bales have been inserted, then the plate is removed.

How to attach ratchet straps for compression

You can appreciate that putting the walls under the amount of pressure required to compress the bales, particularly when using construction-grade bales of very high density, requires a lot of force. This means that whatever you fasten the ratchet straps to at foundation level needs to be very strong. For commercial house building, this is a design detail that needs to be added in at the earliest stages. It can be done in the following ways:

alt For foundations with solid walls: lay a 25mm (1") water pipe through the foundations so that when you are ready you can insert a metal pin (about 25mm / 1" diameter) through it, which projects about 100mm (4") beyond each face of the wall. Ratchet straps can then hook to this pin on either side of the wall, the pin can be removed later, and the hole it leaves can then be jammed tight with insulation eg, Leca™ (blown clay granules) and the last 10mm (½") pointed up with lime both sides, to ensure airtightness.

alt For foundations with Foamglas™ inner skin: the method above cannot be used as the upward force of the ratchet on the pin will tear through the Foamglas™. Therefore, when using a suspended timber floor, with joists perpendicular to the wall, place a metal pin parallel to the inner wall and underneath four joists; use this for the hook on the ratchet. Where the joists are parallel to the wall, use the method described above but make sure the pin is long enough to project inside the building under at least two joists.

alt For foundations of car tyres: the metal pins can go directly underneath the structural box beam, between the tyres.

alt For foundations with a solid floor: fix hooks into the floor that can be removed and filled later.

alt For upper floors: on the inside place, a metal pin parallel to the inner wall and underneath four joists; use this for the hook on the ratchet. Where the joists are parallel to the wall, place the pin under at least two joists. On the outside face of the timber ring beam, securely fix a temporary 50mm x 50mm (2" x 2") batten and hook the ratchet under this. Make sure the base plate and batten are securely fastened with fixings such as TimberLOK screws.

alt You can place ratchet straps and polyester strapping directly below the base plate only. In this case, slots need to be made for them before fixing the plate, and the plate must be fixed very securely to withstand the upward force of the straps. Once completed, the gaps left under the base plate MUST be packed tightly with insulation.

Steps for using ratchet straps. Photographs © Rae Parkinson.

Handy tips for using ratchet straps

alt You will need to use the straps in pairs, with a handle either side of the wall, as the force on the strap pulls towards the handle. Therefore, if you have all the handles on the same side of the wall, it can pull the wall out of true, and it will lean towards you.

alt If you are compressing a tall wall, or the bales are very soft and will compress a lot, the strap will lose the ability to tighten further before the wall is compressed enough. So use two ratchet straps side by side. Tighten the first strap as far as possible, which will bring the height of the wall down. Now tighten the second strap, which should complete the compression. This will also make the first strap loose. n Place the handle quite near to the floor, with the handle action downwards, not up, so that you can put your foot on it for extra strength.

alt Make sure you feed the strap on to the centre of the ratchet. If it starts skewing to one side it will stop the ratchet from working.

alt Be careful not to over-tighten the straps or you will not be able to undo them. If you run out of strap before all the compression has happened, use polyester straps to hold it, then undo the ratchet and tighten again.

Hydraulic car (bottle) jack method

With non-load-bearing structures, beam heights will need to be set before installing the straw

How to calculate the height of beams for non-load-bearing structures

Let us assume we are using construction-grade bales, so these are 365mm (14³/₈") high when delivered to site, and we know we can compress them to 355mm (14") and achieve a very stable wall. The distance between the top of the base plate and the underside of the beam must therefore be a multiple of 355mm. For a 6-bale high wall this is 2.13m (7') and for a 7-bale high wall it is 2.49mm (8'2"). If your bales are less dense than this, you will need to lower the height of the beam, raise the base plate, or allow for one extra bale in the gap.

Hydraulic jacks in use. Photographs © Rae Parkinson.

When there is a horizontal beam central to the straw wall

A central beam can be used to compress the straw with bottle jacks. The height of the beam should be set to accommodate the size of bales to be used, allowing for the compression they will be placed under.

Testing for compression

If you’re not sure how much your bales will compress, you can do a test as soon as you get them by making up a small frame with two posts and a movable top. Use four bales with ratchet straps over the top and compress. When you’ve achieved a stable stack, calculate the difference between the original uncompressed height, and the finished height, and divide by four for the number of bales used. This tells you the compressed height of your bales.

First, the straw is built up apart from the last course of bales. These are dressed and prepared ready for insertion and kept close by. Then a thin rigid metal plate is placed on top of the wall, to prevent the jack from sinking in to the straw. This plate has holes in the sides to which ropes are attached, so that it can be pulled out later when the final bale has been placed on top. The jack is extended between this and the beam above, and used to compress the bales below by the calculated amount. With correct design, whole bales will fit in the space you have created. These are pushed into place on top of the plate between the jacks, and the jacks are removed. The bales are then persuaded along the wall to close up any gaps, and then you can carefully pull out the metal plates. Finally, the last space left by closing the gaps is filled.

In practice, this can be something of a brutish operation, as the bales may need to be belted quite hard with the persuader to move them into the correct position, especially as you will have calculated well to reach optimum compression. Sometimes you will need to repeat the operation and leave the jacks in place for a couple of hours, because the straw may spring back too quickly for you to get the last bales in place.

Handy tips for using bottle jacks

alt The jacks aren’t that tall, so you might have to compress the wall, then take the jacks out one by one, raise them up with extra blocks, then compress again.

alt If you know the bale will fit in the space you’ve created, but the friction between the bale and the beam is preventing this, wrap the bale in tough plastic to reduce friction, push it in, then pull out the plastic.

alt It’s easy to over-compress, as hydraulic jacks have more strength than needed for this job, which is why they also make it easy. The wall may bulge outwards, or the frame joints pop with too much force, and if the force is not vertical it will also cause the wall to bulge.

alt The jack MUST be in the centre of the bale and remain vertical during use, otherwise it may suddenly flip out from under the beam, or create a force that is not central to the wall, which will then start bulging.

When there are vertical posts central to the wall

When the straw is built between framing or fixing posts, it can be pre-compressed using bottle jacks as long as the posts are relatively central to the wall. In this method, it works more easily if the straw is compressed every four bales high. First, build the straw up to this height between two posts, then firmly but temporarily attach two horizontal timbers either side of the posts – the jacks will be pushing up against these. There are two methods of holding the straw in place after compression:

1) Lay polyester straps over the straw and either underneath the base plate or attached to it, so that they can be tightened up once compression has been achieved. This method needs to be prepared in advance, with the straps in place before the plate goes down. Place the jacks on wide timber blocks to spread their load and prevent them from sinking in to the straw, roughly one jack per metre (39") and compress the straw to stability. Now tighten up the polyester straps and remove the jacks and temporary timbers. The straps will dig into the bales but will hold them until you have built the wall up to height and permanently fixed the top. Build up the next four courses of bales above and repeat the same procedure, attaching polyester extensions to the straps below once the jacks are holding all eight bales in compression. In a framework building there is usually a permanent beam at six or seven bales height that can be used instead of the temporary one described above. Once all the bales are compressed under the beam, or ring beam, there is no need to use the polyester straps and they can be removed.

2) Lay a timber on top of the straw, tight between the posts, about 75mm x 50mm (3" x 2"). The dimensions will vary according to the distance between posts – small-dimension timber will bend over longer distances. Place the jacks on top of this, and compress the straw and timber together. Once the straw is stable, permanently fix the timber to the posts and continue to build. The course of bales on top of this timber will need to be notched out the depth of the timber to allow it to settle around it.

Using bottle jacks to compress around windows

The bales above and below windows must also be compressed. A great way to do this, once the ring beam or frame is in place, is to lay the bales below and place the framing sill on top, quite tight between the fixing posts. Now make a jig/temporary frame inside the window space to hold bottle jacks, place the framing header on top with the bale above already in place, and pump up the jacks. This will compress the bales both above and below. Add the framing sides before releasing the jacks, and fix everything to the fixing posts. With a little skill, and knowledge of your bales, you can make your windows the exact size that you want, plus get all your windowsills at the same level!

Alternatively, you can add the bales below, frame out your window but not fix to the fixing posts, and jack between the ring beam or frame above to compress the window frame down on to the lower bales. The top bale above the window can then be inserted in two halves by releasing the jacks one by one.

Hydraulic jacks can also be used to compress the bales above and below windows. Photograph © The Wisharts.

Caution

Always build the whole wall up to the same height before compressing any sections. The force of compression will act sideways as well as vertically, and if the adjoining bales are not in place, the posts could distort, plus the rest of the bales would not benefit from this sideways pressure.

A well-compressed wall will be solid, strong, straight and satisfying, ready for the plaster.

CHAPTER 14

Services, fixings and alcoves

This chapter is about how to attach things to and into the walls. You might want to fix kitchen cupboards or a bookshelf inside the house or a drainpipe to external walls, or to run electricity and water pipes within the walls: all, of course, are possible. Straw bales also allow you to easily create beautiful alcoves and niches in your walls as you build – or even a ‘truth window’ to reveal that your house really is made of straw!

Photograph © Rae Parkinson.

A ‘truth window’ reveals the straw beneath, providing permanent proof that the building really is made of straw! Photograph © Rae Parkinson.

Most things you could imagine wanting to fix to a wall can be fixed to straw walls too. The method harks back to an earlier time when wood was used for fixing – usually between joints in bricks – instead of the modern plastic alternatives.

Internal fittings

These days, we are all familiar with the plastic Rawlplug – a small insert used to line a hole drilled into masonry, into which a screw can be fixed. Fixing things into straw uses essentially the same principle, except that the plug is much larger and made from wood, not plastic. Cupboards, shelves, light switches and sockets, bathroom and kitchen facilities, radiators, etc can all be fixed by using sharpened pins of hazel (about 200mm x 32mm / 8" x 1¼") knocked into the body of a bale, which then gives a fixing point for screws or nails. Heavy items such as radiators and bookshelves can all be fixed this way, with enough hazel-pin fixing points. These fixing points should be placed before plastering, but can be added at a later stage, if necessary, by cutting out a hole in the plaster first. They need to stick out from the bale wall by the thickness of the plaster, so about 20-30mm (¾-1"), but excess can be cut off later, and you can find them after plastering by simply attaching a screw into the end beforehand. It may be useful to add a series of pins in a horizontal line, and attach a timber batten to all of them before fixing cupboards, etc, on to this; this method is particularly useful for skirting boards, as it gives a straight edge to work to for your plaster. Alternatively, vertical battens can be attached to the base plate and wall plate, with hazel pins used as intermediate fixing points between them. Shelves etc can then be hung from these. In a framed construction, the framing posts can be used. If the straw has not been compressed well, it may not be possible to use the hazel pins as fixing points in this way.

Hazel pins knocked into the bales give a fixing point for fittings such as cupboards, shelves, sockets and radiators. Photographs left © Rae Parkinson and above © Barbara Jones.

External fittings

Drainpipes, outdoor lights, etc, can also be fitted using the method described above. In this case it is best to cover the hazel with a layer of meshed render, and to screw through it to protect the end of the hazel.

Services

The general term ‘services’ includes electricity, plumbing, water supply and drainage, telephone, TV, Satellite and internet cables. There are no great differences in the installation of services between either a straw-bale or more conventional building. The layout of pipework and cabling would be similar in all buildings, but depending on the materials the building is made from, the way these are attached to surfaces, and how they pass through walls and floors, may vary. In construction we split the installation into two categories: first fix, which covers all the initial pipe layouts and cabling, and all trenches for them – basically all the stuff that is rarely seen once the building is finished; and second fix, which is all the fancy stuff that we do see, such as covers for light switches, toilets and showers, and plug-in sockets for internet cables. It’s important that first fix is well integrated with the schedule of works, to make sure that access is available at the right time to lay cables, etc, for instance, before a floor is laid or a wall is plastered.

Electricity

In straw-bale walls, electricity cables should be encased in PVC-free conduit sheathing to give extra protection against the (as-yet-unresearched) theoretical risk from heat generated by electricity-carrying wires sited in a super-insulated wall such as straw. There is no need to use fire-resistant cable, except in circumstances where it would need to be used anyway – this would not be normal in a house or most industrial premises. The usual practice is to surface-mount the conduit on to the face of the straw wall and cover it with the internal plaster (lime or clay). These plasters protect against fire. Sometimes cables and more often back boxes for wall sockets, etc, will need to be housed a little into the straw, depending on the depth of the plaster. It is sensible to surround the back box with clay or lime plaster, giving extra fire protection between the straw and box.

A channel can be cut into the straw for electricity cables encased in PVC-free conduit sheathing. Photograph © Barbara Jones

Electric socket fixed to a hazel pin in the wall. © Rae Parkinson.

How to make channels in straw for cables, etc

Specialist tool such as the back of a claw hammer(!) can make channels in the straw to take the conduit. We also find a small electric 100mm / 4" angle grinder works very well, with a wood-carving blade on it (Pro-Tech 4). Alternatively a chainsaw is very effective, although heavy and noisy – not very friendly on a building site with lots of people enjoying the simplicity of straw. Once you’ve cut the channel, the conduit can be fixed using a U-bracket and a short length of sharpened hazel (see above), wherever a fixing is needed.

Plumbing

As far as possible, water-carrying pipes should be designed to be fixed in internal, non-straw walls, to minimize the risk of water seepage to the straw in the event of a leak. However, even if this happened, it is very unlikely to cause a major problem. Water-carrying pipes that pass through straw walls should contain no joints, and be encased in larger insulated plastic pipes for the full width of the wall, just the same as for any other type of wall. An advantage of the deep straw walls is that soil stacks (vent pipes for toilets) and other non-water carrying pipes can be buried in them and taken up through the roof, instead of the usual unsightly series of grey pipes we often see at the backs of houses. Try not to put anything metal directly into the straw or plaster and render, as there is a slight possibility that warm water vapour passing through the wall could condense on the cold metal. But if you do, wrap it in wool pipe lagging to protect the straw from it.

In straw-bale walls, as in any type of wall, watercarrying pipes should be encased in larger insulated plastic pipes. Photograph © Rae Parkinson.

Bathrooms and shower rooms

All walls (whether straw or something else) that will have direct water contact need to be protected, and this is usually done with ceramic (clay) tiles. In general, the tiles themselves will not be strong enough to hold the weight of things attached to them, such as the head of the shower, and will need reinforcement behind them before you lay the tiles. You can simply use hazel pegs, as described above, but in practice it will be hard to find these once the tiles are in place. Instead, it is best to attach wider pieces of durable timber to hazel pegs behind the tiling and draw a map of where you have put these, before they are hidden by the tiles. This way, you have some leeway in terms of where you can fix things that attach later. This is especially necessary if you are fitting out a toilet for disabled use, as this requires several very strong fixings on to the wall for handholds. In this case, it may be best to attach vertical timbers from the base plate to the wall plate (reinforced with hazel pegs at intermediate points) before tiling, to give extremely solid fixing points. Use the minimum amount of solid durable timber (or marine ply) as necessary so as not to impede the breathability of the wall.

Alcoves, truth windows and art in general

To add alchoves and niches to the walls is very simple to do, and it enhances the beauty and individuality of your building immensely. Most self-build houses have a few alcoves, and almost all straw-bale buildings have truth windows (somewhere where the closely clipped straw is always visible). You might want to have a nice little alcove to hold your toilet roll, for instance, or an inset shelf in the study. If you use a small angle grinder, or a combination of handsaw and claw hammer, you can carve out a shape into the straw after compression.

It’s best not to go deeper than about 150mm (6") as you will start reducing the insulation value of the walls, but you should certainly not go deeper than half the bale in order to keep the wall stable. Be aware of where the strings on the bales are. If possible, place your alcove in the body of a bale; but if you want something larger than this, it is fine to cut off the string of the bale, as the straw after compression is held in place by the weight of everything above it, rather than by the strings.

You can add a ledge or shelf of wood to the bottom of your alcove or niche which can stick out beyond the surface of the plaster, to give a good surface to hold things. Make this larger than the bottom of the alcove, and cut a groove into the straw all around at the base of the alcove so you can insert the wood into the straw. This will help to hold it firmly in position, together with the plaster that will sit on top of it within the alcove itself. You can make a larger alcove and add intermediate shelves as well to create a bookshelf.

Truth windows and alcoves are easy to create, and make interesting and unique features. Photograph left © Barbara Jones and right © Rae Parkinson.

A truth window is a great idea. When a straw-bale building is finished you can’t really tell that it’s made of straw unless you have something on show, as these buildings end up looking very like old stone houses that have been limewashed or rendered. The truth window could be a picture on the wall that’s hinged instead of hung, so that when you open it you can see the straw behind; or it may be more like a picture itself with a glazed frame, permanently proving that your building really is made of straw!

Many self-builders and community groups also add frescos and sculptures to their walls, made from lime or clay plaster, or have a section where everyone who has worked on the building leaves their hand prints. This is an expression of the exuberance you feel when working with natural materials, because they’re so easy to use and you feel inspired to be creative and to leave an individual mark on your building.

Marking out alcoves on the bales, before carving these out with a small angle grinder, or handsaw and claw hammer. Photograph © Barbara Jones and right © Rae Parkinson.

CHAPTER 15

Lime and lime plasters

It is important to use plasters made of suitable materials to prevent the straw rotting and to move flexibly with the straw over time. Traditional, natural materials such as lime and clay are ideal. Although knowledge of lime plasters is no longer widespread, there are an increasing number of lime products available, or you can make your own lime putty, and plaster and apply it to the walls by (gloved) hand – hard work but immensely satisfying.

Photograph © Rae Parkinson.

Straw is a vapour-permeable material: it allows the imperceptible passage of moisture vapour and air through it. If it is sealed by a waterproofing material, this cannot happen and it may eventually start to rot: it needs good ventilation around it to stay healthy. In practical terms, this means that anything used to weatherproof or decorate the straw must not impede this breathable nature.

Good finishes for straw-bale buildings

The ideal finishes for straw are traditional lime plasters or renders, or natural clay plasters – since these are also breathable materials – painted with limewash or breathable paints. Both lime and clay are humidity regulators, and help to keep indoor air quality healthy. Apart from them being vapour-permeable, another reason we use lime and clay is because they are flexible – vastly more so than cement, which is classed as a rigid material. Straw and flexible foundations require flexible plasters to go with them: this ensures durability over time, as all buildings move if they stand for many years, and we don’t want them to crack! Finally, a third reason for using lime and clay is that both are sustainable, natural materials that are part of a harmonious cycle in nature, and can be returned to the natural world at any point in their life without negative effect; clay even more easily than lime.

What’s the difference between plastering and rendering?

The only real difference between a plaster (for inside work) and a render (for outside work) is the fineness or coarseness of the sand used. Otherwise it’s just terminology. In the USA, exterior work is generally called ‘stucco’ (from the Italian), but can also be called ‘plaster’ and most people say ‘plaster’ when they mean ‘render’, but not the other way round! So it’s mostly just the English who use separate words (and then not all of us!). Render for areas that experience lots of wind-driven rain may contain aggregate particles up to 10mm (³/₈") or so in size; but inside a smoother finish is usually preferred, using a sand of smaller grain size. In this chapter I’ll use the term ‘plaster’ to mean both an interior and exterior finish, unless I say otherwise.

What are the constituent parts of a plaster?

All plasters have a binder – the glue – that sticks them together. This can be either lime or clay for straw-bale walls (see Chapter 16). They also contain an aggregate which gives them strength. This is usually a type of sand of differing particle size, depending on where it is being used. Many of them also contain fibre, which gives them strength, helping to reduce cracking, and can bring moisture into the plaster during curing to help with the carbonation of lime.

Background to the use of lime

Lime has been used as a binding material (mortar) between stone and brick and as a surface protector of buildings (as mentioned, generally called ‘render’ when used outside, and ‘plaster’ when used inside) for thousands of years. All European countries used lime for building hundreds of years before Portland cement was invented. In all European countries, lime burning was a cottage industry, with local lime pits wherever there was a source of limestone, and most communities had a working knowledge of its uses and how to produce it. There is no doubt that lime plasters are durable and efficient, well able to do the job of protecting our buildings from the weather.

Lime mortar has stood the test of time. Photograph © Barbara Jones.

So, we don’t need to argue the case for the ability of lime to withstand the tests of time and weather. However, in order to use it successfully, we need to understand the processes involved in the slow carbonation of lime back to its original limestone. While it is true that a carefully applied lime plaster can last for hundreds of years, there have been instances of spectacular failure in more recent times, and we need to understand the reasons why if we are not to repeat those mistakes. In essence, the preparation and practice of lime work is simple, but variables in the sand, the lime and particularly in the weather during application and drying time have a critical effect on the overall durability of the material.

Traditionally, knowledge about lime was passed down from one generation to the next. People were used to using it continuously and so built up a wealth of experience based on a sound working knowledge of the material. Two world wars mean that today there are very few skilled craftsmen (we haven’t found any women yet) remaining from those times, and we have to learn as best we can from what we have left, and from remembered histories. Interestingly, we can often find details about lime use in unexpected places, such as in old books about gardening.

To some extent, this can result in an over-technical approach to what was essentially a practical and rather ad-hoc building practice. We may try to specify exact lime/sand mixes when what happened on site was probably fairly rough and ready, except for the most prestigious jobs. As tens of thousands of houses in Europe, hundreds of years old, can testify, this relaxed approach mostly worked! So what follows is an attempt to explain what happens in the lime burning, slaking and mixing process, and what is important to know, so that you can take care of your own limework satisfactorily.

Limestone and lime burning

The raw material for all lime mortars and plasters is naturally occurring limestone, shells or coral, which is calcium carbonate (CaCO₃). It is made into lime putty by a relatively simple process. Traditionally, the limestone is placed in a specially built kiln (sometimes a pit or a heap) and layered with fuel, such as coal or brush, and burnt for about 12 hours. It needs to reach a temperature of 900-1200ºC (1650-2200ºF); 900ºC for carbon dioxide (CO₂) to be driven off, and 1200ºC for the heat to penetrate through to the centre of the stone.

As it heats up, steam is driven off first (water, H₂0), which is always present in the limestone, and the following chemical change takes place:

heat + CaCO₃ → CaO + CO₂

(heat + calcium carbonate → calcium oxide + carbon dioxide)

(heat + limestone → quicklime + carbon dioxide)

At the end of the burning process, calcium oxide in the form of whitish lumps or powder is left with bits of burned and unburned fuel. Over-burnt limestone appears as black, glassy pieces, and these should be removed and discarded. The chemical reaction that takes place is usually more complicated than this, due to other carbonates and silicates being present in the limestone, but it’s important to understand the basic changes that are taking place at this stage. Calcium oxide, called ‘lump lime’ or ‘quicklime’, is very reactive and can be dangerous. It MUST be kept dry, as it reacts very quickly with water – even the water in the air or the moisture in your skin – to form calcium hydroxide, which is the first step to reversing the process back to calcium carbonate. Just as making quicklime needed heat, the reverse process PRODUCES heat. For this reason, never add water to quicklime. Always do it the other way round and add quicklime to water or it could explode!

CaO + H₂O →Ca(OH)₂ + heat

(calcium oxide + water → calcium hydroxide + heat)

So quicklime added to water gives us … lime putty! Protective clothing, goggles and a mask should be worn when working with quicklime. Alternatively, lime putty can be ordered from a supplier, ready-made in airtight sacks or tubs.

How to make lime putty

Ingredients: 2 parts water and 1 part quicklime (by volume)

To make lime putty (calcium hydroxide, Ca(OH)₂), great care must be taken and protective clothes, mask and gloves MUST be worn, as a tremendous amount of heat is generated, producing steam and spitting lime. NEVER ADD WATER TO QUICKLIME; ALWAYS ADD QUICKLIME TO WATER, BECAUSE IT CAN EXPLODE.

First, water is poured into a metal bath (do not use plastic), twice as much by volume as the lump lime to be used. Two people are needed, one adding the lime and the other raking and mixing. Care must be taken when placing the bath, as the heat generated can burn any grass or wood underneath it. Some of the quicklime is added to the water, and is immediately raked and mixed, a garden hoe being the best tool for the job. The process of adding quicklime to water is called SLAKING and great care needs to be taken as the temperature soars and the whole mix starts to bubble and boil.

Slowly add the rest of the quicklime, raking continuously. The purer the quicklime is, the faster the hydration process occurs. Keep on raking and mixing until the lumps have all broken down, and then leave to cool completely. Sieve the resulting putty, which feels like double cream at this point, through a 1.5mm (¹/₁₆") grid to take out any pieces of unburned limestone (which will not react). These can go back into the kiln for burning next time. What you now have is slaked lime, or lime putty, which must be stored for at least three months before use. This is to ensure that most of the calcium oxide has hydrated, which takes time. It’s important not to expose lime putty to the air or it will begin to carbonate before you use it. If this happens, the lime will not be as reactive and will make a much weaker mix than you require.

Traditionally, lime putty was stored in a pit in the ground, where it would remain for several months or years before use. The advantage of a pit is that any excess water rises to the top, leaving good-quality putty below. The Romans forbade the use of any lime putty that was less than three years old; and all the old practitioners say that the older the lime the better it is for the job. Lime putty in pits was a valuable commodity and would often be passed on via will from one generation to the next. Opinions differ about the quality of lime putty if it is subject to frost. Since lime was kept in pits for many years, where some of it was likely to have been subject to frost, this suggests that the frost has no adverse effect and most think the frost improves it.

Sand for plastering

The type and quality of sand chosen for the mix is really important. It’s the sand that gives the plaster its strength and durability. The sand MUST be well graded – that is, contain particle sizes ranging from very small (dust) to quite large (5mm / ³/₁₆" in some cases), and these should be angular (sharp), not rounded. If you use sands that are not like this you are likely to get more cracking in the finished plaster, or parts of it may even crumble off the wall. What happens is that the pointy bits of the sand grains knock together, creating voids, and the smaller grains fill these voids, also knocking together with each other; and the dusty grains do the same thing so that when compacted well there is a strong matrix of different sized grains. The lime putty glues the grains together.

Helpful machinery and tools

It can be a lot of work mixing lime plaster by hand, especially when using lime putty. The temptation is to add water, but this will lead to cracking in the plaster. Instead, a very useful tool is the hand-held plaster mixer (paddle or whizzer), for which you also need a large tub to put your mix in. You can buy a paddle for an ordinary drill but I don’t recommend this, as it is likely to burn the drill out before you’ve finished. There are many products available at reasonable prices. If you’re building your own house then one of these is essential. A step up from this is the roller-pan mixer. They come in different sizes; some can be pulled behind a car or hired from a shop. These squash the mix, lift it and turn it, all while turning in a flat pan. This is ideal for lime putty and sticky mixes including clay, and for reusing old lime mortar as sand. Another type of mixer is the forced action mixer, which is only good when trying to mix powders and fibre together, so won’t do for lime putty but will work with hydraulic limes. The common cement mixer is good for mixing cement but not usually other stuff, especially if it’s got fibres in it. It can be useful for remixing wet ready-mix lime products if they’ve been standing around for some time.

A hand-held plaster mixer is very useful.
Photograph © Rae Parkinson.

How to work out quantities and cost

You will always have to work out the square metreage of the area you have to cover first and then decide how thick you want your plaster. Remember to add in the reveals (sides) of windows and doors. In general, 1 tonne of mixed plaster will cover 20m² (215ft²)at 25mm (1") thickness. So for 100m² (1080ft2) you’ll need 100/20 = 5 tonnes of mixed plaster.

It’s harder to work out quantities when making it yourself, because the ratio is based on volumes, not weight.

So, as a rough guide for 1 tonne of mixed plaster:

36kg (80lb) quicklime + 0.324m³ (11ft³) sand + 108 litres (28 gallons) of water.

Or 108 litres of lime putty + 0.324m³ sand.

Notice the sand is measured by volume.

In 2014, UK prices for quicklime were about £200/tonne and sand about £80/m³ delivered, giving a material price of £33/tonne if you do all the labouring yourself. Lime putty was about £22 per 20 litres delivered, giving a material price for this method if you do all the labouring yourself of £158/tonne. This compared with about £260/tonne delivered for a ready-mixed plaster. So, although these prices are vastly different, you must take into account the enormous amount of time it will take you and your friends to mix this sort of quantity on your own. You should also factor in the cost of hiring machinery, and the fuel to run it if you want to speed up the process, and the water if you’re on a meter. Also consider the peace of mind resulting from the knowledge that each batch you’ve bought ready-mixed is the same quality as every other one – plus the technical back-up of the company you bought it from, should things go wrong.

How to make lime plaster

Lime plaster can be mixed on site from lime putty and local sand, or sometimes from quicklime and sand, depending on local availability. If you are using bags of hydrated lime, see Appendix 6. Well-graded sharp sand can be bought from a builders-merchant, but keep it clean. If you prefer not to make your own plaster, it can be bought ready-mixed from one of a growing number of suppliers (see Appendix 2).

Lime putty in a wooden trough with a stick for beating the putty until it becomes plastic and workable again after maturation. Photograph © Rae Parkinson.

Knowledge concerning the ratio of voids to grains in the sand, and the loss of expertise due to the world wars and rise in popularity of cement, has led to a misconception about the mix for lime putty and sand. In the past, the vast majority of lime plaster was made from quicklime in a 1:3 ratio of lime to sand. As quicklime slakes it increases dramatically in volume by up to 100 per cent, so that if we want to make the same mix using lime putty instead we need to take account of this.

Also lime putty becomes heavier and drier as it matures, which is why using it by volume is important. (1 tonne of mature lime putty should have a volume of about 740 litres / 163 gallons.) Finally, damp sand is more bulky than dry sand, and if it is not well graded it will need extra lime to keep it strong. All these factors tell us how important it is to have experience of making and using limes!! An experienced plasterer will be able to tell by the feel of the mix, its stickiness and workability, whether it is in the right proportions or not.

Making lime plaster by hand (with gloves!) and with the help of a mixer. Photographs © Rae Parkinson.

Lime putty plaster mix

Ingredients: 1 part lime putty to 1½ parts sand by volume The longer a lime putty has matured, the more solid it becomes, and the better plaster it makes. It may seem hard to work with at first, if it has been made for some time, but by pounding and beating it with wooden mallets or posts it soon becomes more malleable and can be worked into the sand. This beating part can be VERY labour-intensive, but should not be missed out. If you are making a lot you could use a specialist mixer. Because it’s so hard to work with, it can be easier to mix the sand with fresh lime putty, which doesn’t need beating. You should then leave this mix to mature for three months, traditionally under a thick layer of sacking and then straw, to keep the air out and stop it carbonating.

Hot lime plaster mix

Ingredients: 1 part quicklime powder to 3 parts sand.

Historically, this is probably the most common method used for mortars. In this method, the quicklime is added to damp sand in a bath and mixed with a shovel. Very soon, the mix starts steaming and becomes warm, as the reactive calcium oxide hydrates with the water in the sand. At this point, the mix can be riddled (passed through a sieve, usually 6mm (¼") for plasters), to get rid of any unburned or over-burned limestone as it’s easier to do it when the quicklime has dried out the sand.

This process is dangerous because the powdered quicklime blows in the air and can get into eyes and lungs, reacting with the moisture there; plus the mix gets hot very quickly and may be difficult to control. It must be raked and mixed continuously and, depending on the dampness of the sand, may not need any extra water added to it. You are aiming for a mix that is nice and sticky, workable, but not sloppy. Ideally, it should be left overnight before using.

Understanding lime products

There are an increasing number of lime plasters and products available on the market. Some are really made from lime, but others are not, and you need to make sure you are not reducing the breathability of your walls without realizing it! It’s important to be able to choose the right type of lime for your purpose. If you’re not sure, ask someone who is. It’s always best to stick to the most simple ingredients and not over-complicate things. They work, and have worked for thousands of years.

For more information on the different types of lime products available, and how to understand what it says on the bag, see Appendix 6.

How to use lime plaster on straw

Lime plaster should be applied in two or three coats of slightly different mixtures and consistencies, as detailed below. The first key coat on to straw is usually lime-rich to make it stickier. The next one or two coats of plaster may contain fibres such as chopped hemp or straw, well distributed throughout the mix to give it greater strength. Many people feel a little sad when their beautiful golden straw walls start disappearing underneath the plaster, but it’s a necessary step to protect the walls and make them durable. Sometime during the plastering process, though, you start to feel excited about the curvaceous, organic look you are creating, and how your home is starting to emerge from all your hard work. Plastering is a lot of fun and very creative – you need to try it to find out!

Preparation

This is an absolutely essential part of the plastering process, but very often doesn’t get done well enough. It can take quite a bit of time, so make sure you allow for it in your schedule. It is often one of those jobs that falls between others – it’s not exactly straw-bale building and it’s not exactly plastering. Professionals in either trade may say it belongs to the other one, so make sure you know whose job it is!

1. Make sure your walls are straight. This should happen as part of the compression process (see Chapter 13).

2. The internal and external faces of the straw walls should be given a very short haircut – trimmed down to a neat finish. All the long, hairy, unkempt bits of straw should be removed. The reasons for this are:

a) to reduce the amount of plaster required by reducing the surface area;

b) to even out any large undulations in the surface of the wall;

c) to minimize flame spread over the surface of the bales, in the event of a fire before plastering;

d) to reveal any joints that still need to be stuffed with dry straw to make them airtight.

3. Stuff any holes that are revealed tightly with dry straw.

4. If you are making your own plaster, it should be stored in airtight buckets until you’re ready to use it. If buying it ready-mixed, make sure you’ve got it all on site before you begin.

5. Use old tarpaulins or clean boards underneath where you are going to work. These will catch any lime that falls off the walls so you don’t waste it (it’s usually expensive!) – you can add it back into the mix later.

6. Make sure you have safety glasses and long-sleeved gloves for protection, buckets of clean water around so you can clean off when you want, and an eye bath in case you get some in your eye.

Mixing your plaster

Lime plaster should be beaten, mixed or worked to a stiff consistency, so sticky that it can be held upside down on a trowel. This takes about 20 minutes. Be careful about adding water, because the more you mix the lime the more plastic, and stickier, it becomes. If you don’t allow time for this mixing and add water instead, you will increase the risk of shrinkage cracks. It will generally become more plastic with lots of beating! Traditionally this was a completely separate trade, being a lime-plaster beater. These days, plaster can be knocked up in a roller-pan mixer to save all that work by hand. Generally, a cement mixer WON’T do the job, especially when mixing from lime putty, as the mix stays in a lump and knocks the machine over – then the tendency is to add water to soften it, and the resultant mix will crack due to too much shrinkage. If the plaster has been matured by the supplier and then delivered, the vibration caused by the delivery lorry can be enough to soften up the mix. In this case, it may be possible to use a cement mixer to complete the process.

Don’ts

alt Do not wrap the straw in stucco or chicken wire first, as many cement-plastered buildings in the USA have been. This is totally unnecessary and a waste of time! Both lime and clay stick extremely well to the straw, particularly if applied by hand or sprayer.

alt Do not wet the straw before applying the key coat – there is enough moisture in the straw already so it doesn’t need any more. The only time you might need to apply a spray of water is if you are working in a very hot country – however, in this case you should wait and do your limework in autumn or spring.

Key coat / first coat / revealing coat

This is usually a mix of sand and lime putty in the ratio of 1:1 and reasonably wet, like thick cream so it’s nice and sticky. It’s usually best to apply this coat by hand (with gloves, as it’s caustic!), because it’s more fun and because the straw tends to flick the stuff back at you if you try to use tools. It needs to be well rubbed in, to get a good key (join) between the straw and the lime – the action is rather like giving a massage! This coat can also be sprayed on, although this can be very messy. In this case, you will still need to follow on behind the sprayer and rub it in well.

A common problem for professional plasterers is that they don’t like to use their hands (at first!) and so use tools. I have seen these guys apply the key coat very quickly by trowel, but this is not good practice! If you do it this way, the lime is sort of stroked on over the top of the straw, it is too thick, uses too much material, and doesn’t key as well as it should into the straw. Also, a lot of it falls off, as the straw is so flexible. Plasterers aren’t used to picking up plaster and reusing it, so wastage is very high. Getting this coat right can be a bit time-consuming, but if there are several of you together the time soon passes!

It’s best to apply the first coat by hand to get a good key between the straw and lime. Photograph © Barbara Jones.

Lime/straw mix

While you’re doing the key coat, the action of rubbing in the mix reveals any holes, hollows or indentations in the wall that couldn’t be seen when the straw was still fluffy. These should be filled first with dry straw wherever possible (dry straw is much more insulating than straw/lime mixed together), then by using a lime/straw mix. Prepare this mix using a very wide, shallow tub. Place some chopped straw in the tub, 50-100mm (2"-4") lengths – the straw you trimmed off the walls will do. Add the key coat mix with a very small amount of water. Mix it all up like a giant salad, making sure that each straw is completely coated with lime. Add more straw then lime till you have a large quantity. Use this mix to fill up any holes, hollows and indentations, rubbing the edges well in to the wet key coat around them, using pressure to make a good bond between the lime/straw and the wall behind.

Reveals

Around windows and doors, use this mix to sculpt the shape you want around the curve of the reveal, which also firms up this section. Start at the bottom and work your way up, creating a beautiful shape. Add jute mesh into the wet plaster to give it even more strength, particularly at the corners above and below openings, where the most stress will occur.

The key coat should dry/carbonate quite quickly on straw, because it has access to air within the straw as well as in the surrounding atmosphere. This usually takes 24-48 hours.

Around windows and doors, work from the bottom to create the right shape. Jute mesh (shown here) can add strength. Photograph © Barbara Jones.

Body coat / second coat

This second coat of plaster contains small chopped fibres, such as straw or hemp, well distributed throughout the mix to give it much greater strength – in the same way that straw is used in mud plaster to give it tensile strength. Usually the same volume of chopped fibre is used as that of lime putty. This is not like the lime/straw mix above. The fibres are a lot shorter and the mix is a body-coat mix, not a key coat.

The second coats contains chopped hemp fibres, well distribted throughout the mix, to give it greater strength. Photographs © Rae Parkinson.

This coat can be applied by hand or with tools and/or a sprayer. It is not a good idea to use a steel float on a lime plaster, as this polishes up the surface and closes up the texture, thus preventing humid air from penetrating into the body of the plaster. If you are skilled with this tool you may be able to put the mix on the wall quickly so this won’t be a problem, and then finish off with a wooden float that keeps the surface open, but most amateurs should avoid this. Instead, either rub small quantities of the mix into an area of the wall, taking care to check that the depth remains about 25-30mm (1"-1³/₁₆") (by sticking your finger into it from time to time) or throw small amounts on (very satisfying!). Alternatively, you can put the mix on using a wooden or plastic float.

Here you can see the difference between the first and second coat. Photograph © Rae Parkinson.

Before you begin, lightly spray the key-coated wall with a mist of water. Once you’ve covered a largish area with plaster – at least 1m² (10.8 ft²) then go over the surface with a long, straight edge – often called a ‘darby’ – to check that you have a flat surface. Add extra mix to any hollows and scrape off any high points. It isn’t generally important to make the walls totally flat and plumb – most people prefer a slightly organic look – but you can do if you want.

The body coat is usually about 25-30mm (1"-1³/₁₆") thick, depending on how undulating the walls are. If it’s an exterior wall then you might want to do this in two coats of about 12-15mm (½"-⁹/₁₆") each to make sure you reach a good thickness.

If you are not experienced at plastering, it is best to use your hands or a wooden or plasic float, not a metal one. photograph © Barbara Jones

Don’t try and get a good surface as soon as you put the lime on the wall, just get it flat. It will be far too wet at this stage to do anything else with it. You’ll need to come back to it later, usually the following day, sometimes after a longer period, when you can rub it up with a wooden or plastic float. The time varies according to weather conditions, the quality of the mix, etc, so a bit of experience here will help! Rubbing up is done to close up the surface, compress the sand together tightly, and squeeze out any air that has come into the mix from an excess of water as this has evaporated. It’s also a surface finish if you are not applying a top coat; but if you are applying one then the surface will need to be left rough, or scratched ready to receive it.

Here a section has been scratched, ready
to receive the finish coat. © Rae Parkinson.

It’s important to encourage the plaster to cure (go off) from the straw out, not to let the outside skin carbonate too fast: this is done by keeping the whole thing MOIST (not wet). The surface should not be allowed to dry out; it will naturally take 2 to 7 days before the plaster feels hard. For the body coat as well as the top coat, keep the plaster moist by misting it (spraying it with water) unless you have ideal drizzling weather! It is often not necessary to do much misting inside the building, as the wind and sun don’t dry the internal air in the same way as they do outside. Keep going over the wall with a wooden float, rubbing in any cracks and misting it further. It is probable that lime plasters on straw-bale walls will carbonate to a greater depth than on stonework, because the straw itself is breathable, and so the back as well as the surface of the plaster has access to the air. Usually lime coats are limited to 10-12mm (³/₈"-½"), as on most surfaces a greater depth would not carbonate fully, but this can be increased on straw.

Protect the plaster from the elements using plastic, sacking or tarpaulins. Photographs © Rae Parkinson.

Finish / top coat

It’s not always necessary to apply a third coat. It depends on how well you finish the body coat, the size of sand you used, what sort of decorative effect you want and what the weather conditions are. Some people choose to use a premixed 3 - 4mm (¹/₈-³/₁₆") layer of weak hydraulic lime / fine sand to give a smooth, flat surface. Others may go for a harled (thrown on) finish of largish stone, about 4-6mm (³/₁₆-¼") in the last coat, which gives better weather protection. Always lightly spray the wall before applying the next coat. A rule of thumb, literally, is to apply the next coat when the first is hard enough that you cannot push your thumbnail into it.

Aftercare for lime plasters

Over the next few days, protect the plaster from direct sunlight, driving rain, forceful wind and frost. Often this can be done by hanging sacking from scaffolding, and keeping it moist to create a humid atmosphere close to the lime. The plaster may crack, particularly if you put too much water into it, or the sand used was not well graded. If it does it needs to be reworked several times over the next few days to squeeze and compress the sand particles together, before the surface hardens. The cracks are caused by shrinkage as the excess water in the mix evaporates. The aim is to compress all the plaster so that there are no air spaces left. The misting is not to add water to the plaster, but to make sure that carbon dioxide can be carried into the thickness of the layer via the medium of water. Lime plasters take in carbon dioxide as they carbonate back to calcium carbonate, but they can only do this if the CO₂ is dissolved in water. They need to be protected from frost for about three months, so exterior lime plasters should only be applied between the end of April and mid September, unless the local microclimate is frost-free for longer. However, the sacking can be taken down after about one month.

Internal plasters can be applied at any time of year, as frost is unlikely to attack them inside. However, no plaster will carbonate if the temperature drops below 5ºC (41ºC), and although this will not damage the plaster, it will mean that it takes longer than the usual three months to carbonate. Internal walls, plastered in winter in an unheated house can still be soft to the touch three months later. This then poses the question, would it have been more sensible to wait until spring?

What happens to lime plaster when exposed to air

It’s essential to understand the chemical change that starts to take place once the plaster is exposed to air, so as to know how best to care for it. Once it’s on a wall, it begins to carbonate: a chemical process begins whereby the carbon dioxide starts to change the calcium hydroxide back into the original limestone (calcium carbonate).

Ca(OH)₂ + CO₂ = CaCO₃ + H₂O

calcium hydroxide + carbon dioxide = calcium carbonate + water

This change happens very slowly; some of our plasters are hundreds of years old, and even now, not all the calcium hydroxide in them is carbonated. Also, the change can only happen in the presence of water, the carbon dioxide must be in solution (dissolved in water); as some of it always is in a humid atmosphere, but too much water inhibits the process. It can take a pure lime putty / sand mix several days to harden, which does not mean that all the lime is carbonated: this process will continue for months or years afterwards. The ideal conditions for a lime plaster are high humidity and good ventilation.

Weather protection and decoration

How limes work to keep the moisture out

Lime plasters are extremely good at protecting buildings from wet weather. They absorb water when it’s raining, and this absorbtion in effect closes up the plaster so that it cannot absorb more, thus providing a weatherproof seal against rain getting through. Isn’t that wonderful? Limewashes on top only serve to increase this natural process. This is in direct contrast to cements, which allow water (but not vapour) to pass through them and therefore are not as good at weather protection. As well as being highly absorbent, limes are breathable, so that they allow the passage of vapour from one side to another, depending on weather and living conditions. This is essential for the long-term durability of straw walls – they must be able to manage moisture by allowing vapour to pass through them and then release it. Limes are at least 10 times more permeable than cement. Finally, limes are flexible, and if they do crack at all, they tend to form a network of small cracks rather than one big one. These cracks will self-heal over time as the lime carbonates, especially if regular maintenance is carried out with lime wash.

Limewash

Limewash is made from lime putty, diluted with water. The best washes are made from very mature lime. It is very thin and watery, and should be applied in many thin coats, left to dry overnight each time, but it’s much quicker to apply than ordinary paint because of this wateriness. Putting it on too thick can make the surface powdery. Applying limewash to the building once it’s been plastered should be seen as part of the plastering process. If there are any tiny cracks left in the finished plaster, the limewash will seal these up. Over time, lime plasters have a self-healing effect. Any cracks that do appear tend to close up as the lime carbonates, because the calcium carbonate molecule is bigger than the calcium hydroxide one. Externally, walls that take a lot of weather, usually the south-west side, should have about five coats of limewash to protect them. The rest of the building may need only three, although the more coats you apply initially the better the weatherproofing will be. How frequently the walls need re-coating will depend on the weather. The sheltered side may need limewashing again only every five years, whereas other parts may need to be done more often. In England there was a tradition to limewash houses on May Day each year. Natural pigments can be added to the limewash, giving a large range of beautiful colours with a visual quality that is quite different from modern vinyl paints. Limewashes are more variegated; less stark and uniform. Lovely!

Limewash, made from lime putty and water, protects the plaster/render and can have pigments added to give a range of beautiful colours. Depending on how exposed the walls are, limewashing will need to be done every one to five years. Photograph left © Barbara Jones, right © Sue Nicol.

Active silicate paints

Water-glass (aka sodium silicate or soluble glass) based natural mineral paints bond with the silica (sand) in the plaster and are an alternative to the limewashes. It’s very important to use an active silicate paint, and to make sure that it does not contain any synthetics (good examples are Beeck and some of the Keim products), otherwise the paint will not be vapour-permeable. These paints differ from the limewashes in that the colours are uniform, not variegated, but they also provide much better weather protection and durability and are recommended for exposed buildings, or those where regular maintenance may be a problem, as they do not need to be reapplied for about 15 years, unless you get fed up with the colour!

Maintenance of lime plasters

As long as a lime plaster is regularly limewashed, or has had mineral paint applied, there should be no other need for maintenance, unless something else is causing problems, eg a broken gutter or an overgrown garden. Otherwise it behaves much like straw; let it breathe, and it will take care of itself. Lime plasters should be limewashed every two or three years at first, and maybe once a year on the most exposed wall. However, as time goes on, the need for this is reduced as the plaster becomes stronger – more like the limestone that it came from – and limewashing becomes a surface decoration rather than a protection.

If you do need to patch a section of plaster, take out any loose material, dampen down the surface of the hole, and apply fresh plaster in thin layers no more than 12mm (½") thick, working well into the sides. The patch will adhere well to the rest of the plaster because it’s all the same stuff. Limewash over the top when done. Old plaster can be pounded up and used as well as / instead of sand for aggregate in the new mix.

Limecrete: Making floors with lime

Limecrete is the name given to solid floor slabs, or bond beams made using lime as a binder rather than cement. It has been used for thousands of years but the knowledge of its use has been almost lost due to the dominance of cement and concrete in the construction industry.

However, it has been brought back into use by companies such as Ty Mawr in Wales (see www.lime.org.uk) and adapted to changing demands by adding particular aggregates to the mix to provide solid floors that are also insulated. Appendix 7 gives a method for making a DIY limecrete floor. Because sands have different properties, it is not recommended that you make a limecrete floor on a large scale without expert advice. It is usually laid 100-150mm (4-6") deep, but with the use of an insulating and load-bearing hardcore underneath (Recycled Foamglas™) this can be reduced to 65-75mm (2⁹/₁₆-3"). Thinner layers (50-65mm / 2-2⁹/₁₆") called screeds can be applied using finer aggregates for different surface finishes, or to contain underfloor heating pipes. Advantages are a lower environmental footprint, no need for expansion joints, and a breathable floor. Costs are comparable with cement floors, if the cost of insulation is also taken into account.

Cement plasters

There are hundreds of straw-bale buildings in the USA and Canada that have been cement plastered. Most of these are doing fine and are not showing any sign of deterioration. Some of them have become very damp as a consequence of using a cement plaster (see Chapter 7).

Cement and lime behave very differently, and are used for different reasons. Lime is a breathable material that holds water within itself while it is raining, and releases it once the rain stops. Moreover, the saturation of water in the lime prevents any further water from passing through. Cement is porous and sucks water into itself unless it has further chemicals added to it to make it waterproof. Therefore lime protects straw better than cement does. Also, lime is very flexible, whereas cement is not. This means that due to its rigidity, it is almost impossible for it not to crack after a short period of time. This problem is made worse when it is applied to a flexible backing material such as straw, so that when it rains, the rain passes through tiny cracks, filters down the inside face of the cement and collects at the bottom of the wall, where it builds up very slowly. A build-up of trapped moisture at the base of the wall over a long period of time causes the rot to set in. This has now been identified as a major cause of damp in old buildings, such that English Heritage has banned the use of cement on older lime-built buildings.

Cement plasters have the same effect as wrapping the straw in plastic, because they are not breathable. This lack of breathability creates an unhealthy, unventilated atmosphere around the straw, which could lead to damp problems in the long term. Typical permeances of various plasters are show in the table below.

In practice, there may be many instances where you can get away with using cement, or at least cements gauged with lime, or where the life of the building is such that a bit of rot developing at the base of the wall does not matter. There is no doubt, however, that in terms of best practice, lime plasters should be used, and not cement.

Type of layer and thickness | Permeance in US units | Permeance in metric units
6 mil poly vapour retarder .006" | <1 US perm | (<60 metric)
1:3 cement-sand stucco) 1.5" | 1 US perm | (60 metric)
5:1:15 cement-lime-sand stucco 1.5" | 4 US perms | (200 metric)
1:1:6 cement-lime-sand stucco 1.5" | 7 US perms | (400 metric)
1:2:9 cement-lime-sand stucco 1.5" | 9 US perms | (500 metric)
1:2 lime-sand plaster 2.0" | 9 US perms | (500 metric)
Earthen (clay) plaster 1.5" | 20 US perms | (1100 metric)

A typical 450mm (18") straw bale has a vapour permeance of approximately 100–200 metric perms (2–4 US perms). Evidence suggests that the total cumulative permeance of the plaster on either side of a straw-bale wall should not be lower than 290 metric perms (5 US perms) to ensure that the walls can dry effectively. Source: The Straw-bale Alternative Solutions Resource.

CHAPTER 16

Clay and clay plasters

Clay is a natural material used traditionally around the world for plastering and building. Like lime, it is ideal for use with straw-bale walls, as it is vapour-permeable and flexible, and excellent at regulating humidity. It is also a great material for decorating with, enabling you to create beautiful shapes and colours on your walls. Because of its natural abundance, you may even be able to mix your own clay plaster from the soil you dig up in the building process – a truly local and sustainable material!

Photograph © Rae Parkinson.

Although these types of plaster are very common in parts of Continental Europe, Scandinavia, the USA, the Middle East and Africa, they are not so well known in the UK and Ireland. Knowledge of their use was almost lost, although we do still have many fine examples of older buildings with a clay mortar binding the bricks or stones together. And, of course, the UK’s rich heritage of cob buildings, built entirely of clay subsoil, are testament to the durability of clay as a building material. In the last 30 or so years, the use of clay for cob building and plasters has seen a revival of interest as people have come to appreciate its quality and advantages, in much the same way as they have come to appreciate straw, and the two materials work very well in combination.

In south-western USA, clay is part of a very old tradition of house building and plastering. It is made into clay blocks, or adobes, and used as unfired bricks which are then plastered. Athena and Bill Steen have popularized clay plastering more than most with their beautiful creations at the Canelo Project, in Canelo, Arizona, and many, many others are doing the same.

In recent years, European countries have been working in partnership to understand and reclaim the knowledge of working with clay, which is still being used in construction in some parts of Europe, particularly the old Eastern bloc countries. While the majority of houses in the world are built with clay in some form or another, in Europe we have almost forgotten about this tactile, beautiful, natural material. Germany has been the lead partner in two Leonardo projects (see www.lernpunktlehm.de for the UK version of these vocational courses), the first to write a course in clay plastering and to train teachers, and the second to agree accreditation for this course across Europe. This work, together with that being done by others in Europe who are using this great material for building, helps to bring the knowledge of working with clay back to ordinary people and those in the construction industry.

Using clay plasters

Why?

Clay is absolutely the best material to regulate humidity in a building; it’s even better than lime. It absorbs excess humidity from the atmosphere, but does not transfer it to whatever’s behind it (eg straw). It holds this humidity within the clay platelets, which expand and make a partially watertight layer that resists further absorption, and releases it as soon as humidity levels drop again, keeping it to pretty much the optimum for human health. So it is perfect for use in a shower room, kitchen or bathroom where it will prevent condensation. It actually works to keep the straw behind it dry, and so could be called a preservative for straw (and wood). Whatever the humidity levels of the surrounding atmosphere, clay will always have a lower moisture content than straw. Minke and Molke have shown that dry clay, as long as it does not have direct water contact, will not move above 7 per cent moisture content even if the surrounding atmosphere contains 100 per cent humidity (quoted in The Straw-bale Alternative Solutions Resource, see Appendix 3). Clay is vapour-permeable, it allows vapour (gases, not water) to pass through it, thus providing a breathable layer on top of other such materials as straw. Inhabited spaces need this vapour permeability for health.

Clay is beautiful. It comes in a range of natural colours, and can be painted with clay paints. It’s tactile and can be used to create curves and textures that will enhance any home or work space.

Clay plaster can be affordable, if you have the time to make your own from raw materials.

And where?

Clay can be used pretty much anywhere, except where it will be in contact with actual water, for example in a shower cubicle or outside. While clay absorbs moisture, a very useful and special quality, in the presence of too much water it will become liquid itself. So it’s great in a shower room on the parts of the wall that do not get direct water contact, or outside under a veranda.

Weatherproofing clay plasters

Clay plaster should never be waterproofed – this stops its natural vapour permeability – but it can be made to withstand some amount of rain without erosion. There are many practices from the past and from other countries that have been used, such as adding 1 per cent linseed oil to the final coat of limewash or clay finish to give it a bit of extra durability. In Arizona they add small amounts of cactus juice to it for the same reason. Or simply limewashing it gives some protection. Internally, mixes using wheat paste or casein from milk are popular. Clay can also be gauged with lime (adding 5-10 per cent lime by weight) to give some extra durability to outside plasters or internal ones that need to be harder and more durable.

So what is clay?

Clay is the finest particle of earth, created by different types of erosion and ultimately made from the weathering of the mountains. When rocks are subjected to wind, rain, ice and heat, they will erode, both in a physical sense and through chemical erosion. Big rocks are broken up into smaller pieces, and in descending order of size we know them as rocks, gravel, sand, silt and clay. Clay and silt can often be difficult to distinguish but there is one very big difference between them, which is that clay particles are extremely cohesive, whereas silt particles are not.

Clay comes in a range of colours and consistencies, and can have very different properties, which is why you will need to test your clays. Photograph © Rae Parkinson.

We tend to be a bit sloppy in our language when talking about clay, and it can be even more confusing when you read books about earth plasters, using dirt for plastering, etc. The fact is that in many parts of the world there is almost no topsoil, and you can basically dig away at the earth, add straw and water to it and make it into a great plaster or use it to build walls with, hence the terms ‘earth plaster’, ‘earth walls’ or ‘dirt plaster’. This is not the case in most of Europe, however, because even in places where we can take away the topsoil and just use what’s underneath, it’s almost always far too wet and sticky to use without some sort of processing. It’s always important to take away the topsoil first. This is the growing medium that contains organic matter, such as plant roots and worms, and can’t be used to make a plaster. If you dig a hole, underneath the topsoil you should find a layer of undisturbed ground, and this is the subsoil. If it’s clay-rich, then you can certainly use it for construction, both for house building and for renders and plasters, but it will probably need some work done to it first to make it useable. In the UK, pretty much any type of clay that you find can be used for plastering, usually with the addition of some sand and fibre. It’s common to find it in riverbanks in the slow parts of the stream, and in areas that thousands of years ago were estuaries, because, as a very small particle, clay takes a long time to settle out of water, so needs almost still water to do this.

As clay is made from local rocks, weathered by rain, as well as being carried long distances by water, it’s quite possible to find a pocket of clay in one area that is completely different from another pocket only half a mile away. Depending on the geology of your local area, you may find a clay subsoil that is ideal for plastering, or pockets of clay that can be added to sand to make a good mix. Clays have different properties because of the rocks they are made from. China became famous for its little yellow clay pipes, made from ‘China clay’, a type of clay that can be fired at high temperatures and makes very fine scientific instruments. This is also the source of the clay in the old-fashioned digestive remedy kaolin and morphine; it comes from the area in China near Mount Kao-ling.

In everyday life, we are also familiar with another clay, but one that would definitely not be good for plastering – Fuller’s earth, better known as natural cat litter. This is a clay that expands dramatically on contact with liquid – not something we would want our plaster to do! A third type of clay becomes totally impervious to water once it’s mixed. We call this puddling clay, and it’s what the canals of the industrial revolution were lined with, and many ponds. It’s also used for roofs in parts of Spain, but imagine what might happen if you used the wrong type!!! Understanding the nature of clay is a science all of its own, as clays are made up of many different substances. For our purposes, it is enough to say that all home-made clay plasters should be tested before use to work out exactly what mix of subsoil, sand and straw should be used for an optimum mix.

Testing clays

If you’re pretty sure you’ve got a clay subsoil, you still need to test it to decide on the optimum mix for your plaster. Clay subsoil is usually a mix of clay, sand, silt and aggregate but you will need to know what proportions they are, and you will need to add fibre (usually straw) to it. To test your subsoil, make up several mixes using exact proportions of subsoil, well-graded sharp sand (sand that includes a good range of sizes) and finely chopped straw, with enough water to make a workable plaster. Usually, you will need the same volume of straw as clay. Vary the amounts of sand and fibre according to the table below, marking up the mixes as shown in the boxes with an X. Then make some test panels on a few spare bales, leave them to thoroughly dry, plastering them in panels about 10mm (just under ½") thick and 300mm (12") square so that you can see the differences between them.

Fibre helps the mix to bind together, giving it tensile strength. Sand makes it strong and durable. Too much clay (glue) without fibre will make the plaster crack and drop off; too much sand will make it crumbly and dusty. In general, a plaster or render needs about 20-50 per cent clay but different clays will have very different properties, which is why it needs to be tested. Decide which is your optimum mix from your test panels.

Clay | 1 clay, ¹/₃ fibre | 1 clay, ²/₃ fibre | 1 clay, 1 fibre
2 sand | X |  | X
3 sand |  | X | X
4 sand |  |  | X

Mix up different proportions of clay subsoil, sand, straw and water, and create test panels to work out the optimum mix. Photographs © Rae Parkinson.

Making your own clay plasters

To be usable, clay or clay subsoil needs to be either very wet or very dry. In Europe it’s almost always somewhere in the middle – which means that making your own plasters, renders and cob is going to be hard and time-consuming if you just dig the clay up from your back garden. On the other hand, this is tremendously rewarding, and there are some ways you can make it a bit easier. Think about all those lovely villages in Devon, all with their duck ponds – the original source of the material for their houses. In this area, the geology is such that the subsoil is perfect for building when certain other ingredients are added. People would dig what they needed and dump it in the farmyard, then let the cows in to mix it (and add manure), add straw and water, and then build. Below are some ways you can mix your own clay subsoil to make a plaster.

From wet clay

First find your source of clay, test it as described above, and dig it up either by hand or with a tractor. Then start mixing it with water and straw. The following are some suggestions for how to do this.

alt Make a paddling pool by using a strong tarpaulin draped inside a circle of bales. Place your clay and sand (according to the mix you already tested) inside this and trample the whole lot with your bare feet or wearing wellies, adding water and straw as necessary once the rest has mixed thoroughly. By grabbing the sides of the tarpaulin you can turn the mix over. This is a lot of fun if done in a group, but takes a long time and can be tiring.

alt Use a paddle mill or roller pan. A local potter may have one of these you can borrow. It’s an electric or diesel round pan with heavy wheels inside that turn and squash the clay mix; as the pan is turning the clay is scraped off the bottom and sides, and dumped back into the mix to be squashed again.

alt Use a tractor to drive continuously over the mix, scooping up the stuff and turning it from time to time, adding water and straw as necessary.

alt Use other clay plaster-making machines. They have really brilliant ones in Germany and other European countries, and we’ve found a great home-made one in the UK.

alt Or, make your clay thoroughly wet first, like a slurry, by putting it into a bath of water and whizzing it up from time to time with a hand-held mixer. Once it’s become like lovely thick cream, you can mix it with sand and fibre in a big tub using the same plaster mixer or a mixing machine.

Making clay plaster from subsoil is very labour intensive but can be fun and rewarding too. Photograph © Sarah Leather.

From dry clay

You can usually get free or relatively cheap milled clay from a brickworks, if you collect it yourself. This is dry clay that has been processed ready to make bricks or clay pipes, dug from the ground, dried and ground up into small pieces or sometimes powder. It’s called clay but it often has some sand or grit in it as well, so will always need testing. Other sources are from mineral companies that produce dry powdered clay in large quantities for industrial processes. These fine clays make great finish plasters. In this dry form it can be mixed with sand and fibre in a cement mixer, although not very well, but another way to mix up your plaster is in a large tub using a hand-held plaster mixer (both dry and claggy clay can be done this way). ¾ tonne of milled clay will cover about 100m² (1,100ft²) when made into a 30mm (1¼") thick plaster by adding sand and fibre.

From a manufacturer

There are an increasing number of sources of ready-made clay products available throughout Europe and the USA. Clayworks have made a name for themselves in the UK as specialist suppliers, and there are many general ‘eco-merchants’ that stock products from all over the world. You can buy ready-mixed key coat, body coat and finish coat, or pure clay in lumps to mix yourself. There is also a vast range of dry products that just require mixing with water. When you buy wet products, you will need to tip them out into a bigger tub and mix with a hand-held mixer before you use them, as the water in them tends to separate out.

Always mix your clay and clay plaster at least 24 hours before use. One of clay’s tremendous properties is that it attracts water to itself, and this makes it lovely and sticky, due to the chemical attraction created between the platelets of clay. But this needs you to mix it up, and to give it time to happen. Plasters that are not mixed and stood for long enough will not be as easy or satisfying to use as those that are.

How to use clay plaster on straw

Clay is often used for inside and internal walls, but would not generally be used on the outside as a finish except in dry climates and sheltered positions, or where you have a veranda, because rain will erode it fairly quickly.

Preparation

Exactly as with lime plasters, you need to:

alt Make sure your walls are straight, trimmed, and any holes have been tightly stuffed with dry straw. (Stuffing holes with a clay/straw mix reduces the insulation of the walls.)

alt Use old tarpaulins or clean boards underneath where you are going to work. These will catch any clay that falls off the walls, and you can add it back into the mix.

alt Make sure you have safety glasses and gloves for protection.

As with lime plasters, don’t wrap the straw in stucco or chicken wire first, or wet the straw before applying the key coat (see page 164).

Then make your plaster, or mix it up if you’ve bought some, and leave it at least overnight before using.

Key coat / first coat / revealing coat

This is a slurry made from pure (or almost pure) clay, sometimes with a mix of sand to clay in the ratio of 1:1. Mix up the clay at least 24 hours before you want to use it to make sure it has absorbed all the water it needs to and is really nice and sticky. Dry clay can just be put in a tub with water and mixed using a hand-held plaster mixer; claggy clay should be broken up into small lumps and may need to be soaked for a day or so before it can be mixed. Often it will need mixing a bit, leaving to soak, then mixing again before it is all a uniform thickness. This clay slurry should have the consistency of thick cream. (Lovely on your hands!) Apply it by hand (with or without gloves; it’s not toxic but the straw can be harsh on your hands) direct to the straw, rubbing it well in so that all the straw is covered. It’s rather like massaging the straw. You’re not trying to hide the straw with this coat, only to cover it well and provide a good key – something for the next coat to hang on to. It’s more fun to do it by hand, and the straw tends to flick the stuff back at you if you try to use tools. This coat can also be sprayed on, although this can be very messy. In this case, you will still need to follow on behind the sprayer and rub it well in. Getting this coat right is time-consuming but important.

The aim of the key coat is not to cover the straw and create a smooth finish, but to provide a good key for the next coat. Applying the key coat can be time-consuming, and may require a few rests! Photograph © Rae Parkinson.

Clay/straw mix

While you’re doing the key coat, the action of rubbing in the mix reveals any holes, hollows or indentations in the wall that couldn’t be seen when the straw was still fluffy. These should be filled at the same time with dry straw if at all possible, and then with a clay/straw mix. Prepare this mix using a wide, shallow tub. Place some key coat in the tub followed by chopped straw, 50-100mm (2-4") lengths – the straw you trimmed off the walls will do. Add more key coat mix with a very small amount of water. Mix it all up like a giant salad, making sure that each straw is completely coated with clay and the whole thing is very sticky. Add more straw then clay till you have a large quantity. Use this mix to fill up any holes, hollows and indentations, rubbing the edges well in to the wet key coat around them, using pressure to make a good bond between the clay/straw and the wall behind.

Mix straw and clay in a wide, shallow tub and use the mixture to fill any holes revealed when doing the key coat. Photograph © Rae Parkinson.

Reveals

Around windows and doors, use this mix to sculpt the shape you want around the curve of the reveal, which also firms up this section. Start at the bottom and work your way up, creating a beautiful shape. Add jute mesh into the wet plaster to give it even more strength, particularly at the corners above and below openings, where most stresses will occur.

The key coat should dry quite quickly on straw and usually takes 24-48 hours.

Clay plaster works by gripping on to the backing and then drying, not by making a chemical change (like lime). So the surface behind it must be rough, or must be more clay but sticky (wet) so that the plaster melds with the key coat clay.

Body coat / second coat

This can be applied before the first coat has dried. Or, if it has already dried, wet it down so that it’s a bit sticky again before adding the second coat, but don’t wash it off! This is the coat you made your tests for, so mix up lots of it and call all your friends in. The mix should have lots of finely chopped straw in it to give it tensile strength and stop it cracking. You can use a fairly gritty sand, with 4mm (¹/₆") or even 6mm (¼") maximum sized grains as long as it is sharp and well graded (ie has a good range of sizes in it – see Chapter 15). Use this coat to even out any undulations: you can apply it quite thickly (25-30mm / 1-1¼") and if you add lots of straw, you can put on a greater thickness – but remember that the thicker it is the longer it will take to dry. Aim for a flat surface. Everything can be done by hand, but using tools will give a flatter finish. By hand, rub small quantities into the key coat, making sure each handful is well attached to the surface behind. Alternatively, throw it on in small quantities and then flatten out the surface from time to time. If you want to or are used to using tools, it’s best not to use a steel trowel too much, as it can draw the clay out of the mix and leave the surface dusty. A wooden or plastic float is better – use it to rub the plaster in well.

How to make a flat or flatter surface

alt Use a darby. This is a long, straight steel tool that you can swing over the surface of your body coat. It will catch on the high points (use the darby to scrape them off) and not touch the low points (add more plaster).

alt Fix horizontal timbers top and bottom to the ring beam / base plate the same thickness as the plaster you want. Apply your plaster, then use a straight timber that is long enough to reach between the two horizontal timbers, and run it along them just like a darby, scraping off anything that’s too thick, and adding more plaster where it’s needed.

alt Ask a professional plasterer to work with you, following on behind what you’ve done, as it does take skill and years of practice to be able to make a flat wall quickly.

Apply the second coat before the first has dried (or wet the first coat if it has already dried). This can be done by hand or with non-metal tools: aim for a flat finish. Photograph © Rae Parkinson.

How to make regular curves

It’s always lovely to have curves at the sides of windows and doors. Curves are better for the plaster: it’s less likely to crack than if you include right angles. And if you splay the inside faces of the reveals, it lets more light in as well as being more pleasing. It can look great if each reveal is a bit different from all the others too, but sometimes you’ll want to aim for all the reveals to match. This is quite easy to do!

alt One way is to build up the reveal into a curve you like using a long straw / clay mix, starting at the bottom so the upper mix has something to sit on. Use jute mesh in the wet plaster to help reinforce it. Then, after it’s dried a little (but is still wet), use an old plastic bag, hold it around the curve and wriggle it from side to side up and down. This helps to make it the same all the way up.

alt Another method is to make the shape you want out of a piece of plywood, apply the plaster as above, then use the wooden shape to work up and down the reveal, taking off high points and adding more plaster to the low points.

If you’re happy with an organic look, just do the best you can. Clay won’t dry before you’ve had time to really play with it, move it around, and flatten it out. Generally, it isn’t important to make the walls totally flat and plumb – most people prefer a slightly organic look.

THe second coat is usually about 30-40mm (1₃/₁₆-1⁹/₁₆") thick depending on how undulating the walls are. Don’t try and get a good surface as soon as you put the clay on the wall, just get it as flat as you want. It will be far too wet at this stage to do anything else with it. You’ll need to come back to it later – sometimes the following day, sometimes after a longer period – when you can rub it up with a wooden or plastic float. The time varies according to how warm it is, the quality of the mix, etc, so a bit of experience here will help! Rubbing up is done to close up the surface, compress the sand together tightly and squeeze out any cracks. This can also be a surface finish if you are not applying a top coat; but if you are applying one then the surface will need to be left rough, or scratched ready to receive it.

Clay plaster, drying and mould

It will take a long time for the plaster to dry out. If the plaster is thick and the air is cold and still, it will take longer. In the UK in winter it may not fully dry out for three months or more. In summer with the windows open (assuming it’s not one of those summers where it rains all the time!) it could be dry in four weeks. It is normal for the plaster to grow some mould while drying – don’t panic! This is just an indication of the moisture content of the plaster, and the fact that it contains some organic material. As the plaster dries, the moisture content drops and the mould dies, never to return. It often looks like a white fuzz or bloom on the plaster. You can brush this off, or just wait for it to die off and disappear. You might also get some green shoots appearing from some of the seed heads in the straw fibre. Again, these will die off and are nothing to worry about.

You’ll know when the plaster dries, because its colour changes. It will be hard to the touch, and become lighter coloured, in patches at first and then all over. You should let this coat dry out completely before adding a top coat.

Finish coat (top coat)

It’s not always necessary to apply a final coat. If you’re happy with the finish on your body coat, you can simply rub it up with a sponge the next day, or smooth it off with a wooden float, or rub it up with a small round stone. There are lots of different finishes and textures you can add to your plaster. Hopefully you’ll be inspired to read and practise more on your own. It’s really beneficial and a lot of fun to go on a course and play and practise with everything that’s possible before you make your final decisions about your own plaster. Top coats generally use a fine sand (2mm or less) to make them smooth, have no fibre, and can contain other substances to make them more attractive, eg natural colours or shiny bits of mica. They are generally 4-5mm (³/₁₆") thick

Decoration

You can choose from a range of clay pigments to make coloured plasters that won’t need further decoration, but if you do want to paint the plaster then limewashes (see Chapter 15) are perfect, giving a variegated hue and plenty of natural colours. There are also clay paints, and other breathable paints in many colours. It’s important to use vapour-permeable paints and finishes to keep the plaster (and straw behind) healthy. To protect outside surfaces from the weather, you may want to use several coats of limewash, or add linseed oil into the final coat (see page 175). Using texture is a great way to decorate clay plasters with or without adding a top coat. And graffito has been used for thousands of years – using a different colour of clay underneath the top coat, and scraping away parts of the top coat to reveal it in beautiful patterns. Once you get hooked on clay you’ll discover all sorts of interesting facts about it.

Clay is very versatile and lends itself to many different decorative finishes, including sgrafitto, imprinting, and molding/sculputing. Photographs © Rae Parkinson.

Professional use of clay plasters

Clay is being specified as a plaster in many high-class designs all over the world, as well as being used by self-builders, and continues to grow in popularity. Many professional plasterers are adding clay work to their list of skills and becoming familiar with its properties; building colleges are offering clay plastering as an option. We are seeing a more widespread take-up of clay and clay products throughout the construction industry as more people become aware of the need to use sustainable materials and learn exactly what this means.

Clay plasters work brilliantly well, people feel happy working with them because they are truly natural, and they make sense as a material. You can see where you’ve been; there’s no time pressure, as they dry quite slowly; you can play around with them, make different textures, shapes and colours; there’s no waste, as anything that is dropped or dries out can just be remixed with water; and they’re not harmful so you can work without protective clothing, as long as you don’t mind being spattered with clay!

We use them because they are natural, non-toxic, beautiful, affordable, come from the earth and go back to the earth. They regulate humidity in the air at pretty much the optimum level for human health (unless you’re stuck in an airtight room with a drying source of heat, eg radiators!). They prevent condensation, deal very well with humid conditions, and help generate that cosy, genial ambience that is associated with straw-bale houses.

Clay plasters and paints create a beautiful, warm finish inside and out. Photograph left © Rae Parkinson, right © The Wisharts.

clay pigments can be added to the top coat, or use clay paints, limewashes or other natural, breathable paints – there are many available in a huge range of colours.

CHAPTER 17

Safety and tools

Building with straw bales requires almost no specialist equipment, although a number of good basic tools are essential. Following sensible safety guidelines when building, and wearing protective clothing, will help ensure everyone remains safe. In general, there will be fewer accidents and mistakes on site if everybody is happy and well motivated. It is good practice to encourage the whole team to be aware of each other, of each other’s roles, and of how each separate action is part of the whole achievement.

Photograph © Rae Parkinson

It can be very useful for everyone to meet together at the start of each day, to share thoughts or reflections on the work to be done or the manner of its doing, and discuss possible problems or areas that will need cooperation or greater care. If we can recognize our interdependence with each other rather than be in competition, this will help to engender a caring and considerate workforce, where each individual is responsible for their own self but also has regard for others’ welfare. This is especially necessary on a self-build site where people have different levels of skill or knowledge.

Straw-bale building sites have become renowned for their ethos of working together, sharing knowledge and skills, equality for women, enjoyable learning and fun. But there is no reason why these qualities cannot be encouraged on all building sites.

Safety on site

The following basic guidelines will help to ensure that nobody has an accident.

alt Every building site, even if it’s your own home, should have a first-aid kit available and everyone should know where it is!

alt There should be no smoking anywhere on site or around stored straw.

alt Always keep the site tidy. Scaffolding and working areas should be cleared and tidied each evening before finishing. It is helpful to store materials and rubbish, etc, in specific places to keep a sense of order. Since loose straw is a fire hazard, it is essential to clear loose straw from the site daily.

alt Have a central place for tools so that any that are not being used can be put back, and can be found when needed.

alt Extra care should be taken when using heat or open flames, eg when connecting plumbing pipes together.

alt Never leave tools lying around. Each person should be responsible for the tools that they are using, should know where they are, and should put them away when not in use.

alt Sharp tools should be sheathed or put away when not in use.

alt All electrical tools should be unplugged when not in use.

alt Do not leave electrical leads trailing across the site.

alt All ladders must be securely tied.

alt Never move a ladder temporarily and then leave it unattended.

alt Stepladders and scaffolding towers, etc, should always have a firm footing.

alt Hard hats should always be worn when anyone is working overhead.

Clothing and safety equipment

Always buy good-quality equipment, check it regularly for wear and tear and replace when necessary. If you’re small it can be hard to find equipment that fits, but persevere, as it is essential kit. Try to find stuff that makes you feel good when wearing it, even if it costs a bit more, as you will use it more often. Here are the most essential items you’ll need;

alt When working with straw it is advisable to wear a long-sleeved shirt and long trousers because straw stalks can be quite prickly.

alt Gloves are also a must, at least for some aspects of the work. Get close-fitting ones and some that are waterproof.

alt Overalls can be handy when doing particularly messy jobs, such as plastering or painting.

alt Eye protection – wrap-around safety glasses are good.

alt Ear defenders – discreet earplugs with a band are good.

alt Dust masks may be needed for certain jobs.

alt Hard hats must be worn if anyone is working overhead (in 30 years I still haven’t found one that fits me).

alt Stout boots are needed to protect your feet on uneven surfaces and from falling objects. Invest in some with steel toecaps and wear them all the time – you’ll soon get used to them.

alt Knee pads – work pants with integral knee-pad patches are more comfortable to wear than strap on knee pads.

alt Hi-visability vest – can be useful when new people are around so you can be seen clearly.

alt Make sure there is a first-aid kit on site, with plenty of plasters.

alt Waterproof clothing.

alt Tool belt – a smaller one is often more handy than a large one, but it saves using your pockets.

alt As loose straw is a fire hazard, have a long enough hosepipe ready in case.

Some of the essential items you will need include hard hats, overalls, dust masks, sturdy boots, eye protection and gloves. Photographs © Barbara Jones except top left © Rae Parkinson.

Tools for straw-bale building

Straw-bale building requires almost no specialist tools. A basic toolkit of items can often be found around the home, but if you don’t already have them, they are relatively cheap to buy and not too difficult to find. As you become familiar with using tools, you’ll find that particular ones suit you better than others, and you’ll develop favourites. This may be because the handle is just the right size, or it does its job well, or it’s well balanced, or because you like the look of it and it’s familiar. Developing a feel for tools can be one of the unexpected joys of working with straw.

As with most things, there are tools of different standards available on the market. On the whole, it is best to buy better-quality tools even if they cost a bit more, because poor-quality tools may be inaccurate, wear out quickly or stretch! They will certainly frustrate you and make you feel inadequate.

It is often more cost-effective to buy tools and equipment rather than to hire them, due to the length of time you need them for. Expensive items can be sold on secondhand afterwards if still in good condition – but you may have fallen in love with them by then! If you have been on some training courses, you may also know other self-builders who are willing to share or pass on their equipment when they’ve finished with it.

Some basic but essential hand tools include claw hammer, baling twine, tape measure and Stanley knife. Good-quality tools will last and make your job easier. Photographs © Rae Parkinson.

The following are some of the reasons for investing in good tools:

alt You will be using them over a long time period, so they need to last.

alt They must withstand normal wear and tear on a building site, so need to be robust.

alt Some will probably get lost in the straw at times, and should be able to withstand this, at least overnight.

alt Using tools that don’t do their job well can make you think it’s you not the tools that’s the problem.

alt Good tools, if well maintained, can last a lifetime, not just while you build your first house.

If you are building your own house, these are the tools I recommend as part of your tool kit.

Essential tools | Preferred type | Comments
Tape measure | 5m (16') long, with metric and imperial units together. | The hook on poor-quality tape measures may come loose easily, bend or break, or not be set correctly. Such tape measures may also stop returning into their holder, rust easily or snap!
Stanley knife | With retractable blade for safety. | Any small hand knife will do, preferably on a string to avoid losing it!
Claw hammer | 400g (14oz) weight or less, unless you are used to them. | Cheap hammers bend when pulling nails out, or become pockmarked on hitting nails. Their handles may pull off too!
Lump hammer | 900g (2lb) is sufficient unless you’re trying to impress someone. | The head can fly off cheap ones!
Handsaw | Cross-cut panel saw. | A sharp saw is a dream to use, but a blunt one may put you off carpentry for life.
Axe | A side axe is best, but a splitting axe is useful too. | Get one that fits your hand size and strength.
Persuader | Long handle, large, lightweight head. | Otherwise known as a fencing hammer. No straw-baler should be without one!
Baling needles | Lightweight steel, pointed, with two holes at one end. | Should be long enough to go through the width of a bale, and sturdy enough not to bend too much.
Gloves | Close-fitting fabric perhaps with some water protection | Most work gloves are made for men with big hands, so choose carefully.

Essential power tools | Preferred type | Comments
Electric (combination) drill | Battery powered, at least 18V. Powerful, with hammer action. | Buy a very good-quality one – you’ll use this a lot. See ‘Power tools and electricity on site’, page 191.
Power Driver | Battery powered screwdriver, at least 18V. | Buy a very good-quality one – you’ll use this a lot. See ‘Power tools and electricity on site’, page 191.
Electric circular saw | Battery powered, at least 18V.100mm (4") blade. |
Electric circular saw | Mains supply, 110V 225mm (9") blade. |
Alligator saw | Mains supply 110V. | Must be very sharp, but can get very close to the straw for a nice neat finish and can create lovely details around window and door openings. Very good for making notches.
Transformer and extension leads | Reduces 240V to 110V. | Essential for electrical site safety.

Other useful tools | Preferred type | Comments
Combination square | For marking 90º and 45º angles. | Cheap ones may bend or not measure angles accurately.
Bevel | For marking angles from 0º to 180º. | Most common problems are the tightening screw handle getting in the way when marking, or not staying tight.
Hay knife | Large single blade. | A traditional tool that is extremely effective, but may be difficult to find. Can be made by a blacksmith from an original.
Earth trip | Plugs directly into a socket. | An essential protection when using 240V power tools on site.
Screwdrivers | A selection of sizes, both straight head and Pozidriv or Phillips. | Can also use battery powered. The heads on cheaper screwdrivers or bits are made of softer metals that will wear out rather than turn a difficult screw.
Spanners and wrenches | A selection of sizes. | May need both metric and imperial sizes.
Crowbar | Long but not too heavy. | Can be used for many different jobs.
Sledgehammer | Short and/or long-handled. | Possible alternative to the persuader, but can encourage too much macho behaviour.
Garden strimmer | Best with a metal cutting blade. | All types will work, but be careful not to get one that’s too heavy, as it has to be used sideways.
Chainsaw | Petrol-driven or electric. | A very dangerous but effective tool, needs to be kept sharp. May need constant cleaning, as straw will clog it up.
100mm (4") angle grinder | Electric. | Use a wood-carving (Arbutus or Protech) blade. Again, a very dangerous tool: be careful!
Hedge cutters | Petrol or electric. | Must be very sharp. Don’t hire them at the end of the season because they’ll be blunt and frustrating!
Billhook | Any type will do. | This is an alternative to the axe, and extremely handy if you’re used to using one.
Hedge clippers | Any type will do. | These need to be really sharp, as straw is particularly tough to clip.
Chop saw | Mains supply 110V capable of cutting up to 225mm (9"). | For cutting larger timbers accurately, eg floor joists and noggins.
Table saw | Mains supply 110V. | For cutting boards and sheet materials.

Power tools and electricity on site

Electricity, at the voltage that is supplied to us domestically, is powerful enough to kill outright if we make direct contact with it. This is why there are so many safeguards in the home, to ensure that this never happens. On construction sites, the risk of cutting through a cable is so much greater than at home that on major sites power tools are usually used at a lower voltage than the normal 240 volts, so that any potential shock is not life-threatening. Tools such as drills are connected via a yellow round pin socket to a transformer, which is connected to the mains electricity by a short lead. The transformer converts the usual supply of 240 volts to 110 volts. When using power tools at home, or on a self-build site, it is not common to use 110V equipment, and in fact 240V equipment is sold as standard in all DIY shops. When used according to manufacturers’ instructions, this is not dangerous. However, with the increased risks on a building site, it is a sensible precaution to use an earth trip plug for each electric tool. This looks like an ordinary adaptor that plugs into the socket; the electrical tool plugs into this. What it does is to cut off the power supply if there is a short in the circuit, such as would occur if the cable were cut or wires were immersed in water. Electrical tools from a hire shop will be supplied to work at 110V with a transformer.

Battery-powered tools with rechargeable batteries are increasingly popular and useful. The main ones in use are drills and screwdrivers, but you can also buy circular saws, jigsaws, etc. They have a huge advantage over standard plug-in tools in that they do not have any trailing leads to trip over or cut through.

The main issues to consider are as follows:

alt Batteries should store enough charge to give a reasonable length of time for usage.

alt The tool must be powerful enough for the job it is required to do or it can be very frustrating: 18 volts or higher is a good power rating.

alt Two batteries are best, then one can charge while the other is being used.

Handy tips

alt Buy brightly coloured tools, and/or mark them with coloured tape or paint: they will be easier to find when you have lost them somewhere in the straw!

alt Buy tools from a manufacturer that has been recommended by someone who uses them all the time – some of the big-name brands are not worth having.

alt Never stick baling needles into the ground, as the holes get clogged with soil. For safety, and so you can find them again, stick them upright in a spare bale.

alt The right tool for the job can save you time and money, but having lots of tools lying around that rarely get used just makes you look silly.

When using power tools, take care not to cut through the cable. It is sensible to use an earth trip plug for each electric tool. Photograph © Rae Parkinson.

CHAPTER 18

Planning permission

Planning policy is a political subject that is determined broadly at national level, and in specifics at local level. Even within one country, there may be differences in policy locally that reflect local circumstances. The fact that a house is built with straw walls is actually of very little concern to the planners but, as with most building projects, you will need to go through the requisite planning processes. It is important to know what the planning policy is for your area, and to understand why the planning authority has made these decisions

Photograph © Rae Parkinson.

This chapter deals with general aspects of planning permission that are not country-specific. For readers based in the UK, there is more information about UK-specific planning regulations and processes online at www.greenbooks.co.uk/bwsb-uk-planning

Planning permission processes and regulations vary across and within countries. In most places, the planning department, guided by local elected councillors or officials, will have worked out a comprehensive plan for the area that specifies where new housing can be built, which areas are to be kept as green belt, which is agricultural land, etc. Within each area, different types of building will be allowed or not allowed, according to guidelines that have been set by political considerations. In densely populated countries and those less used to self-build (like the UK), rules on where you can build may be quite stringent. For instance, if you wish to build a three-bedroom house in a local farmer’s field in England, you are unlikely to get permission to do so, because the field is probably designated as agricultural land and therefore no domestic buildings will be allowed. However, if you wish to build on a site next to other houses, you probably would get permission. In countries that are less populated, or have more of a tradition of self-build than the UK, it is usually easier to get permission to build. In France, for instance, as long as you keep your house relatively small, you will be able to build just by negotiating with the local mayor.

In many places, an application for planning permission has more chance of success if the relevant official (eg a planning officer) supports it. Most planning decisions are subjective and political, and your planning officer can be of invaluable help in informing you of basic policy, and of particular circumstances in which there may be room for negotiation. It is sensible to recognize your planning officer as someone who has useful knowledge that can be shared with you to enhance your project. It is always best to find a way to work together, if at all possible. It can also often be very helpful to talk to local elected officials such as councillors. Elected officials often sit on planning committees and may have no prior knowledge of your project, so inviting them to your site, explaining what you want to do and why, and enlisting their support can really help your application. The degree to which decisions are subjective, rather than strictly adhering to national and local policy, will vary by country and even by local area. In some cases authorities may just say no because they don’t like the look of what you want to do in their area, or they may think it will attract ‘the wrong sort of people’ – although they will find other reasons to object that sound more reasonable. However, decisions can usually be appealed and this has often been successful for sustainable projects in the UK.

Different types of planning permission

In the UK and many other countries there are different types of planning permission, either:

i) ‘In principle’ (or ‘Outline’) – it has been agreed in principle that a dwelling can be built on the site, but the size and exact location has not yet been determined. You will need to submit a planning application and have it approved before you can build.

Or:

ii) ‘Full’ – detailed plans have been agreed for the size, style and location of a house, and whatever is built must conform to this. If you want to make alterations you will probably need to reapply for planning permission to do so.

Areas of concern for planners

Different planning authorities will have different concerns and priorities but some typical considerations include:

alt What does it look like? Will it fit in with local surroundings?

alt What will it be used for? What you do in the building has implications for wider services and the impact you’ll make on the social and physical environment.

alt What do the neighbours think? The opinions and concerns of neighbouring property owners and local people may be taken in to account.

alt Environmental and sustainability issues: European laws say that countries must plan for sustainable development, meeting the needs of the present without compromising the ability of future generations to meet their own needs.

The planning authorities were concerned about the appearance of the building. They wanted it to look like a barn (even though there were no barns on the farm), hence the 2-storey long roof which hangs down to the ground floor. We risked going against their advice to have dormer windows and were given permission for rooflights instead. Photograph © The Wisharts.

What drawings and documents are needed?

The documents required by the planning authority will vary, of course, but typical requirements may include:

alt site location plan;

alt site layout plan;

alt floor plan (for each floor);

alt elevations;

alt roof plan;

alt drainage layout;

alt cross-sectional drawings;

alt information about why the building should be built (how it conforms to local and national policies on sustainability disabled access and other issues);

alt surveys depending on your site and local requirements (eg flood risk);

alt transport assessments if your design has transport implications for the local area.

Planning considerations for straw-bale buildings

While planners may not be at all interested in what your house is made of, you do need to think about what makes your straw-bale house different from a conventional one. Many architects new to designing with straw assume that they can just design the sort of building that they’re used to, make the walls wider and call it a straw-bale house. If you’ve got one of these designs, several things might happen:

alt You can’t build it at all with straw.

alt You have to pay more for the Building Regulation part of the work because some of the design has to be done at this later stage (since it wasn’t done at design and planning stage).

alt The straw-bale build part of it takes a long time and is fiddly instead of being simple and straightforward.

When designing a straw building, as oppose to a building made from other materials, different considerations apply and this affects what the building looks like. Also you need to be sure that your design can actually be built in straw. This means you need to begin thinking about construction during the design process, which requires a bit more work than if the planning drawings were for a standard building. For instance, we were asked to do the Building Regulation work and the actual build for a café for one of the London boroughs after they had already got planning permission for a ‘straw-bale’ design. We noticed straight away that there was no plinth shown below the straw, and that there were no opening windows in the roof or at the top of the building. The plinth, or some sort of upstand, is essential for all straw houses and should be seen on the design. Straw-bale buildings are very well insulated so, especially for a café, would need opening windows high up to let out the excess heat. We went back to the planners and asked for an amendment to allow us to change these details, but they refused. This meant that in order to go ahead, we had to redesign the building so it looked the same but worked as a straw-bale building. In this case, we were fortunate in having a good chalk soil below that drained well, so we could put our plinth below ground (not advised!) in a dugout with a fall-off to the rear. Unfortunately, we couldn’t do anything about the lack of windows, so the heat in this building has to be regulated with an air-conditioning unit instead of naturally. So much for reducing the need for energy!

Other planning considerations

There may be other planning considerations to take into account, depending on local policies. These may include:

alt Are you in an area that is considered to be of special architectural or historic interest and may therefore have particular planning rules?

alt Large projects may have to pay a levy to the local authority towards local services, transport, etc.

alt Small projects, those on your own land and those considered temporary (straw-bale buildings are considered by some authorities to be temporary, despite their potential 200-year lifespan) may not need planning permission – it is worth checking local rules.

CHAPTER 19

Building regulations

Most countries in the world have regulations about constructing buildings. They are used to make sure that buildings do not pose a threat to anyone or anything in terms of health and safety either while being built, or in use, so regulations are very useful when thinking about good practice. They also include requirements to ensure energy conservation, and to provide for the welfare and convenience of all people who might use or be affected by the building

Photograph © Rae Parkinson.

This chapter deals with general aspects of building regulations that are not country-specific. For readers based in the UK, there is more information about UK-specific building regulations online at www.greenbooks.co.uk/bwsb-uk-building-regs

Regulations (also called codes or standards) around the world fall into two broad categories: proscriptive and guidelines. Where regulations are proscriptive, then buildings must conform to proscribed norms, standards or codes: this is the case in the USA and Germany, among other places. Where they are guidelines, then if you can show an alternative to the regulation that works just as well, you can be allowed to do that instead: this is the case in the UK and occasionally in Continental European countries.

Regulations are set by governments, sometimes by local areas, and in the past were often created as a result of catastrophe such as fire. Deaths in buildings led to regulations about using materials that could withstand fire, and ensuring there were means of escape. Other times they came about as new products started to be used – this is why building regulations contain lots of information about how to use cement, and not much about how to use lime, as the knowledge of lime was widespread (in earlier times). Nowadays, new regulations are created largely due to directives from national and international governmental bodies, such as the European parliament, and/or after lobbying or application by an interest group.

Large product manufacturers are able to pay for research to prove how well their product performs. They will also be able to lobby government and may persuade them of how important it is that people use this new product over other ones.

How regulations can work against good building practice and new ideas

Proscriptive regulations can lead to bad practice in some instances, or can prevent experimentation and new ideas from flourishing. When I first learned straw-bale building in the USA, I came across local codes that said you must wrap the first course of bales in plastic to protect it from splashback. Most people were dealing with this by wrapping the bales until the building regulations official had made the inspection, then taking the plastic off just before plastering. (This code has since been changed.) In Germany and many other European countries, they are not officially allowed to build load-bearing straw-bale buildings, because there is nothing written about them in the regulations.

It is extremely costly to take a new technique through the procedures required, and so grassroots ideas like this, which don’t profit a company, are penalized. Practitioners have got around this in Germany by adhering to existing regulations for timber-frame buildings, and using the straw as an infill material, with horizontal boards every fourth course; or they have been able to build ‘experimental’ buildings.

So you can see that if we get it wrong, and a method becomes enshrined in the regulations that actually doesn’t work or has a detrimental effect on the building in the long term, we still legally have to do adhere to it. All an official has can base their opinion on is what the regulation says, even though common sense would tell us otherwise. In this way, regulations can be restrictive, especially, as mentioned, for a new building technique with no financial backing. And if a product becomes featured within the regulations and is subsequently found not to work, there is tremendous pressure to keep these findings from the public because of the enormous investment the company has made to prove its product.

The advantage of guidelines

I have already pointed out that in the UK and some Continental European countries, regulations are guidelines and not proscriptive. This means that if you have a really good idea for building, such as load-bearing straw-bale walls, which is not described in the regulations, you can still do it! It depends on having a strong and sensible argument, backed up with evidence or precedents (showing that a similar project has been achieved successfully), and a sensible building inspector who can not only understand how the method works, but is also brave enough to step outside their comfort zone. This is how I managed to get full legal approval for load-bearing buildings, and several new designs for foundations too, basically just by explaining how they would work to open-minded inspectors, engineers and clients, with evidence to back my claims.

It’s good to talk! It’s always worth talking to the building control inspector, structural engineer, client or contractor and explaining methods that might be new to them. Many are willing to listen to a sensible argument and try new things when they can. Photograph © Rae Parkinson.

Can a straw-bale house get building regulation approval?

Yes! But in some countries your choice of design may be limited – some don’t accept load-bearing designs. Research and tests, plus experiential evidence, have shown that straw-bale buildings can meet all sorts of requirements for regulation in relation to the straw-bale element. This includes structural stability, thermal efficiency and airtightness. The regulations also cover many areas of the building that have nothing to do with the straw, and you will need to make sure that all these are up to standard as well. Other areas you will need to consider may include adequate means of escape in a fire, adequate ventilation, full disabled access, safe electricity and plumbing, structurally sound roof, enough insulation and more.

Responsibility

It is the responsibility of the designer or architect to make sure that the building meets regulations at the drawing stage. During construction, it may be the responsibility of the self-builder, the project manager, the designer, or the builder; it varies from country to country. Sometimes these may be the same person, sometimes there is a main contractor who acts as the project manager, or an architect does this, or the project manager may be an independent person. It’s important to be clear who is taking on this role. Then there will be a building inspector or code official involved also. They are appointed either by the local authority or by a private company to first check your drawings to make sure they comply, and then to visit the site at specific stages during the build to make sure the construction process is following the drawings and that it is being carried out to a sufficiently high standard.

Where to find the regulations/codes

USA

There have been some very sensible lobbyists for straw-bale building in the USA who have recently managed to write a code that works for straw-bale buildings. This work began with David Eisenberg of the Development Centre for Appropriate Technology (DCAT), and was continued by Martin Hammer, a Californian architect. This has come about by extensive fundraising to pay for tests that prove how straw behaves; in fire, under loading, with different plasters, etc. You can find more information here: www.ecobuildnetwork.org/projects/straw-bale-code-supporting-documents

The UK and Ireland

UK Building Regulations are contained in a number of documents, called Approved Documents, and can be downloaded from www.planningportal.gov.uk/buildingregulations/
approveddocuments/downloads

Scotland and Wales currently use the UK regulations but have some national differences from the main UK ones. You can find out about Scottish Building Standards in the Technical Handbooks that can be downloaded here: www.scotland.gov.uk/Topics/Built-Environment/Building/Building-standards and Welsh regulations here: wales.gov.uk/topics/planning/buildingregs/

Irish Building Regulations are contained in a number of Technical Guidance Documents and can be found here: www.environ.ie/en /TGD/#Current Technical Guidance Documents and Supporting Documentation

Building regulations and straw-bale buildings

Building regulations, codes and standards cover all aspects of building, but for our purposes, although the whole building is built predominantly of sustainable, natural materials, the only areas that are substantially different from other building techniques are the walls and foundation design. So the areas of concern for building regulations with straw-bale buildings are:

alt thermal efficiency (insulation);

alt acoustic efficiency (sound insulation);

alt fire resistance;

alt structural performance;

alt durability (including degradation due to moisture);

alt airtightness;

alt energy ratings.

Thermal efficiency

In brick or block walls, insulation often takes the form of expanded polystyrene or foam (high in embodied energy, and using pollution-intensive production methods) stuck to the back of the blocks inside the cavity of the wall. Other types of eco-construction may use timber and sheep’s wool, wood fibreboard, hemp or recycled paper as insulation. With straw-bale walls, the insulation (straw) can also be the building block – and the surface for plaster – thus reducing the need for extra materials. The amount of insulation of a material is measured by its U-value.

The U-value (or thermal transmittance coefficient) is a measure of how much heat will pass through one square metre of a structure when the temperature on either side of the structure differs by 1°C (34°F). It is measured in units of watts per square metre per degree of temperature difference (usually measured in kelvin): W/m²K. Put simply, the U-value is a measure of how much heat a material allows to pass through it. The lower the U-value, the greater the insulation of the material.

Building regulations will usually give maximum U-values for different elements of new extensions or new builds. For example, UK regulations give a maximum U-value of 0.30 W/m²K for walls, 1.50 W/m²K for doors and 2.00 W/m²K for roof windows.

Straw-bale walls have a U-value of between 0.13 W/m²K and 0.20 W/m²K, depending on the quality of the bales used and the degree of skill used in installing them. This doesn’t include the render or plaster on either side, which can lower the U-value to 0.11 W/m²K.

The high insulation value of straw is achieved because of the width of the bales. In the table below, compare the U-values of other common wall building materials.

There is no doubt that straw-bale walls exceed the requirements of building regulations, standards and codes for thermal insulation. Further information on the thermal tests conducted so far on straw-bales is available in Appendix 4 and at www.ecobuildnetwork.org/projects/straw-bale-code-supporting-documents

Material | U-value (W/m2K)
100mm (4") brickwork, 75mm (3") mineral fibre, 100mm (4") light concrete block, 13mm (½") lightweight plaster | 0.33
100mm (4") heavyweight concrete block, 75mm (3") mineral fibre, 100mm (4") heavyweight concrete block, 13mm (½") lightweight plaster | 0.40
100mm (4") lightweight concrete block, 75mm (3") mineral fibre, 100mm (4") lightweight concrete block, 13mm (½") lightweight plaster | 0.29

Source: Chartered Institute of Building Service Engineers (CIBSE) (2002) Guide A: Environmental Design, Chapter 3, ‘Thermal properties of building structures’.

Sound insulation

In the Netherlands, research has been done into the sound insulation provided by straw bales (as detailed in volume 53 of The Last Straw journal (see page 209)), and we have overwhelming experiential evidence that straw walls offer far more sound insulation than that offered by walls built with conventional techniques. People who live in, use or visit straw-bale buildings remark on the quality of the atmosphere found inside them. They are cosy, calm and quiet. They offer a feeling of peace. There are at least two recording and rehearsal studios in the USA built of straw because of its acoustic properties, one in Wales (www.pindropclub.co.uk/strawdio/index.htm), and several more meditation centres. Straw-bale walls are increasingly being used in airports and motorway systems as sound barriers to reduce traffic noise. Plastered straw-bale walls have achieved Building Regulations approval in the UK as thermal and acoustic walls in semi-detached houses, with a special less-dense wall built in front.

Fire resistance

There is no question that straw-bale walls fulfil all the requirements for fire safety as contained in the UK Building Regulations and the standards of other countries. Straw-bale walls are not a fire risk and they have passed all the fire tests they have been subjected to in the USA, Canada and Europe.

In 2004, Amazon Nails conducted fire-resistance tests with the UK Building Research Establishment (BRE), which showed that lime-plastered straw-bale walls will withstand fire for a minimum of 2 hours 40 minutes. A conventional plasterboard and scrim finish on a timber stud wall, which withstands fire for 30 minutes.

A straw-bale wall built for fire testing at the Building Research Establishment (BRE) in 2004. Photograph © Barbara Jones.

The same tests also showed that it took an hour before there was any temperature difference at all on the side of the wall away from the fire, which is an indication of its insulation value. The ‘fire side’ of the wall was fired with the same furnace that all building materials are tested with – subjecting the wall to naked flames and temperatures of over 1,000ºC (1,800ºF). Unfortunately, there was not enough money available to extend these tests up to British Standard (the only difference being that the wall system would have to be larger, at 3m² (32ft²), rather than the 1.5m² (16ft²) walls tested). However, the results are the same as those demonstrated in many other countries: fasba.de/content/view/206/325/

It is a popular misconception that straw-bale buildings are a fire risk. This misconception seems to come partly from the confusion of straw with hay, and the collective memory of (relatively rare) spontaneous combustion in hay barns (from large haystacks baled too wet and green). Straw is a very different material from hay, and there are no known cases of spontaneous combustion with straw, even when stored in poor conditions.

There is a risk of fire with straw, however, during the storage and construction process. It is loose straw that is the risk, since it combusts readily. If you were to cut the strings on a bale and make a loose pile of the straw, it would burn very easily, as it contains lots of air. Therefore it is essential to clear loose straw from the site daily, store straw bales safely, have a no-smoking policy on site and protect the site from vandalism. If you are going to leave a wall for a while before plastering, be sure to trim it, getting rid of the ‘fluffy bits’ that would encourage the spread of flames.

Once the straw is built up into a single bale wall it tends to behave as though it were solid timber, particularly when it is load-bearing and therefore under compression, but also when used as infill. In a fire, it chars on the outside and then the charring itself protects the straw from further burning. It’s like trying to burn a telephone directory – if you tear loose pages from it, they will burn easily, but if you try to set fire to the whole book, it’s very difficult. When the wall is plastered both sides, the risk of fire is reduced even further, as the plaster itself provides fire protection.

For the purposes of Building Regulations, a wall built of any material that is covered with half an inch of plaster has a half-hour fire protection rating (this is the requirement for domestic buildings in the UK). All the fire-testing research on straw-bale walls concludes that this type of wall-building system is not a fire risk.

Research in the USA and Canada reached the following conclusions:

“The straw-bales/mortar structure wall has proven to be exceptionally resistant to fire. The straw-bales hold enough air to provide good insulation value but because they are compacted firmly they don’t hold enough air to permit combustion.”
Report to the Canada Mortgage and Housing Corporation by Bob Platts, 1997

“ASTM tests for fire-resistance have been completed. The results of these tests have proven that a straw-bale infill wall assembly is a far greater fire-resistive assembly than a wood frame wall assembly using the same finishes.”
Report to the Construction Industries Division by Manuel A. Fernandez, State Architect and head of Permitting and Plan Approval, CID, State of New Mexico, USA.

Structural performance

The building codes of many countries do not give any guidance on the structural performance of straw-bale load-bearing walls; and in countries like Germany, where regulations are proscriptive, load-bearing designs may not be allowed at all. However, just because regulations do not give guidance on building straw-bale walls, this does not mean it cannot legally be done! Straw-bale walls have been shown to have excellent structural performance. Research in the USA has shown that structural load-bearing straw-bale walls can withstand loads of more than 10,000 lb/ft², equivalent to 48,800kg/m². (See ‘Results of a Structural Straw-bale Testing Program’ by Ghailene Bou-Ali, 1993.)

There is no doubt that load-bearing straw walls can withstand greater loads than will be imposed on them by floors, roofs and possible snow-loading. It is the design of associated timber work, the even spread of loads around the walls, and the quality of building that is crucial here, not whether the straw is strong enough. With infill walls, in framework structures, the straw does not take weight anyway and there are conventional calculations available for the structural strength of other types of framing.

Durability

This is the area of most concern when designing straw-bale houses in order to comply with building regulations. Will the straw-bale walls retain their structural integrity over time, or will they suffer material degradation caused by moisture from either condensation, rain or ground water? While this is a consideration for all house builders, all that most building regulations require is that the walls of a building pose no threat to health and safety. There has been little research to date on the durability of straw-bale houses in the weather conditions experienced in the UK. Research carried out in the USA and Canada shows that there should be no need to be concerned that straw-bale walls will not withstand the test of time and the rigours of the UK climate. (See www.ecobuildnetwork.org/projects/research/straw-bale-test-program.) The key to durability lies in good design, good-quality work and maintenance. In countries where regulations are for guidance rather than proscriptive, past experience may be allowed as a method of establishing the fitness of a material, as it is in the UK.

A building that is designed well and built well should not experience any long-term effects of degradation due to moisture. There are plenty of examples in the USA of straw-bale houses enduring for over 50 years with no signs of deterioration. The oldest known ones still inhabited today were built in 1903 in Nebraska, and the oldest in Europe is near Paris, built in 1921. In the UK, straw-bale building goes back only around 20 years, and there is not yet empirical evidence that straw-bale buildings will survive for long time periods in the UK climate. We do, however, know that even if there is degradation of the straw, it is easily repaired, and that it degrades slowly and therefore poses no risk to safety.

Airtightness

An airtightness test is used to determine a building’s air leakage, and basically determines whether your building has been designed well, and then built well. Obviously, heat will escape from anywhere that air does. To carry out the test, you have to close all the windows and doors and use a machine that sucks air out of the building. The rate at which air moves, and is therefore being sucked into the building through unintended gaps, is then measured. Most problems in standard designs occur at the junction between the roof and walls, around windows and doors, and around electrical boxes. The building regulations of most countries require a minimum airtightness for new buildings. For example, in the UK, buildings larger than 15m³ (530ft³) need to be tested and their air leakage proven to be no worse than 10m³/hr/m² (33ft³/hr/ft²) at 50Pa. The houses we designed and built with a local firm for North Kesteven District Council in 2009 achieved an airtightness of 2.62m³/hr/m² (8.6ft³/hr/ft²). This was without making a particular effort to be airtight other than through following our own designs and building with quality. We built to Passivhaus standards in 2013, which means an airtightness of 0.6m³/hr/m² (2.0ft³/hr/ft²), and in this case taped joints were used. However, given that we need at least 4 air changes/hour for human health, I am not convinced that PassivHaus standards are the way to go, because they mean we must also be dependent on mechanical ventilation with heat recovery (MVHR).

Most problems with airtightness occur because of poor-quality work and a lack of understanding of the reasons for following designs accurately, coupled with out-of-date designs that can’t really be improved, such as the cavity wall system. It’s common for electricians installing electrical back boxes to break through the internal concrete block skin into the cavity of the exterior wall, thus exposing the building to air leakage via the cavity. Monolithic wall systems such as straw-bale ones can never suffer from this defect and are therefore inherently better, reducing the potential for poor-quality work. Not only that, but strawbale builders are usually conscientious people who have chosen this type of construction for sensible reasons, and when we run courses to help them build, we use a process that involves each person in a thoughtful and responsible way. Building with this sort of awareness can create much better-quality buildings. We pay attention to the process of building as well as the function, and it produces amazing results!

Log book and energy ratings

New buildings often have a log book, although this may not be a legal part of the local building regulations. This details lots of information about the building, including the type of construction. It also shows the calculations based on type of insulation, floor-to-window area ratios, U-values of different components, etc, which combine to produce a figure showing how energy-efficient the building is (known as the Standard Assessment Procedure rating in the UK). The log book for a straw-bale building would also include details of how to maintain the walls and plaster/render, what to do in case of accident (eg water penetrating the bales due to storm damage on the roof), how to fix things to the walls, and how to make extensions/alterations.

And finally…

Lastly, a word of caution about building codes, regulations and building practice in general.

You need to be careful about what you read in books and on the internet about straw-bale building and how it must be done. USA Building Codes and methods of building are not necessarily appropriate for European countries and vice versa. There is an increasing body of knowledge available in Europe, but I would not necessarily agree with all of it, because I am a passionate advocate of natural materials,. Moreover, I like to keep everything as simple as possible. I encourage you to inform yourself as fully as possible of the principles of good straw-bale design, and then make up your own mind.

When faced with a choice about trying a new or different construction technique, always ask yourself first: ‘Does it work?’ But don’t stop there. Ask the most important question next: ‘Is it necessary, or does it over-complicate?’ And finally, ask yourself: ‘Do I have the knowledge, skills and support required to carry out my dreams?’ The best straw-bale homes are straightforward, beautifully simple (often simply beautiful!) and created as the result of many people’s efforts.

Building with straw bales: simple and beautiful! Photograph © Rae Parkinson.

Appendix 1: Frequently asked questions

What about mice and rats?

There is no greater risk of encouraging mice and rats into your straw-bale house than there is for any other type of building. Straw is the empty stem of a baled grain crop and, unlike hay, it doesn’t contain food to attract furry creatures. Any home where food is left out in the open is a potential lure for vermin. Once your straw-bale house is plastered, the walls seem no different from other plastered walls to a mouse. Mice and rats like to live in spaces between things, as they are very sociable animals. In barns, they live in the gaps between bales and in houses they live in cavities and under floors. If you build straw walls and then clad them in timber, with an air gap between, this might attract mice: but it’s the gap they like, not particularly the straw. If you build straw walls, plaster them with clay or lime and maintain them; then there are no gaps to invite in mice or rats, and no cavities in which they can live. Lime plaster can also help deter your furry friends – they don’t like it!

How long will it last?

No one can completely answer this question, because the first straw-bale house was built only about 150 years ago. In the USA there are about a dozen houses more than 100 years old that are still inhabited and showing no problems. They have an increasing stock of houses built since 1980 that are also enduring with no problems. In the UK we started building in 1994, and in France there is a house that was built in 1921. As with any other technique of house building, if your straw-bale house is constructed with a good design, with quality work and is properly maintained throughout its life, there is no reason why it should not last at least 200 years.

Isn’t it a fire risk?

No. It may seem strange, but when you stack bales up in a wall and plaster them either side, the density of the bales is such that there isn’t enough air inside the bales for them to burn. It’s like trying to burn a telephone directory – loose pages burn easily, but the whole book won’t catch fire. Straw-bale walls have passed all the fire tests they have been subjected to in the USA, Canada and Europe. And, regardless of whether the bales themselves are a risk, if you plaster any wall with a half-inch of plaster it gives sufficient fire protection to satisfy UK Building Regulations. In fact, plastered straw-bale walls are so much not a risk that they are being used as fire walls between semi-detached houses.

Is it really cheap to build?

The short answer is no, not for most people, but it is possible and affordable. It depends on your approach to building. If you can put lots of time into collecting recycled materials, or do the drawings yourself and keep the design simple, or organize training workshops to build the walls and plaster them, or get your friends and family to help, then yes, it can be cheaper to build. For most people, it is more sensible to think of doing the simple bits yourself (initial design, foundation, straw and plaster) and employing others to do the rest (carpentry, roofing, plumbing and electrics). A small office in the garden might cost £35,000 (although much less with recycled materials and volunteer labour), and a self-built two-bedroomed house could be £50,000. Affordable council houses were built for £110,000. And a warehouse near Oxford cost £537/m² (£49/ft²). Savings are greater on bigger buildings. Remember that the straw walls represent only 16 per cent of the total build cost – everything else may be just like any other house.

Can I do it myself?

Yes, parts of a straw-bale house are quite easy to build. Other parts, such as roofing and carpentry, are more difficult. It depends on how much time, relevant experience, determination and dedication you have. But the straw-building technique is simple, straightforward and accessible to almost anyone. Most self-builders take a couple of years to build their own house, working on it most of the time: it’s a slower process than having a contractor doing it for you.

What about temporary buildings?

The design of straw-bale buildings is very versatile, and can be adapted for a more or less durable function. If a building is required for only a few years, then there may be no need to build elaborate foundations, or plaster it inside or even outside. Or you can buy an off-the-peg building and sell it on when you’ve finished with it. You can also build a high-quality straw-bale building that can be classed as temporary, because it could be taken down again easily if required, but the same building could also last for 200 years.

What else can be built with straw?

Straw has been put to many uses. Apart from houses, studios, offices and community spaces, straw is also used for schools, warehouses, retail units, holiday homes, salerooms, restaurants, barns and stables, sound studios, meditation centres, acoustic barriers for airports and motorways, business premises, food storage and farm buildings.

What if some of my bales do get wet?

It depends on where, and how badly. Generally, if a bale gets wet through the top or bottom into the centre, then it will not dry out before it starts rotting, unless you make holes in it to provide ventilation. So any bales that are rained on, or stand in water while in storage, should be discarded. This also applies to any bales already in the walls that are not covered against the rain. But if you have covered the tops of the bales, and the sides get wet from the rain, this usually presents no problem, as they will quickly dry out once the rain stops. The only time this may not be the case is if the walls are exposed to severe wind and rain at the same time for prolonged periods, as the wind may drive the rain into the bale, where it cannot dry out.

Is it possible to repair straw walls?

It is not only possible, it’s very easy! The hardest part is making a hole through the straw. This can be done with the claw on a hammer or crowbar, and by just pulling at the straw. It can be quite difficult to make the first hole, due to the density of the bale. However, once this is done, wedges of the bales can be pulled out quite easily. Hazel pins can be cut through if necessary, and fresh straw wedges can be packed tightly back to fill the gap.

Experience has shown that if a section of wall does get wet, damp remains remarkably localized. It tends not to spread further through the straw, and so wedges or flakes of the bale can be removed and replaced.

What if I want an extra window?

Again, it’s fairly easy to cut through the walls to create a window-sized hole. Usually there is no need to support the rest of the wall, as the wall plate carries most of the load of the floor above, and the straw bales act together as an integral material because of the way they are pinned. Either follow the method described above for repairing straw walls, or you can use a hay knife, even a chainsaw (although power tools like this tend to clog up very quickly). Once you’ve cut the hole, a structural box frame can be fixed into the gap, with the window inside this.

Can I use straw to add an extension to my house?

Yes, both load-bearing and framed systems work well here. You may need to think carefully about settlement, and not make the final attachments from the straw to the house wall until after the walls are compressed. Better still, use a slip joint by attaching a timber post to the existing wall and notching the bales around it.

You can also easily add an extension to your straw-bale house by cutting a doorway through, in the same way as described on page 206 for making a window. Families have sometimes encouraged their children to build their own add-on spaces once they’ve reached a suitable age!

Can I insulate my existing house with straw?

It’s perfectly possible to wrap your existing house with straw to give it better thermal efficiency. You will need to extend the roof over the top, and add a small foundation for the straw, plus think carefully about details around windows and doors, but you could transform your ugly concrete bungalow into one that looks like a beautiful old Devon cottage – at least from the outside! It will reduce your heating costs while increasing your comfort.

Can I use straw in the roof or floor to insulate my house?

Yes, especially for a new build, as you will need to design the rafters and joists to suit the size of bales. In fact it can be a very cost-effective solution to achieve very good insulation levels. If used in the roof, it will need to have fire protection on both sides. This could be a 25mm (1") layer of clay plaster above, and plasterboard below. You will also need to make sure that the roof structure is strong enough to take the weight of the bales. If used in the floor, it should not be used where there is a risk of local flooding (eg by overflowing the bath) because extensive water damage will make the straw rot.

What if I don’t want to build it myself?

There are several companies or individuals who can do this for you, or you could work together with a local building contractor. Or you can buy a small straw-bale building ‘off the peg’ and have it delivered!

Appendix 2: Resources

Books

Especially recommended by the author

Gruber, H..et al. (2012). Neues Bauen mit Stroh in Europe [New Building with Straw in Europe]. Okobuch: Germany.

Holmes, S. and Wingate, M. (2002). Building with Lime: A Practical Introduction. Practical Action Publishing: Rugby, UK.

Wiesman, A. and Bryce, K. (2015) Clay and lime render, plaster and paints. Green Books: Cambridge, UK.

King, B. et al. (2006). Design of Straw-bale Buildings: The state of the art. Green Building Press: San Rafael, USA.

Lacinski, P. and Bergeron, M. (2000). Serious Straw-bale. Chelsea Green Publishing: Vermont, USA.

Magwood, C. and Walker, C. (2003) Straw-bale Details. New Society Publishers: Ontario, Canada.

Minke, G. (2009). Handbuch Lehmbau: Baustoffkunde, Techniken, Lehmarchitektur [Earthbuilding Handbook: Building materials, techniques, earthern architecture]. Okobuch: Germany.

Nitzkin, R. and Termens, M. (2010). Casas de Paja [Straw Houses]. EcoHabitar Sustainable Visions.

Steen, A. S. and Steen, B. (2000). The Beauty of Straw-bale Homes. Chelsea Green Publishing: Vermont, USA.

The Straw-bale Alternative Solutions Resource pdf (2013) available from www.asri.ca

Wanek, C. (2009). The New Straw-bale Home. Gibbs Smith Publisher: Utah.

Other books on straw-bale building

Bainbridge, D. et al. (1994) The Straw-bale House. Chelsea Green Publishing: Vermont, USA.

Snell, C. and Callahan, T. (2005). Building Green: A complete how-to guide to alternative building methods – earth plaster, straw bale, cordwood, cob, living roofs. Lark Crafts.

Corum, N. and Goodall J. (2005). Building a Straw-bale House: The red-feather construction handbook. Princeton Architectural Press: New York.

Elpel, T. J. (2011). Living Homes: Stone masonry, log and straw-bale construction (sixth edition). HOPS Press.

Hodge, B. (2006). Building Your Straw-bale Home: From foundations to the roof. Landlinks Press.

Hollis, M. (2005). Practical Straw-bale Building. Landlinks Press: Victoria, Australia.

King, B. (1996). Buildings of Earth and Straw. Ecological Design Pres: USA.

Law, B. (2010). The Woodland House. Permaculture Press: London.

Macdonald S. O. and Myhrman, M. (1998). Build it with Bales. [version two]. Treasure Chest Books.

Magwood, C., et al. (2005). More Straw-bale Building. New Society Publishers; Ontario, Canada.

Minke, G. and Mahlke, F. (2005). Building with Straw. Birkhauser: Basel.

Roberts, C. (2002). A House of Straw (2010). Kindle edition, self-published. Chelsea Green Publishing: Vermont, USA.

Steen, B. et al. (2005) Small Straw-bale. Gibbs Smith Publisher: Layton, Utah.

Wanek, C (2010). The Hybrid House: Designing with sun, wind, water, and earth. Gibbs Smith Inc.

Inspirational books

Anderson, W. and Chadwick, R (2007). Green Up: An A-Z of environmentally friendly home improvements. Green Books: Cambridge.

Bee, B. (1998). The Cob Builders Handbook: You can hand-sculpt your own home. Groundworks: Oregon. Available from www.beckybee.net.

Easton, D. (2007). The Rammed Earth House  second edition). Chelsea Green Publishing: Vermont, USA.

Broome, J. (2007). The Green Self-build Book: How to design and build your own eco-home. Green Books: Cambridge.

Lynne, E. and Adams, C. (eds) (2005). Alternative Construction: Contemporary natural building methods. (second edition). John Wiley & Sons.

Evans, I. et al. (2002). The Hand-sculpted House: A practical guide to building a cob cottage. Chelsea Green Publishing: Vermont, USA.

Kahn, L. (2004). Home Work: Handbuilt shelter. Shelter Publications: California, USA.

Khalili, N. (1996). Ceramic Houses and Earth Architecture: How to build your own. Chelsea Green Publishing: Vermont, USA.

McCloud, K. (2009) Grand Designs Handbook: The blueprint for building your dream home. Harper Collins: London.

Norton, J. and Oliver, P (1997). Building with Earth Practical Action Publishing. Rugby: Warwickshire.

Olsen, R. (2012). Handmade Houses: A free-spirited century of earth-friendly home design. Rizzoli International Publications.

Stanwix, W. and Sparrow A. (2014). The Hempcrete Book: Designing and building with hemp-lime. Green Books: Cambridge.

Taylor, J. S. (2008). Shelter Sketchbook. Chelsea Green Publishing: Vermont, USA.

Weismann, A. and Bryce, K. (2015). Building with Cob. Green Books: Cambridge.

Natural plasters

English Heritage (2012) Mortars, Renders and Plasters (Practical Building Conservation) Ashgate Publishing Limited.

Fromme, Irmela. Herz, Uta. Lehm – und Kalkputz (2012).

Guelberth, C. R. and Chiras, D. (2002) The Natural Plaster Book. New Society Publishers; Gabriola Island, BC, Canada.

Henderson, James & Angliss (illus) (2013) Earth Render - The art of clay plaster, render and paints. Python Press

Henry, Alison & Stewart, John (ed) (2012) Mortars, Renders & Plasters (Practical Building Conservation) Ashgate Pub Co

Handbook on ECVET clay plastering lernpunktlehm.de/wpll/?page_id=90

McAfee, Pat. (1998) Lime Works

Reichel, Alexander (2005) Detail Practice: Plaster, Render, Paint and Coatings: Details, Products, Case Studies (Detail Practice Series) Birkhäuser GmbH

Schofield, Jane. (1998) Lime in Building: A Practical Guide. Black Dog Press: Devon.

Soens, M. (2005) Italian Plaster Techniques. Sterling Publishing, New York, USA.

Articles

Alcorn, A. and Donn, M. (2010). ‘Life-cycle potential of straw-bale and timber for carbon sequestration in house construction’. Second International Conference on Sustainable Construction Materials and Technologies, Università Politecnica delle Marche, Ancona, Italy, 28–30 June 2010.

Offin, M. (2010). ‘Straw-bale Construction: Assessing and minimizing embodied energy.’ MSc diss., Queen’s University, Canada.
www.queensu.ca/ensc/graduate/messtudents/mariaoffin.html

Journals

The Building Limes Forum. News available from
www.buildinglimesforum.org.uk

The CASBA Journal Archive. Citu.

The Newsletter of the California Straw Building Association.
www.strawbuilding.org/casba_journal

Green Building Magazine available from
www.greenbuildingpress.co.uk

The Last Straw, the international journal of straw-bale and natural building since 1993. www.thelaststraw.org

Passive House Plus magazine available from www.passivehouseplus.ie

Sustainability, a practical journal for green building, renewable energy, permaculture and sustainable communities available from www.sustainability.ie

Websites

The following websites contain resources relevant to straw-bale and natural building.

www.strawworks.co.uk Design & build, training, online course, free technical details

www.baubiologie.at A major European site for natural building and the European straw-bale network

www.buildinglimesforum.org.uk Encourages expertise and understanding in the appropriate use of building limes

www.buildingwithawareness.com Dvd on straw-bale building

www.cat.org.uk Visitor centre in Wales offering solutions on climate change, pollution and the waste of precious resources. Free information service and a wide range of residential courses, events for schools etc.

www.greenspec.co.uk A directory of sustainable construction products and contact details of manufacturers and suppliers

www.lernpunktlehm.de European website of the Leonardo project on clay plastering

www.lowimpact.org A non-profit organisation that aims to help people reduce their impact on the environment, improve their quality of life and gain new skills.

mha-net.org/html/sblinks.htm Surfin’ straw-bale

www.naturalbuildingcoalition.ca Ontario straw-bale Building Coalition

www.naturalhomes.org A resource for finding courses and workshops related to natural home building worldwide.

www.oekofilm.de Films on straw-bale building and fire tests

www.strawworks.co.uk/the-school-of-natural-building/ Provides training in straw-bale building, lime and clay plastering, low impact foundations and other natural materials.

www.scotlime.org Promotes traditional building skills and provides advice and training in the use of lime.

www.strawbale.com Information, courses and dvds

www.strawbalecentral.com Information on straw-bale building

www.strawbuild.org Building, design and training
en.wikibooks.org/wiki/Straw_Bale_Construction

Other websites and organisations by country

Austria: www.baubiologie.at

Australia: www.ausbale.org

Belgium: www.casacalida.be

Canada: www.naturalbuildingcoalition.ca Ontario, information, support and research

Czech Republic: www.jakubwihan.com Kuba Wihan, design, build and training

www.slamenedomy.cz

Denmark: www.lob.dk National Association for Ecological Construction

Estonia: www.equilibre.ee Sustainable Living, ecological construction and consultation

France: www.compaillons.eu Reseau Francais de la construction paille www.lamaisonenpaille.com straw-bale building and mass stoves

Ireland: www.cultivate.ie Information, shop, courses and events

www.easca.ie Environmental and Sustainable Construction Association, to promote the viability of sustainable construction in Ireland.

www.sonairte.ie An interactive visitor centre promoting ecological awareness and sustainable living

Italy: www.laboa.org Stefano Soldati, La Boa Permaculture Centre, Nr Venice straw-bale building, lime and clay plastering, training and design.

Germany: www.fasba.de National organisation for straw-bale building

www.stroh-unlimited.de Friederike Fuchs, architect, straw-bale, clay and lime design, build, engineering and training.

www.earthbuilding.eu

The Netherlands: www.strobouw.nl

New Zealand: www.earthbuilding.org.nz/ straw-bale

Pakistan: www.paksbab.org US organisation for Straw-bale & Appropriate Building

Poland: www.cohabitat.net Social network

Slovakia: www.ozartur.sk Straw-bale building, clay and lime training and information

Spain: www.casasdepaja.org Straw-bale building network, training

www.hortadelaviola.org/taller-de-palla Maren Termens Technical architect for ecological buildings, straw-bale and clay training

rikkinitzkin@earthlink.net Rikki Nitzkin straw-bale and clay training.

UK: www.aecb.net The Sustainable Building Association. Promotes environmentally responsible practices within building. Has a forum, newsletter and magazine

www.ebuk.uk.com Raising awareness of earth as a building material

www.alternativetechnology.org.uk

www.sustainablebuild.co.uk A reference point for sustainable building, development and eco-construction techniques

www.spab.org.uk A membership organisation fighting to save historic and listed buildings from decay, demolition and destruction.

www.wamt.org UK organisation for tradeswomen and women training in the trades.

USA: www.builderswithoutborders.org An international network of ecological builders who advocate the use of straw and earth in construction.

www.caneloproject.com A non-profit organization dedicated to the exploration and development of living systems, including growing food and building that creates friendship, beauty and simplicity.

www.dcat.net The Development Center for Appropriate Technology Works to enhance the health of the planet and our communities by promoting a shift to sustainable construction and development.

www.ecobuildnetwork.org Works to transform the way we build shelter.

www.nbnetwork.org Natural Building Network A not-for-profit membership association promoting natural building principles, materials and practitioners worldwide.

www.sbregistry.greenbuilder.com International Straw-bale Registry

Research

Comprehensive results of tests on fire resistance, thermal resistance, sound insulation, structure, moisture and earthquake resistance can be found at: www.baubiologie.at/wp/strohballenbau/zertifikate-und-studien/ And at: www.ecobuildnetwork.org/projects/research/straw-bale-test-program

Other tests that cannot be found above:

Acoustics:

Air-sound-insulation of clay-plastered non-load-bearing sb-wall (Danish Research 2004). Available at:
en.wikibooks.org/wiki/Straw_Bale_Construction/Resources/Technical_Studies

Earthquake resistance

http://nees.unr.edu/projects/straw-house results from Paksbab, accessed 10/8/14: Truong, B. et al. (2008). ‘Shear Strength of Straw-bale Plasters.’

The 14th World Conference on Earthquake Engineering, Beijing. Available at http://nisee.berkeley.edu/elibrary/Text/2009012814. Accessed 15/01/14.

Fire tests

FASBA (2008). ‘Classification of the behaviour in case of fire, in accordance with DIN EN 13501-1’: (2007). Institute for Baustoffe, Massivebau und Brandschutz: Hannover, Germany. Available from: http://fasba.de/content/view/24/220/

Structural tests

Walker, P. (2004). ‘Compression-load-testing straw-bale walls.’ Dept. Architecture & Civil Engineering: University of Bath. Available from: http://people.bath.ac.uk/abspw/straw%20bale%20test%20report.pdf. Accessed 15/01/14.

Building Regulations, Guidelines and Codes

Europe: Most countries except France and the UK have proscriptive Codes and only infill techniques are legal.

France: http://www.compaillons.eu/reglementation Load-bearing, infill and hybrid techniques all legal for single houses.

Germany: http://fasba.de/content/view/24/220/ Only infill techniques legal

New Zealand: Building with straw requires a Designer to do the analysis to apply for a Building Permit as an ‘Alternative Solution’ under the Building Code.

UK: No specific written Regulations on straw-bale building but Standards available. Load-bearing, infill and hybrid techniques all legal.

USA: www.dcat.net/about_dcat/current/codes.php. Load-bearing, infill and hybrid techniques all legal

Embodied Energy, Performance and Life-cycle Analysis

www.buildingscience.com/doctypes/digests Useful information about building physics Embodied Energy Coefficients – Alphabetical. Victoria University of
at http://www.victoria.ac.nz/architecture/centres/cbpr/
resources/pdfs/ee-coefficients.pdf Accessed 10/8/14

Appendix 3: Design and construction drawings

A successful self-build design must be relatively easy for a novice to understand and construct. It should involve simple, easy-to-follow techniques that do not require expensive machinery, specialist equipment, or non-standard building products to achieve. As well as these considerations, it should be readily adaptable to changing personal / family circumstances, be built of materials that do no harm to our planet, and be possible to build between the beginning of spring and the onset of winter.

The load-bearing straw-bale building technique (that requires no extra framework to support floors and roofs) has proved that it can meet these criteria, and also produce buildings with a lifespan of 100 years or more. The following designs fulfil the above criteria, have been tried and tested, and the basic two-bedroom house option can be built for under £50,000.

Appendix 4: Humidity in straw-bale walls

By Jakub Wihan

The doubts that generally come to mind in connection with straw-bale walls are always related to fire, rodents and moisture. While worries about fire and animals can easily be dispelled by the application of good plaster on to the straw, the question of the vulnerability of straw to humidity is potentially more serious and it is worth looking into this in detail.

Even in the often-wet climate of the UK and Ireland, straw will become damp only in certain situations. When a farmer finds some old bale in a forgotten corner, it is likely to still be in very good shape after many years or even decades unless it sits on damp ground, is covered so that fresh air can’t get to it, or is exposed to leaks, condensation or direct rain. So, according to the experience of any farmer, a wall of stacked bales that is built on a tall and draining plinth and covered by a generous overhang will last for a very long time, even in a very humid climate such as that of the UK and Ireland.

Questions start to spring to mind when contemplating building a straw-bale house on a site exposed to severe winds. For example: will the render shield the straw enough from driving rain? And if the rendered straw gets wet for some reason, doesn’t the render then prevent the straw from drying? And, most importantly, doesn’t the rain cause condensation within a straw-bale wall?

Research – where are problems most likely to occur?

Houses built out of straw and hay bales more than 100 years ago in the Nebraskan desert are failing to give us convincing answers to the above questions, because there is not much that can go wrong with a straw-bale house in a desert climate. But since people in the late 1980s started building in a more humid environment, in the UK and Continental Europe, the effect of moisture on straw-bale walls has been the subject of numerous investigations. This has given us an idea of:

alt the way different plasters and renders influence humidity levels within the straw;

alt the relationship between humidity and potential straw decomposition;

alt the most common causes of straw rot in walls.

A questionnaire sent to professional straw-bale builders all around the world in June 2004 by Jakub Wihan was used to create a database of 174 moisture problems in straw-bale walls that those builders had repaired during their careers. Almost a quarter of all the cases of serious moisture damage related to bad window or door detailing, where leaking windowsills were the most frequent causes. The survey reported that 1 in 6 moisture problems were related to a leaky roof, and only slightly more – 30 cases out of the 174 – had rotting straw in their walls due to wind-driven rain.

The window detailing can’t ever be underestimated in straw-bale building, because the window and door surfaces tend to collect large amounts of concentrated water during rain. This water is then usually driven by wind into the bottom corner of the recessed opening, where it is likely to find its way into the wall through any cracks between the render and the frame. Figure 1 shows window detailing designed for extra protection against water ingress.

Figure 1. Extra corner protections embedded in window frames and generous (sometimes doubled) windowsills with drip edges will guarantee the owners of any straw-bale building sound sleep at night. Picture: Carol Atkinson’s straw-bale cabin in Howden, East Yorkshire. Photograph © Jakub Wihan.

The roof above straw-bale walls needs to be finished with the utmost care, and the same applies to all external straw protection. The render quality, for example, can’t be compromised. For this reason, it is best practice to use traditional lime render (using well-graded sharp sand and mature lime putty in the ratio 3:1 – see Chapter 15), applied when there is no risk of frost and to the best possible standard.

What we know about straw is that it gets wet quickly – but it also gets dry quickly!

One case study that demonstrates serious moisture damage caused by the application of lime render too late in the year comes from the southern ocean front of Brittany, France. A pair of straw-bale semi-detached houses had been built in a village only about 100 metres (330') away from the cliff edge, where severe, long-lasting storms make local stone dwellings extremely wet during the winter. Insufficiently carbonated, vulnerable and soft lime plaster didn’t manage to protect the straw from heavy winter rain that had been driven by very strong coastal winds into the exposed wall constantly for two days. As a result, straw in the wall on one house got wet all the way through, so that wet patches appeared on the interior plaster of the building. Almost immediately after the incident, the house started to smell of rot.

The owner of the house took a risk and, rather than trying to dry the walls, he decided to clad the entire house in timber to prevent any further damage. Wet straw, protected by cladding, although covered in soaked plaster, dried out to a satisfactory level in about four months. Decomposition stopped and, since the disastrous storm, the house has been used for its original purpose without any trace of bad smell or straw rot. This shows us that even saturated straw can dry out fairly quickly in the right conditions, resulting in no long-term problems. Timber-clad straw-bale walls on this house are performing in this extreme climate, where outdoor humidity levels rarely drop under 75 per cent relative humidity (see box on page 226) remarkably well.

According to the experience of building with straw in similar conditions in the UK and Ireland, properly made and executed traditional lime render, if sufficiently carbonated (the finishing layer applied in mid September at the latest), has proved able to protect straw from driving rain in most situations. However, in areas of extreme exposure (on gable walls, for example), it is recommended that the plastered straw-bale walls are clad in timber, or at least that some trees are planted in the direction of prevailing winds to protect the walls.

Relative humidity

The amount of water vapour in the air at any given time is usually less than that required to saturate the air. The relative humidity is the percentage of saturation humidity, generally calculated in relation to saturated vapour density.

Relative humidity (RH)
=

actual vapour density × 100 saturation vapour density

Moisture monitoring

In order to bring peace of mind to straw-bale home owners, it is also recommended that they monitor humidity levels in any potentially worrying section of their straw-bale walls. Generally, the area of interest is the bottom of the most exposed wall, as the moisture tends to gather at the baseplate. Good monitoring points are towards the bottom of the bottom bale, just above the baseplate, aligned with the sides of the windows. The data indicating the greatest values of relative humidity or moisture content generally come from the exterior side of the wall (see box right). The exterior monitor is usually buried in the straw, about an inch away from the inner surface of the exterior plaster. The central monitor, placed halfway through the wall, will collect other useful data. This way it is possible to check the depth of any moisture intrusion and thus assess the seriousness of a problem. The central and exterior monitors should never share the same hole in the wall, but they should be as close to each other as possible. It is reassuring for homeowners to know the pattern of moisture activity within a straw-bale wall over the year; this way they can be sure that their straw protection is doing the job it’s supposed to do.

The humidity profile in a wall

The relative humidity (RH) gradually changes through a wall, from the exterior humidity level to the interior humidity level. The closer to the exterior we monitor, the closer we get to the exterior RH value. If we draw a graph showing the change of RH across the wall, the steepness of the graph depends on the difference between the exterior and interior RH and on the water vapour diffusion coefficient (WVDC) of the material. If the render is closed – not breathable or vapour-permeable – it has a high value for its WVDC and the curve over the thickness of the render tends to be steep. That is why we usually find quite different RH levels just behind the render compared with the exterior environment, although this difference gets smaller the more breathable a plaster we use.

Clay plaster generally has the lowest WVDC of all renders and plasters, followed by traditional lime render. Cement has the highest WVDC value, which might suggest that the cement render is the best in protecting the straw from any very humid environment. However, the opposite is in fact true. Cement render, because of its high WVDC, prevents straw from drying out. It has been proven by many years of experience in the USA, where cement renders had been used on straw-bale buildings initially, that it fails to protect the straw from high humidity levels outside the wall despite a high WVDC, because of the shrinkage cracking of cement. Shrinkage cracking happens inevitably during the drying process of cement mortar. Shrinkage cracks are invisible capillaries leading the moisture deep into the cement, making it wet all the way through, and having a disastrous effect on straw, which is then left wet and unable to dry.

There are several options when choosing appropriate monitoring equipment. Depending on your budget and the aim of your research, you can build a moisture probe of your own or buy a factory-manufactured data-logger. These tools measure either moisture content or relative humidity – see page 228 for more about how these relate to each other.

Make your own moisture probe

Canadian and UK research shows that home-made wood-disc moisture probes (see Figure 2), if built properly, are capable of giving the owner of a straw-bale building fairly accurate readings of moisture content in a straw-bale wall. They are cheap and easy to make. They require a disciplined collection of regular data and any kind of cheap wood moisture meter to view the readings. One handheld wood moisture meter will serve all the probes installed.

Figure 2. The wood-disc moisture probe is like the moisture meter’s extended arm. It allows the moisture meter to extend its sensors towards the desired monitoring position within the wall (in this case towards its centre), where it measures the moisture content in a 5mm (³/₁₆") thick, 25mm (1") wooden disc that always has the same moisture content as the surrounding straw in that particular location. The couple of screws and the cap sticking out of the wall need to be perfectly sealed against air infiltration, as do any cracks in the plaster around the protruding cap. © Participants of the ‘Humidity in Straw-bale Walls’ course run by amazonails in France.

Or buy a datalogger

While wood-disc moisture probes are capable of showing directly the moisture content of straw in a measured location, data-loggers (see Figure 3) are generally calibrated to measure relative humidity in the air surrounding the straw. The advantage of a data-logger is in automatic data collection – a data-logger can be set up to store thousands of readings in its memory. The data is automatically collected at regular intervals (from one minute up to an hour). However, the data-logger doesn’t give an immediate picture of measured data, unless it is pulled out of the wall and plugged into a computer. Once displayed on a computer screen, readings show the development of humidity levels in a straw-bale wall over time, in a colourful chart with satisfactory precision.

Home-made wood-disc moisture probes are the most convenient option, as they give the homeowner instantaneous readings and thus an early warning of any moisture problem.

Data-loggers, on the other hand, are best at providing consistent, reliable readings for more in-depth research into humidity development within straw-bale walls.

Before deciding on the best monitoring approach for you, it is useful to have a clear understanding of the difference between relative humidity and moisture content.

Figure 3. A section through a straw-bale wall with a data-logger in the external monitor position.

Moisture content and relative humidity

Hygroscopic materials

Some materials, such as glass, are indifferent to changes of humidity that surround them. If we weigh a piece of glass that has been in an environment with very dry air (in a desert, for example), and then compare its weight after it has been rained on for a while, this will hardly change. Materials such as glass or steel are called ‘non-hygroscopic’ – indifferent to moisture.

Most building materials, however, are hygroscopic, which means that their weight gradually increases if, for example, they are suddenly transferred from the Sahara to West Wales. The extra weight relates to the quantity of liquid water adsorbed (see box above) from the air into the porous structure of the hygroscopic material. Straw, wood, concrete, most renders and plasters are hygroscopic and porous, and the amount of adsorbed water – the moisture content – is closely related to the humidity of the air that surrounds them, the relative humidity. The more humid the air, the more the hygroscopic material will adsorb liquid water.

Adsorbtion by hygroscopic materials

Hygroscopic material surrounded by water vapour in the air attracts water molecules to its surface by tiny electromagnetic forces. The surface is often very rough, with – from the water molecule’s perspective – lots of caves and canyons. The tinier the crack, the more attraction, thus water molecules are trapped in very small capillaries, where they tend to stick together to form miniature liquid drops in a process called adsorption. That is why the material gets wet when surrounded with humid air and that is why the weight of a hygrosopic material changes in these conditions – it is influenced by the presence of liquid water within it. Adsorption: The attraction process initiated by electromagnetic forces between a material surface and water molecules. Absorption: The attraction process initiated by electromagnetic forces between the surface of water and water molecules.

Relative humidity and moisture content in different materials

The exact amount of moisture absorbed by a material depends on the material’s microscopic structure, particularly on the number of pores and their sizes. At the same level of atmospheric humidity, materials such as straw and wood will tend to absorb more moisture than brick, for example, because straw and wood are more sponge-like in the sense of having much more microscopic pores.

The relationship between relative humidity and moisture content is unique for each material, and can be determined by a simple test. A sample of a material is placed in a room with stable relative humidity for a couple of days. Once the sample’s weight is stable, the material’s moisture content is in equilibrium with the room’s relative humidity, and the sample can be thoroughly dried in an oven. The completely dried sample is weighed at intervals until the weight stabilizes, and the difference between the weight of the sample before and after drying will reveal the exact mass of moisture absorbed by the material in the room with that particular relative humidity. Moisture content can be calculated on a wet-weight basis or a dry-weight basis.

Example 1
A 1.11kg (2lb 7oz) bunch of straw that has been in an environment with 50 per cent relative humidity for a couple of days weighs 1kg (2lb 3oz) after thorough drying in the oven. This means that the sample had absorbed 0.11kg (4oz) of water at 50 per cent relative humidity.

The percentage of dry-basis moisture content refers to the weight of water divided by the weight of dry material.

Dry-basis straw moisture content at 50 per cent relative humidity = 0.11/1 = 11 per cent.

The percentage of wet-basis moisture content refers to the weight of water divided by the weight of wet material.

Wet-basis straw moisture content at 50 per cent relative humidity = 0.11/1.11 = 10 per cent

Example 2
The same sample of straw has been placed for a couple of days in an environment with 80 per cent relative humidity. It gained 0.2kg (7oz) of water, hence:

Dry-basis straw moisture content at 80 per cent relative humidity = 0.2/1 = 20 per cent. Wet-basis straw moisture content at 80 per cent relative humidity = 0.2/1.2 = 17 per cent.

Researchers carrying out tests on the same sample of straw repeatedly, each time in an environment with different relative humidities, plot the results of their investigation as a curve, known as a ‘sorption curve’ or ‘sorption isotherm’ (see Figure 4).

Straw decomposition

An important investigation‡ on samples of straw has shown that there is a certain borderline at which straw starts to significantly decompose. And, as UK and Irish farmers might expect, this borderline relates to an environment with very high relative humidity. Even in very humid air, with relative humidity up to 95 per cent, straw decomposes at rate that can’t be noticed; but once relative humidity gets higher than about 98 per cent, straw starts to rot.

What does an environment with 98 per cent relative humidity look like? Air with 100 per cent relative humidity is saturated with water vapour, which starts to show on all material surfaces as liquid water. 100 per cent relative humidity in nature refers to dew, fog or rain. It is not surprising that straw at 100 per cent relative humidity – at this point practically covered all over with liquid water – starts to rot. What is reassuring, however, is that according to the research, it doesn’t significantly decompose until almost at that very point.

Figure 4. The sorption curve, or sorption isotherm, is different for each material and can be usually found among basic building material characteristics, provided by the manufacturer. Sorption isotherms of different straw samples are well known as a result of agricultural research. The thick dotted curve shows levels of safe moisture content and relative humidity. The dashed portion of curve shows the danger zone, with potential moisture problems. The continuous thick black line refers to levels of relative humidity and moisture content that cause significant decomposition of the straw.

The same investigation has shown that very slow straw decomposition, although unnoticeable, can be recorded by very sensitive laboratory equipment in an environment with 84 per cent relative humidity. Straw-bale builders have taken this value as a limit for a healthy condition within a straw-bale wall. It has been generally agreed that data-loggers showing relative humidity readings above 84 per cent in a straw-bale wall are usually referring to a potential moisture hazard.

Measurements of relative humidity have one great advantage. A data-logger measuring relative humidity in the air shows clearly how close the environment is to creating a moisture problem. This level is 98 per cent relative humidity, or, to be on the safe side, a benchmark of 84 per cent relative humidity.

In order to assess the danger of a potential moisture problem with measuring equipment showing moisture content values, rather than relative humidity values (using a home-made moisture probe, for example) one needs to have the relevant sorption isotherm handy. The straw sorption isotherm shows that a handheld wood moisture meter referring to a dry-basis moisture content above 25 per cent, or a wet-basis moisture content above 20 per cent, shows readings in the danger zone. This is because it can be seen from the sorption isotherm that the level of relative humidity in the air surrounding straw with this moisture content is above 84 per cent. The sorption isotherm also reveals that significant decomposition starts when straw has absorbed as much moisture as is equivalent to 39 per cent dry-basis moisture content, or 28 per cent wet-basis moisture content, which relates to 98 per cent relative humidity.

Note: The building industry prefers to use the dry-basis moisture content, which is what the handheld moisture meter would probably be showing. However, most of the moisture meters used by the food industry and agriculture will give wet-basis moisture content readings, although this isn’t a rule. Before using any moisture meter data it is crucial to find out which moisture content the meter had been calibrated for.

When moisture conditions change quickly

Although the difference in meaning between moisture content and relative humidity is that the former relates to the weight of water contained within the material structure and the latter refers to the saturation level of water vapour in the air, the most important difference for practical purposes – that is, when monitoring moisture – is in the speed of reaction of the material to changing conditions.

A data-logger measuring relative humidity will respond to sudden changes in humidity levels within the environment quickly with great accuracy, while it can take hours or sometimes even days for straw to settle to its final weight with a relative humidity change. In fact, the perfect balance between relative humidity and moisture content can never be fully established, because the percentage of relative humidity in a wall changes all the time on a daily basis. In reality, monitors of straw moisture content just can’t react this quickly. Moisture content might almost seem like relative humidity’s slower sibling. Observations show that in an environment with changing humidity levels, the straw moisture content always tends to rather lazily correspond to lower levels of relative humidity, while ignoring its peaks (see Figure 5).

This fact needs to be fully understood when measuring relative humidity by data-logger. Even though the data will peak daily at a certain level, it is actually the troughs that are giving us a picture of the true amount of moisture absorbed by straw in given fluctuating conditions.

What we know about humidity in straw

alt Mould growth and rot occur only when relative humidity levels are at 98 per cent or above (where 100 per cent = rain, fog or dew).

alt Even when relative humidity around straw reaches this level, it has to remain there for a considerable time (2 days) for it to affect the straw bale, because straw absorbs moisture quite slowly from the air. This level of moisture in the air remains at 98 per cent for as long as 2 days only in a rainforest.

alt Straw-bale builders generally take a level of up to 84 per cent relative humidity to be a safe threshold.

alt A dry-basis moisture content of 25 per cent, or a wet-basis moisture content of 20 per cent, is equivalent to a relative humidity reading of 84 per cent.

alt Straw will absorb moisture quickly if it is in direct contact with water – for example if it sits in a puddle, if wind drives rain through unprotected straw, or there is a hole in the building’s roof.

alt Straw will dry quickly once the source of water has been removed and ventilation is good.

alt Even if mould growth has occurred, it will stop and the straw will become healthy again once it has dried out.

Figure 5. Datalogger readings (solid line) in an external position (in straw, a few inches away from the outdoor environment) reflect the pronounced daily fluctuations of outdoor relative humidity. At the same time, measured straw moisture content in the same location (dotted line) keeps relatively stable, corresponding to lower values of relative humidity.

Guidelines for healthy straw houses

To keep straw houses healthy and free from mould growth or rot they need to have:

alt good ventilation – all plasters, renders and surface finishes should be vapour-permeable, and there should be at least 500mm (20") of clear space around each exterior wall;

alt protection from the damp of the earth and rain splashback – straw should be raised up at least 300mm (12") and preferably 450mm (18") from the ground on self-draining non-porous foundations;

alt protection from leaks – a good overhang of at least 450mm (18") on the roof and all maintenance carried out when necessary;

alt protection from driving rain – fully carbonated lime render, timber cladding and rain screens in areas of extreme exposure;

alt protection from condensation – use of vapour-permeable plasters and finishes with good ventilation.

There are some circumstances in which it is good common sense to adapt the best practice guidelines above to suit particular conditions. For example, it is sensible to tile bathrooms and kitchens to protect plastered straw walls from direct water contact. Similarly, timber cladding may be required in areas of extreme exposure outside. However, all these areas will benefit from having as much vapour-permeable material around them as possible, as straw needs good ventilation to remain healthy, so keep tiling to a minimum and use clay or lime plasters in the rest of the room.

If you want to have a truth window with a glass front, or to show off your straw walls through a perspex cover, as long as you have enough ventilation around nearby, this should not present a problem.

Intuition

None of the studies conducted to date have made any groundbreaking discovery. The results support the common knowledge of natural materials that had been, until recently, carried through generations by handing down experience. Straw, like wood or other plant materials, consists mainly of cellulose and lignin, and that is why it is vulnerable to decomposition. It is our common sense and our basic intuition that have advised us for millennia with regard to how to keep organic materials, such as wood, when used in construction, away from water. Nothing can go wrong if we apply the same knowledge to straw.

‡ Summers, M. D., Blunk S. L. and Jenkins, B. M. (2003) How Straw Decomposes, EBNet Straw-bale Test Program, DRAFT 12/8/2003. Available from EBNet database: www.ecobuildnetwork.org/pdfs/How_Straw_Decomposes.pdf

* Detailed analysis of this questionnaire is included in: Wihan, J. (2007). Humidity in straw-bale walls and its effect on the decomposition of straw, thesis written for University of East London School of Computing and Technology, Dagenham, London. Available at www.jakubwihan.com/pdf/thesis.pdf.

Online appendices

The full text of these appendices can be found online at www.greenbooks.co.uk/bwsb-appendices

Appendix 5: How to build rammed car tyre foundations

Recycled car tyres rammed with stone make excellent, low cost foundations with a low impact on the environment. Appendix 5 explains why you should use car tyres to make foundations and guides you through the process for different sorts of soils and building designs.

Appendix 6: Understanding lime products

There are an increasing number of lime plasters and products available on the market and it’s important to be able to choose the right type of lime for your purpose. Appendix 6 gives information on the products available, and explains how to understand what it says on the bag.

Appendix 7: How to make a limecrete floor

Limecrete is the name given to solid-floor slabs, or bond beams, made using lime as a binder rather than cement. This appendix gives a method for making a DIY limecrete floor (although, because different sands have different strengths and properties, it is not recommended that you make a limecrete floor on a large scale without expert advice).

Index

The index from the print edition of the book is included in this ebook for completeness and reference. In the ebook, each index entry points to the top of the page in the printed book where the indexed item occurs. Especially if you are reading with very large font, this may be a few pages before the true location of the item. Because of this you might find it easier to use your ebook reader’s search facility when looking up items.

Page numbers in italic refer to illustrations
*indicates the page where a term is defined

A

adsorption 1*, 2

affordability 1, 2, 3, 4

see also build costs

airtightness 1, 2, 3, 4, 5, 6, 7, 8

air changes/hour 1, 2

tests 1, 2

alcoves and niches 1

allergy sufferers 1, 2, 3

Alligator saws 1, 2, 3, 4

angle grinders 1

artistic creations 1

axes 1

B

bale frenzy 1, 2, 3

bales see straw bales

baling needles 1, 2, 3, 4, 5, 6

baling twine 1, 2, 3, 4

base plates 1, 2, 3, 4, 5*, 6, 7, 8

bathrooms and shower rooms 1, 2, 3, 4

bevels 1

billhooks 1

box beams 1, 2, 3, 4, 5, 6, 7

box frames, structural 1, 2

bracing 1, 2, 3, 4

build costs 1, 2, 3, 4, 5, 6, 7

building inspectors 1, 2, 3, 4, 5, 6

Building Regulations and Codes 1, 2, 3, 4, 5, 6

building techniques

Greb method 1*

hybrid techniques 1

infill method 1*, 2

keeping straw dry during

construction 1

load-bearing construction 1*, 2, 3

Mortared Bale Matrix 1*

pre-fabrication 1, 2, 3, 4

C

cables 1

capillary breaks 1, 2*, 3, 4

car tyres 1, 2, 3, 4

carbon footprint 1, 2, 3*, 4

carpentry for load-bearing designs 1

costs 1

durable timber 1, 2

engineered timbers 1

lintels 1

nails 1, 2

ring beams 1

cavity wall system 1, 2, 3, 4

cement mortars, plasters and renders 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

chainsaws 1, 2, 3

chicken wire 1, 2, 3

cladding 1, 2, 3, 4, 5

claw hammers 1

clay mortars, plasters and renders 1, 2, 3, 4, 5

making your own clay plasters 1

ready-made clay products 1

testing clays 1

vapour permeability 1, 2

weatherproofing 1

see also plastering

clay paints 1, 2

clay subsoil 1, 2, 3, 4, 5, 6

clothing, protective 1

CO₂ pollution 1, 2

cob buildings 1, 2, 3

cold bridges 1, 2, 3, 4

combination squares 1

commercial buildings 1, 2

Community Infrastructure levy 1

compression 1, 2, 3, 4, 5, 6, 7, 8, 9

amount of 1, 2

compressed enough, judging 1

see also pre-compression

compressive frame method 1, 2, 3

concrete foundations 1, 2

condensation 1, 2, 3, 4, 5, 6

corner braces 1

corners 1, 2, 3, 4, 5

cranes 1, 2, 3, 4

crowbars 1

cupboards 1

curves and circles 1, 2

D

damp 1, 2, 3, 4, 5, 6

causes 1, 2

damp-proof course (dpc) 1, 2

Darby 1

dataloggers 1, 2

design

bale plans 1

foundations 1

general design principles 1, 2

roofs 1

simplicity of design 1, 2, 3

timber framework design 1, 2

windows and doors 1

doors see windows and doors

draughts 1, 2, 3

drip moulds 1, 2, 3

durability 1, 2, 3, 4, 5

E

earth plasters 1

earth trips 1, 2

electrical installations 1, 2

elevations 1, 2

embodied energy 1*, 2, 3, 4, 5, 6, 7, 8

energy ratings 1

environmental benefits 1, 2, 3, 4

environmental impact, low 1

extensions 1, 2

F

fire hazards 1, 2, 3, 4

fire resistance 1, 2, 3, 4

fittings and fixings 1, 2, 3

fixing posts 1, 2, 3*, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19

flexibility 1, 2, 3, 4, 5, 6, 7, 8

flitch beams 1*

floor plates 1, 2*, 3

floors

insulation 1, 2

limecrete 1, 2

foamglas 1, 2, 3, 4, 5

foundations 1, 2, 3

capillary breaks 1, 2*, 3, 4

concrete 1, 2

designing 1, 2

engineering input 1

foundation loads 1

good bearing soil 1, 2, 3

gravel bed foundations 1

gravel trench foundations 1, 2

insulation 1, 2

low-impact 1

natural 1

piers/pillars 1, 2, 3, 4, 5

piles 1, 2, 3

raft foundations 1

strip 1, 2*, 3, 4

subsoil 1, 2, 3

framework constructions

lightweight frame method 1

off-site construction 1, 2

pinning 1

windows and doors 1

see also infill method

Frequently-Asked Questions 1

frost 1, 2, 3

frost heave 1

fuel costs, savings in 1, 2, 3

G

gabions 1

garden strimmers 1, 2

glulam beams 1

gravel bed foundations 1

gravel trench foundations 1, 2

Greb method 1*

greenhouse gas emissions 1

gypsum plasters and renders 1, 2, 3

H

hand saws 1, 2

hay 1, 2

hay knives 1

hay-fever 1, 2

hazel pins 1, 2, 3, 4, 5, 6, 7, 8, 9

hazel staples 1, 2

hazel stubs 1, 2, 3, 4, 5

health benefits 1

hedge clippers 1

hedge cutters 1, 2

hemp 1, 2, 3, 4

hessian 1, 2, 3, 4

high wind loading 1

history of straw bale building 1

humidity 1, 2, 3

moisture monitoring 1, 2, 3

regulators 1, 2, 3

relative humidity 1, 2, 3*, 4

hygroscopic materials 1

I

I beams 1

infill method 1*, 2

compressive frame method 1, 2, 3

drawbacks 1

lightweight frame 1

off-site frame construction 1

post material 1

posts, location of 1

posts, size of 1

timber framework design 1, 2

insulation 1, 2, 3, 4

acoustic 1, 2, 3, 4

floors 1, 2

foundations 1, 2

insulating an existing house with straw 1

roofs 1, 2

U-values 1, 2, 3, 4, 5, 6, 7

J

joists 1, 2, 3, 4, 5, 6

jumbo bales see mini Hestons

jute mesh 1, 2, 3, 4, 5

K

kerto beams 1

kitchens 1, 2

L

land, availability of 1

leca 1, 2

lifespan of straw-bale buildings 1, 2, 3

see also durability

light switches and sockets 1, 2

lightweight frame technique 1–1

lime burning 1, 2

lime mortars, plasters and renders 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14

aftercare 1, 2

hydraulic limes 1, 2, 3

making lime plaster 1

mould inhibition 1

see also plastering

lime putty 1, 2, 3, 4, 5, 6

limecrete 1, 2, 3

limestone 1, 2

limewash 1, 2, 3, 4, 5, 6, 7

linseed oil 1, 2

lintels 1, 2, 3, 4

load-bearing construction 1*, 2, 3

carpentry 1

construction drawings 1

with jumbo bales 1

pre-fabricated parts 1

structural performance 1

in a wet climate 1

windows and doors 1

see also compression

log books 1

lump hammers 1

M

made up ground 1, 2*

mainstream construction 1, 2, 3, 4, 5, 6, 7, 8, 9

maintenance 1

metal 1, 2, 3, 4, 5, 6, 7

mice and rats 1

mini Hestons (jumbo bales) 1*, 2

moisture damage 1, 2

see also damp

moisture probes 1

mortar 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13

Mortared Bale Matrix 1*

mortice and tenon joints 1, 2

mould 1, 2, 3, 4

MVHR (mechanical ventilation and heat recovery) 1

N

Nebraskan method 1, 2

see also load-bearing construction

nitrates 1, 2

noggins 1, 2, 3, 4, 5, 6

O

off-the-peg buildings 1, 2

overhangs 1, 2, 3, 4, 5

P

paddle mills 1

partnership working 1, 2, 3

PassivHaus 1, 2, 3, 4, 5, 6, 7

pea shingle 1

persuaders 1, 2, 3, 4, 5

pier/pillar foundations 1, 2, 3, 4, 5

piling 1, 2, 3*

pinning 1

external 1

frame designs 1

internal 1

windows and doors 1, 2

plan view 1

planning permission 1

plastered straw panels, pre-fabricated 1

plastering 1, 2

curves, shaping 1

flat walls 1, 2

machinery and tools 1

sand type and quality 1

technique (clay plasters) 1

technique (lime plasters) 1

wall preparation 1, 2, 3

plasters and renders

aftercare 1

breathability 1, 2

cement 1, 2, 3, 4

clay 1, 2, 3, 4, 5

coloured 1

cracks in 1, 2, 3

drying out 1, 2

earth 1

fire resistance 1

flexibility 1, 2, 3

gypsum 1, 2, 3

lime 1, 2, 3, 4, 5, 6, 7, 8, 9

long straw mix 1, 2

maintenance 1

making 1, 2

patching 1

quantities and costs 1

plumbing 1, 2, 3

post and beam method see infill method

pre-compression 1, 2, 3, 4, 5, 6

methods 1

pre-fabrication 1, 2, 3, 4

props 1, 2, 3, 4, 5, 6

Q

quad bales 1*, 2

quicklime 1, 2, 3, 4

R

radiators 1

raft foundations 1

rafters 1, 2, 3, 4, 5, 6, 7, 8

rain 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11

ratchet straps 1, 2, 3, 4, 5, 6

reed matting 1

renders see plasters and renders

ring beams 1, 2, 3, 4, 5*, 6, 7, 8, 9

fitting 1

see also base plates; floor plates; roof plates; wall plates

roof plates 1*, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13

roofs 1

circular/curved 1, 2

construction 1, 2

coverings 1

cut roofs 1, 2

design 1

dropping the roof 1

duo-pitch 1, 2

flat 1

hipped 1, 2, 3, 4

insulation 1, 2

load 1

mono-pitch 1, 2

overhangs 1, 2, 3, 4, 5

planted 1, 2, 3

pre-fabrication 1

temporary 1, 2

truss roofs 1, 2

tying down 1, 2, 3

rye 1

S

safety on site 1, 2, 3

sandy soil 1

saws 1, 2

scaffolding 1, 2, 3, 4, 5

screwdrivers 1

self-builders 1, 2, 3

services, installation of 1

settlement 1, 2, 3, 4, 5, 6, 7, 8

settlement gaps 1, 2, 3

sgraffito 1

sheepswool 1, 2

shelves 1, 2

shingles 1, 2, 3

‘sick-building syndrome’ 1

silicate paints 1

slaking 1*

slate damp proof course 1

sledgehammers 1

sloping sites 1, 2, 3

smartply 1*, 2, 3, 4, 5

soil heave 1*

soil stacks 1

soil types, foundations and 1, 2, 3

solar panels 1

sorption isotherm 1, 2, 3

sound insulation 1, 2, 3, 4

spanners and wrenches 1

splashback 1, 2, 3

stanley knives 1

staples 1, 2, 3

stone pile foundations 1

strapping see tie-downs

straw

age of 1

annual production (UK) 1

characteristics 1, 2, 3

decomposition 1

drying out 1, 2, 3

flexibility 1

length of 1, 2

organic/non-organic 1

rotting 1

working with 1

straw bales 1

bale plans 1, 2

bale standard 1

compression see compression

construction-grade 1, 2, 3, 4, 5, 6, 7, 8

costs 1, 2

curving 1, 2

customising 1, 2, 3, 4

dressing (trimming) 1, 2

flakes/wedges 1, 2, 3

half bales 1, 2

laying 1, 2, 3

mini Hestons 1*, 2

moisture content 1, 2, 3, 4, 5

notching 1, 2, 3, 4, 5

orientation of the straw 1, 2

pinning 1, 2

quad bales 1, 2

size and length 1

storage 1, 2

three-string bales 1

two-string bales 1, 2, 3

straw-bale building

beneficial qualities of 1

empowerment potential 1, 2

history of 1

techniques see building techniques

structural performance 1, 2

stucco 1, 2, 3

sustainability 1, 2, 3

sweet chestnut pins 1

T

tape measures 1

tarpaulins 1, 2, 3

temporary buildings 1

thatch 1, 2, 3, 4

thermal efficiency 1, 2, 3, 4, 5, 6, 7, 8, 9

thermal mass 1

threaded rod 1, 2

tie-downs 1, 2, 3, 4, 5

timber folding wedges 1, 2, 3

timber frame method see infill method

timber pile foundations 1, 2, 3

tools 1, 2

power tools 1, 2

trusses 1, 2, 3, 4, 5, 6, 7

truth windows 1, 2, 3, 4, 5

U

U-values 1, 2, 3, 4, 5, 6, 7

V

vapour permeability 1, 2, 3, 4, 5, 6, 7, 8, 9

ventilation 1, 2, 3

vermin 1

W

wall plates 1, 2, 3, 4, 5, 6, 7

wall ties 1

walls

in load-bearing construction 1

raising 1

repairs 1

straightening 1

tie-downs 1, 2, 3

see also plastering; settlement

water pipes 1, 2, 3

water vapour diffusion coefficient (WVDC) 1

wattle-and-daub buildings 1, 2

weatherproofing 1, 2, 3, 4, 5, 6, 7, 8

wet ground, building on 1, 2, 3, 4

windows and doors 1

creating extra windows 1

design 1

fixing posts 1, 2, 3, 4, 5, 6

in framework constructions 1

headers 1, 2

in load-bearing constructions 1, 2, 3

pinning 1, 2

plastering around 1, 2, 3, 4

roof windows 1

secondary glazing 1

settlement gaps 1

sills 1, 2, 3

sizes 1

structural box frames 1

sub framing 1, 2

truth windows 1, 2, 3, 4, 5

weatherproofing 1, 2, 3, 4

winter wheat 1

woodfibre board 1, 2, 3, 4, 5, 6

About the Author

Barbara Jones is well known internationally as a pioneer of straw-bale building and designer of innovative foundations (including using car tyres), and has been designing and building with natural materials for 20 years, and has worked on literally hundreds of different buildings. Barbara was part of the famous Strawbale Tour of the USA in 1999, organized by Judy Knox and Matts Myrhman, that brought European straw-bale pioneers to the USA, and she has worked with Bill and Athena Steen, Kelly Lerner and Becky Bee.

Barbara has also developed a uniquely empowering method of practical training on building sites, for which she is in demand throughout Europe. She is the Principal of the School of Natural Building, which she established in order to provide high quality training and experience in the practical and theoretical uses of natural materials, encouraging women in particular to embark on this very rewarding career path. Given a Lifetime Achievement Award by Women in Construction in 2011 and a Woman of Outstanding Achievement Award in 2009, she continues to deliver high-quality buildings that break conventional boundaries.

Her practical skills include straw-bale building, clay and lime plastering, carpentry and roofing, and she runs her own company, Straw Works, designing natural buildings of all shapes and sizes, as well as supporting self-builders. In 1989 she founded Amazon Nails, a women’s roofing company, and in 2007 she developed this into a social enterprise called amazonails, which closed in 2011 when Straw Works began. Barbara now works with many Associates, continuing her ground-breaking work. Her current projects include designing Passivhaus structures, visitor centres and terraces of load-bearing houses, working with self-builders, housing associations and community groups, and consulting with architects. Her focus is to make building accessible to everyone, and to develop designs and building practices using natural materials that are affordable for ordinary people.

Find Straw Works on Facebook! www.strawworks.co.uk @strawworks

All buildings cited in this book (except where stated) were designed by or in consultation with Barbara Jones, and/or were built by Barbara Jones or amazonails/Straw Works trainers, or with the help of their on-site training. Where ‘we’ is used in the text in the context of building practice, it refers to Barbara Jones’s team.

Copyright

Published by

Green Books
An imprint of UIT Cambridge Ltd
www.greenbooks.co.uk

PO Box 145, Cambridge CB4 1GQ, England
+44 (0)1223 302 041

Copyright © Barbara Jones 2015. All rights reserved. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no part of this book may be reproduced in any manner without the prior written permission of the publisher.

First published 2015

Barbara Jones has asserted her moral rights under the Copyright, Designs and Patents Act 1988.

Front cover photograph © Rae Parkinson
Back cover photographs left and centre © Rae Parkinson, right © Barbara Jones.

Design by Jayne Jones

Illustrations on pages 36, 54, 68, 69, 70, 72, 92, 100, 114, 115, 130, 132, 133 by Juliet Breese of deaftdesign.co.uk

ISBN: 978 0 85784 227 5 (hardback)
ISBN: 978 0 85784 228 2 (paperback)
ISBN: 978 0 85784 229 9 (ePub)
ISBN: 978 0 85784 230 5 (pdf)
Also available for Kindle

Disclaimer: the advice herein is believed to be correct at the time of printing, but the author and publisher accept no liability for actions inspired by this book.

About Green Books

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