There are many reasons we like to build with straw bales: we have listed the main reasons we like straw bale as a construction material below. There is also a wealth of information available on straw bale building on line; we won’t repeat all that good information but we can point you in the right direction should you be interested in further reading.
- Durability – when built right this method of construction can be exceptionally durable. In New Zealand the bales in straw bale houses are generally not load bearing. Timber frames or posts carry the whole load of the roof or upper floors. In an earthquake situation the cracks that will inevitably appear in the earth or lime plastered walls (as with nearly any plaster system) can be repaired with out the need to replace whole wall sections. They are less likely to delaminate from the substrate due to the particularly good key of a furry straw bale. Fire resistance tests have also been performed on straw bale walls in both the US and Australia and they achieved a 90 minute rating (even unplastered bales smoulder rather than burn due to the lack of oxygen in a compressed bale). Simply put as long as the bales themselves are kept below about 80% humidity they will last indefinitely. As far as a tried and true cladding system nothing else even comes close to earth and lime.
- Thermal performance – whole wall system performance of R4.5 to R6 depending on bale thickness, wall plastering system and structural system. This compares to the R2.26 whole wall performance of a standard 90mm stud frame with R2.8 batts (under clause H1 of the NZ building code walls are required to only achieve R1.9-2.0 in non-solid construction)
- Aesthetics – The thickness of half a metre walls gives a very secure feel and the rounded, deep window plastered reveals reflect light deep into the interior particularly is lime washed, while creating a place to sit.
- Indoor environment – Using earth plasters as a base coat provides for exceptional humidity regulation 1.
- Environmental factors – where to start! As far as building materials go we can’t think of many more benign than straw bales. The earth plasters used in straw bale best practice are also extremely low impact, particularly if bales are sourced locally, which is usually the case. All buildings have a finite life, and a ‘grave’. When a straw bale building comes to the end of its life there a few far fewer materials that need to be disposed of in a landfill. The walls themselves can simply be returned to the soil from whence they came as they are 100% biodegradable. As long as all the other elements in a straw bale building have a similarly low thermal conductivity, a straw bale building will use far less energy, and therefore produce far less CO2 than a conventional building built to ‘code’. Fewer products need to be used in a straw bale building and earth, lime and straw are all non-toxic. In addition, both straw and timber sequester carbon when they are used as building material.2
- Economic factors – Straw is in itself a by-product, not the primary crop. Using more local materials in construction can only be a good thing for those involved in producing them.
There are also some common sense rules of thumb when it comes to straw bale building. We believe in a good hat (eaves) and boots (raised foundation) for bale walls and strongly caution against parapet walls. Permeable and softer plasters (lime or earth) should always be used in preference to relatively impermeable and more brittle (cement) plasters; we have checked our construction methods in our WUFI hygrothermal modelling software. Passive solar design is also a no brainer, and we take these principles into account wherever possible.
1 Evrard, A (2013) ‘Thermal inertia and moisture regulation of straw bale buildings with earth plasters’, PLEA2013 – 29th Conference, Sustainable Architecture for a Renewable Future, Munich, Germany 10-12 September 2013
2 Alcorn, A & Donn, M (2010) ‘Life Cycle Potential of Strawbale and Timber for Carbon Sequestration in House Construction’, Victoria University of Wellington