Key Properties of Straw-bale Building
Fire Resistance - sealing straw with lime or clay plasters ensures fire-, pest- and rot-resistance
Vapour-permeable/Breathable - for excellent indoor air quality and longevity of building fabric
Exceptional Insulation Performance - naturally super-insulated, due to the air trapped in the thick straw layer
Better-than-zero-carbon - a net sequestration of atmospheric carbon in a straw bale
Flexibility - used in a range of applications: load- bearing, infill, or pre-fab systems
Affordable - Build cost similar to conventional construction, but running costs much lower
Straw-bale buildings often come with the unfounded stigma of issues such as fire, rodents and rot. Straw walls have been tested across the world including the UK, Australia and the USA and found in all cases to exceed the fire standards of traditional construction types. This is mainly because fire needs oxygen to continue to burn, and a compressed bale has had the oxygen removed. For the same reason rodents find it difficult to burrow through the straw which is of little interest to them once the grain has been removed as it does not provide a nutritional food for them. Whilst it is essential to keep the straw dry prior to, and during, construction, once the external render or cladding has been installed it does not become an issue as the straw’s hygroscopicity is its own moisture management system.
Straw-bale construction is well suited to both residential and commercial contexts, and self- builders find it easy to learn the basics with a little motivation and energy.
Straw-bale’s vapour-permeability, thermal performance, ease of construction, and typical finishes, all make it an ideal material for new build, or extensions to existing buildings. It is an inherently sustainable solution and provides healthy, warm and biodegradable buildings with low running costs.
Different Uses and Contexts
Load-bearing - Straw-bales can be used a bit like giant lego bricks; a load-bearing straw bale building is constructed by placing the bales in an interlocking pattern, and the imagination of the designer is the main limitation. Straw allows for both simple and complicated structures to be designed and built with the same performance characteristics.
Timber frame - Straw bales can be notched around a timber frame as a non-structural alternative.
Pre-fabrication - For larger, commercial-scale projects, off-site construction methods of pre-compressed straw panels bring predictability to the schedule. The advantages of straw panel systems can outweigh the additional design work needed to resolve the thermal bridging and air tightness challenges which any prefabricated assemble-on-site system brings.
Finishes for straw-bale should be vapour permeable, and include; lime or clay plasters, lime render, and timber cladding. Internally a clay plaster is most commonly applied leaving a healthy and breathable finish. Research shows that the presence of a lime render finish significantly reduces the propensity of nearby straw to rot. Finishes on straw bale must be well maintained throughout the building’s life, as they are important for fire-, rot- and pest resistance.
Materials and Process
Straw is an agricultural by-product that is used for a limited number of purposes with more than 60% of it ploughed back into the soil as a waste material. Straw is the dry stalk of a cereal crop that has had the grain and chaff removed. Bundled into a bale and compressed evenly, straw can be designed to support a roof structure.
Straw is an ancient building material that has been used for thousands of years, often mixed with binders such as clay or dung to build walls. The use of straw in bale form is relatively modern and came with the invention of the baling machine. The earliest documented straw-bale building was built in the 1890s, and examples now exist across the world.