Straw has been used as a building material for centuries for thatch roofing and also mixed with earth in cob and wattle and daub walls. Straw bales were first used for building over a century ago by settlers in Nebraska, USA, shortly after the invention of baling machines. Here is a Performance Summary to help you decide if you want to use Straw Bale in your next project.
Finished strawbale walls are invariably rendered with cement or earth so that the straw is not visible. The final appearance of rendered straw bale can be very smooth and almost indistinguishable from rendered masonry, or it can be more expressive and textural.
The structural capability of straw bales is surprisingly good. In the loadbearing (‘Nebraska’ style) strawbale method, walls of up to three storeys have been constructed. Most strawbale construction uses a frame of timber or steel for the building structure to comply with the Building Code of Australia (BCA).
There are many examples of multi-storey buildings in framed strawbale construction.
Straw bales have very low thermal mass, being composed, by volume, mostly of air. However, the cement and earth renders typically used on straw bales result in finished walls having appreciable thermal mass in the masonry ‘skins’ either side of the insulated straw core. With earthen renders a thick render skin of up to 75mm can be achieved, providing significant thermal mass.
Straw bales have excellent insulation properties, among the most cost effective thermal insulation available.
• Centimetre for centimetre, straw has similar insulation value to fibreglass batts.
• A typical strawbale wall has an R-value greater than 10.
• Dollar for dollar, the insulation value of a strawbale wall exceeds conventional construction.
The design goal in any structure must be to complement the insulation performance with the performance of the rest of the building. Thus, it is essential to insulate roofs and windows to maintain the overall performance of a strawbale building.
Straw bales also provide cost effective sound insulation, which contributes to the liveability of this kind of construction and can be quite marked. Even walking into the space created by an unfinished strawbale structure, one can appreciate the quietness and hear the difference from conventional buildings.
Straw bales are tightly packed and covered with a skin of render. Fire can’t burn without oxygen, and the dense walls provide a nearly airless environment, so the fire resistance of compacted straw is very good. Laboratory fire tests conducted at the Richmond Field Station in 1997 by students at the University of California Berkeley rated a strawbale wall at two hours. Strawbale homes survived Californian bush fires that destroyed conventional structures. A fire that started in a Whyalla strawbale building did not take hold, as it would have in a conventional structure, and the damage caused was repaired with the cost covered by insurance.
Because the dense walls provide a nearly airless environment, the fire resistance of compacted straw is very good.
Tests undertaken by the CSIRO on behalf of Ausbale and the South Australian fire authority in July 2002 on three kinds of standard size rendered straw bales (earth; lime and sand; lime, sand and cement) produced a two hour fire rating. Samples were subjected to a simulated bushfire front with a maximum heat intensity of 29kW per square metre — an accepted standard under AS 3959, Construction of buildings in bushfire prone areas. Since the 2009 Victorian bushfires new standards have been set for testing materials for bushfires.
Straw bales can burn (slowly) but the potential for fire to take hold can be minimised. Try to cap walls by continuing render over the top of the bales and plates so that an inadvertent flue effect does not support combustion by bringing in air to fuel the fire.
Strawbale structures attract interest: sometimes that interest is not positive. Maintain vigilance during construction and ensure that loose straw and sawdust or other combustibles are not left in or around the structure at any time. Some trades use fire, such as oxy cutters and welders. Take special care to manage activities that are of high fire risk.
A completed wall has excellent resistance to vermin and the normal termite protection measures required in the BCA are generally sufficient. However, prevent infestation of mice during construction when the bales are relatively unprotected. Most strawbale construction is coated with plaster or render which is adequate to keep animals out, and if they do manage to get inside, densely packed straw makes it hard for them to navigate through the space. During construction, consider using traps and baits to ensure the finished structure is sound and vermin-free.
Durability and moisture resistance
Provided the straw is protected and not allowed to get waterlogged, strawbale buildings may have a lifetime of 100 years or more (Amazon Nails 2001).
The most detrimental factor affecting strawbale wall durability is long term or repeated exposure to water. After two or three weeks the fungi in bales produce enzymes that break down straw cellulose if the moisture content is above 20% by weight. The best way to prevent rot in a finished structure is to create a waterproof, breathable wall. The survival of historic strawbale structures in Nebraska and Alabama demonstrates their durability in climates with variable moisture and temperature.
Rice straw can make more durable bales than wheat straw because its high silica content improves rot resistance. Rice straw bales are slightly denser, and therefore heavier, but can otherwise be treated the same as straw bales.
Toxicity and breathability
The natural materials of strawbale construction are safe and biodegradable. Some people are allergic to the dust created during strawbale building. No toxic fumes are released when straw burns and there is no toxic end to the strawbale construction cycle. Strawbale walls have good breathability, allowing air to slowly permeate the structure without moisture penetration. Earthen and some earth-lime renders may allow walls to ‘breathe’ better than cement render, especially renders that have a high cement to sand ratio.
Straw is a waste product; it cannot be used for feed, like hay, and much of it is burned at the end of the season. Using straw for building reduces air pollution and stores carbon. The straw left over from building can be used as mulch so that, overall, there is minimal waste from using the material (see Waste minimisation).
Straw bales contain a high level of renewable material. Straw has a six month growing cycle and is biodegradable. To be sustainable in the long term, straw would need to be grown in a way that maintained the soil quality and ecological integrity of its provenance.
Fertilisers and pesticides associated with industrial farming practices increase the environmental impact of straw bales, as does the use of baling twine made from petroleum products.
Straw bales are inherently low in embodied energy but most are produced by fossil-fuelled machinery, tied together by plastic twine and transported long distances — increasing their embodied energy. Strawbale walls often require concrete footings that add further to the energy cost of their construction.
Rice straw is a by-product of irrigation agriculture that changes the flow and water balance of catchments in Australia’s major river systems. Wheat straw is less water-intensive.
Greenhouse gas emissions associated with straw bales are very low. One tonne of concrete requires more than 50 times the amount of energy in its manufacture than straw. Using straw for building stores carbon that would otherwise be released but the amount sequestered per dwelling is relatively small.
Straw’s primary value is as an insulating material that enables houses to use less energy and have lower carbon dioxide emissions over the building’s life.
Buildability, availability and cost
Strawbale construction rates highly for buildability because it can be very straightforward and is well suited to workshop and volunteer based building programs. Many volunteer and workshop-based bale raisings happen around Australia. The very active and informed network of straw balers constantly explores ways to improve and quantify bale building technology. In 2002 the non-profit association Ausbale was formed to develop and provide information on strawbale building techniques and performance.
Strawbale construction rates highly for buildability because it can be very straightforward and is well suited to workshop and volunteer based building programs.
The general availability of straw bales is good, with many settled parts of Australia being within an hour or so of wheat or rice straw supplies. Straw bale is a low cost material but requires labour-intensive construction techniques. Projects that incorporate some volunteer or workshop-based construction can deliver cost savings. A large detached dwelling, with a high standard of fittings and finishes and built through conventional building contractual arrangements, costs about the same as if it were in double brick, but with a better cost-saving thermal performance.