Straw bale homes seem to tick all the boxes of the ideal passively designed home, yet for all their apparent benefits they are still not that common in building structures.
Straw is cheap, plentiful, eco-friendly and an excellent insulator. Modern straw bale houses deliver excellent levels of energy efficiency, meet the Australian Building Code of Australia, are approved through Councils and comply with the CSIRO fire tests for building in bush fire prone areas.
Despite this, it has been hard to change the public mindset around straw’s fire resistance, while there are also concerns about its poor durability, the way it attracts rodents and its lack of structural integrity.
The main perceived advantage of straw is its environmental credentials, but how do they stack up?
Straw is a waste product; it is just the dried stalks of plants stripped of their grain. It cannot be used for feed, like hay, and much of it is burned at the end of the season.
Recent research led by the BRE Centre for Innovative Construction Materials at the University of Bath has shown that In the UK more than seven million tonnes of straw remains after the production of wheat, and up to half of this amount is effectively discarded.
The research adds that an average three-bedroom house needs 7.2 tonnes of straw to build, so this ‘leftover’ could be used for some 500,000 new homes with a change of mindset.
Straw bales contain a high level of renewable material. Straw has a six-month growing cycle and is biodegradable. 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.
Straw has very good thermal insulation properties with a similar insulation value to fibreglass batts. A typical straw bale wall has an R-value greater than 10 and dollar for dollar exceeds the insulation value of conventional construction.
Recent research led by the BRE Centre for Innovative Construction Materials at the University of Bath in the UK has shown that straw bale buildings reduce energy bills by 90 per cent compared to conventional housing stock.
Some of the negative environmental effects to consider are the fertilisers and pesticides associated with industrial farming practices and the fact that the bales are produced by fossil-fuelled machinery.
Structurally, straw bales are less limited than one might think. In the load bearing straw bale method, walls of three storeys have been built.
Similar to brick construction bales are laid in an interlocking manner to ensure a stronger, more stable wall.
Footings are typically concrete but there have been successful examples of using rubble trench and rubber tyre footings, and several straw bale buildings in Australia are built on piers, bearers and joists.
Wall heights are limited, however, to a recommended maximum height of 2.5 metres when using standard sized bales. In addition, openings for windows and doors should not exceed 50 per cent of any given wall surface area and the maximum unbraced wall length is limited to around six metres.
“Although it is possible to build strong and effective single storey straw bale structures, it is often easier to ensure BCA compliance and predictable engineering outcomes if the walls are constructed as infill elements between load bearing frames,” recommends the ‘Your Home’, Australian Government website.
To ensure compliance with building codes, most use frames of timber or steel. Framed construction gives more freedom for wall and opening placements, even enabling partly cantilevered floor construction not possible in a load bearing straw bale structure.
Protection from rain, fire and rodents is crucial. Traditional wall claddings such as corrugated steel sheets are not sufficient as they do not seal the bale surface. Therefore, render is the only really viable solution – either cement, lime and sand; lime putty and sand; or earthen render.
Long-term or repeated exposure to water is one of the biggest risks to the durability of the structure. If the moisture content is more than 20 per cent by weight, then fungi will produce enzymes that break down straw cellulose within two to three weeks. In this regard, rice straw is a better than wheat straw as its high silica content improves rot resistance.
Provided the straw is protected and not allowed to get waterlogged, straw bale buildings may have a lifespan of 100 years or more.
Perhaps the biggest perception issue is that of fire safety. But here straw gets an unnecessarily bad name.
Fire can’t burn without oxygen. Because the dense walls provide a nearly airless environment, the fire resistance of compacted straw is very good.
In July 2002, CSIRO undertook tests on three kinds of standard size rendered straw bales (earth; lime and sand; lime, sand and cement) which produced a two-hour fire rating.
More recently, sustainable designer Joost Bakker tested, with the help of CSIRO, what he said was Australia’s first bushfire resistant straw house. Made with straw bale insulation and set in a recycled steel frame with magnesium oxide cladding, using a bushfire simulator in Moggo, NSW, he found the house could withstand temperatures of more than 1,000 degrees.
“The Austrian government rates a straw bale house as a one-and-a-half day burn time,” said Bakker. “Whereas a conventional house is 90 minutes.”
So what of the future?
Researchers and building product manufacturers are continuing in their quest to make straw bale more accessible to the mainstream market.
This has seen the development of prefabricated wall panels using straw bale for insulation. A prefabricated product can be certified as fit for use by industry bodies, making it much more acceptable to builders, financiers and insurers. And now countries from the UK to Belgium to Canada are making prefabricated straw bale systems more widely available.
The potential not only for more straw bale homes but for entire straw bale communities is certainly there.