There are so many reasons to build with straw bales: we love them! We have spent the last 15 years studying, learning, experimenting, modelling, designing and building ecological, low energy buildings, and we decided long ago that strawbale was the most appropriate construction type for our region.
Straw is the stalks, of grain grasses such as wheat, rice, barley, and rye. It was used by Native Americans as structural elements before the European settlers learned the techniques of stacking bales like bricks to form homes more than 100 years ago. During the twentieth and twenty first centuries, straw bale construction has become an increasingly common method of building warm, environmentally sound buildings.
There is also a wealth of information available on straw bale building online; we won’t repeat all that good information but we can point you in the right direction should you be interested in further reading.
Here are some of the main reasons we like straw bale as a construction material:
- 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 can be repaired with out the need to replace whole wall sections. Studies following the Kaikoura earthquake demonstrated the resilience of these buildings: many buildings that undergone severe shaking escaped with only superficial cracks. Fire resistance tests have also been performed on straw bale walls in the US, the UK and Australia and they achieved a 90 minute rating (even unplastered bales smoulder rather than burn)
- Thermal performance – whole wall system performance of R5 to R7 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 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 A UK research paper found that a small strawbale house sequestered 43 tonnes of atmospheric CO2.
- Economic factors – Straw is in itself a by-product, not the primary crop. Here in Central Otago straw of good quality can be sourced relatively easily, and using more local materials in construction can only be a good thing. Clay is dirt cheap (if you’ll excuse the pun) but the cost comes in getting it on the wall.
- Natural plasters are inherently airtight and leak free if the connections are detailed and built correctly
- Strawbale walls have a great acoustical characteristics and are sometimes used in recording studios because of this
- Unlike hay, which contains grain, straw does not attract mice or insects and will make an inhospitable home for them if sufficiently dense and well plastered.
- Straw does not rot unless exposed to excessive humidity for prolonged periods. We have modelled our wall construction in WUFI to verify longevity and to give building officials confidence in granting consent. Strawbale is now a recognised and accepted construction method in most regions of New Zealand.
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 plasters (lime or earth) should always be used in preference to impermeable (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