Straw bale: An enduring building technique
Whether you’re planning a sleek or rustic-looking house, straw bales are a low cost and sustainable building material with flexibility.
What has an accredited fire resistance of two hours, is more resistant to vermin and rot than most timber, and provides insulation values that comfortably exceed the most stringent demands of the building code? The answer is rendered straw bale – and as a sustainability bonus, when the building’s life is over, most of its walls can be turned into mulch.
The rise of straw bales
Straw bale building has been around for over a century. The oldest surviving buildings date back to the early twentieth century, when pioneering farmers in the Nebraska Sandhills facing a dearth of trees found the local soil made poor building material. The most famous historical example of a straw bale building is a church, built in 1928, listed in the US National Register of Historic Places – it is still in use today.
The modern straw bale movement dates from the early 1980s, when the rediscovery of bale building was fuelled by renewed interest in ecological building. In the early 1990s a book called The Straw Bale House by Athena Steen, Bill Steen and David Bainbridge became a manifesto for the growing movement towards healthy, affordable, environmentally responsible building.
Straw bale buildings range from tiny houses to large commercial facilities and their appearance ranges from slick to rustic. Straw bale structures can be built onto existing buildings and joined to other construction systems. Walls can be finished as smoothly and evenly as a concrete wall or as lumpily as rough stonework; corners can be sharp arrises or smooth curves. A straw bale house can look “alternative” or blend in completely with neighbouring suburban properties.
Internationally, straw bale buildings like the Austrian S-House meet the “Passivhaus” standard – the highest accredited energy performance rating in domestic construction. [Ed note: see Sanctuary 17 for more on the Passivhaus (Passive House) standard.] In Australia, most straw bale buildings are individual dwellings and owner-built, but straw bale buildings also function as community facilities, eco-tourism developments, wineries and educational buildings. These include the Australian Wildlife Hospital in Beerwah, Queensland, the Adelaide Zoo Environmental Education Centre, major winery buildings (some built using jumbo bales), eco-villas in South Australia’s Flinders Ranges, the Hall of Conciliation in Ganmain, New South Wales, and houses ranging from huts to mansions.
The mainstream building industry is beginning to recognise the material’s value as a general purpose, high performance building medium.
Most straw bale buildings use the bales to create thick infill panels in timber or steel framed structures and rarely use the bales to take any major structural loads. However, straw bales can be used like giant bricks to not only support their own weight as they do in framed construction, but also roof structures as they did in the old Nebraskan examples. Used in this way, there are more design limitations (particularly on the size of windows and other openings) than when major loads are carried by structural frames.
Building with straw bales
Straw bales weigh about the same as five or six conventional bricks. In a typical domestic construction they are stacked in a similar way to brick walls around a structural frame, all secured to a concrete slab. They are sometimes pinned through with vertical metal or bamboo rods but are invariably compressed after stacking – usually with high tensile wire that wraps around the height of the wall or at every six to seven bales high. Trimming, cutting and shaping is done with a chainsaw. A wire or perforated metal mesh is pinned and sewn to the bales and rendered with cement or earth to a thickness of 30–40 millimetres.
When building with straw bale wall thickness is determined by the bale size, generally 900 x 450 x 350 millimetres. Although bales can be laid on their edge to make a thinner wall, the bale laid flat is typical and, with render, produces a wall thickness of just over 500 millimetres. On a rectangular slab of seven by 10 metres straw bales give a floor area of 54 square metres, compared to a brick veneer wall that results in a floor area of around 60 square metres. However, in this example the slab only needs to be made one metre longer to get the same floor area in straw bale.
The danger period for straw bales is during construction and it is important to maintain a tidy building site and to sweep up and remove all loose straw from around the construction area. Straw bales rot quickly and disintegrate if they contain too much moisture, so keeping them dry during construction and measuring their moisture content regularly during the building process is vital. Compacted bales do not burn easily because they don’t allow air to enter freely enough to support combustion. Because of this, and the fact that the bales are completely sealed and covered by render, they have very high fire resistance. CSIRO testing has demonstrated that rendered straw bale construction can satisfy Australian standards for building in moderate risk bushfire prone areas. Their use in more extreme fire areas can be approved on a case-by-case basis. Loose, dry straw burns well, however, and it is during construction that damaging fires can occur.
Straw bale buildings also limit noise levels during construction. Notwithstanding the occasional whine of a chainsaw cutting and trimming bales, the physical process of building with straw is quieter than with conventional materials. Communication on building sites is usually achieved by shouting but on a straw bale build normal conversation is possible, and in a self-build project a sense of community emerges. Inside a finished straw bale home, the noise insulation provided is excellent. Their acoustic properties also contribute to excellent spatial sound quality appreciated by musicians – not echoey, like typical plasterboard walls, and not totally “dead” either.
Straw bale buildings are often built as passive solar or climate-responsive homes, making use of high insulation levels to contain winter heat captured through glazing on the sunny side of the house. Because straw bales provide such good insulation it is important to maintain a high level of insulation of the rest of the building’s skin. Areas of poor insulation like single glazed windows create cold spots that suck heat out of the building in winter and, in hot weather, act as hot spots that allow heat to enter the structure.
There is little thermal mass in straw, which is essentially tubes of air contained in natural cellulose, but the render that has to be applied to the walls – it may be cement, lime putty or earth – does add a layer of high mass material that provides a substantial and useful amount
of thermal mass.
Indeed, Australian straw bale homes are rarely cold, but without careful design they may become too warm. Once a house is well insulated the need for solar orientation for heating becomes less critical, because the amount of heat generated by occupation and everyday use of the home can be sufficient to keep the whole house warm. With their capacity to hold heat, the challenge with straw bale buildings switches to cooling and letting warm air out of the house – as well as finding a source of cooler air to bring into the building.
The benefits of bales
Unless space is at a premium, straw bale is a good option for its thermal performance, visual appearance, tactile and acoustic qualities. Straw bale structures can bring an overall ambience to a building that other materials cannot provide, with details like curving reveals which generate a rich, low glare quality of light that simply can’t be achieved with brick veneer. Recently, straw bale pioneers John Glassford and Susan Wingate-Pearse have also developed 150 millimetre thick prefabricated straw bale panels to add additional options and flexibility to straw bale construction.
After designing and working with straw bale buildings for over 15 years, I have become more and more enamoured of the merits of the material. It is ideal for owner-builders, not just because it can keep costs down, but also because it offers the opportunity to customise details of the house during the building process, for instance by adding niches or sensuous curves to otherwise featureless walls. At the other end of the spectrum, straw bale can be used to create an elegant, more formal aesthetic. In design terms, straw bale walls can be celebrated for their funkiness, or treated as mute, smooth surfaces in stark, modern architectural compositions. It is an adaptable and responsive construction medium that has much potential still to be realised in modern building design and sustainable development.
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