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Last edited 11 Dec 2018
Straw bale construction
Straw bale construction is a method of building that uses bales (or bundles) of straw. Straw bales can be used as structural elements, insulation or both. The use of straw bales is typically found in 'natural' building projects, or earthen construction, and while historically it has been a common form of building material in many parts of the world, it has been received increased attention more recently due to its sustainable properties.
Two different types of straw bale are in common use:
- Bales bound together with two strings.
- Bales bound together with three strings (these are larger in all three dimensions).
 Construction techniques
The techniques used for straw bale construction usually involve stacking rows of bales on a raised footing or foundation. A capillary break or moisture barrier is inserted between the supporting platform and the bales. Pins made of bamboo or timber can be used to tie bale walls together, or surface wire mesh can be used.
The bale wall is then stuccoed or plastered using either a lime-based formulation or earth/clay render, depending on the local climatic conditions. For example, in wet climates, a vapour-permeable finish may be preferred to a cement-based stucco.
Straw bales can be designed to provide load-bearing structural support to a building, as well as lateral and shear resistance to wind and seismic loads. They can also be designed to serve as an insulation substrate, with a separate, load-bearing structural frame, typically made from timber. By building a skeletal framework first, a basic roof structure can be installed which protects the bale wall during construction when it may otherwise be at risk of water damage.
 Advantages and disadvantages
Some of the advantages of straw bale construction include:
- Straw bales are made from a low-cost product.
- Insulation values of R-30 - R-35 can be achieved.
- The thickness of straw bale walls can help reflect sunlight into a room at openings, and provide space for window seats.
- They are relatively easy and cost-efficient to install.
- Straw bales have a low-embodied energy.
- With good maintenance, straw bale construction is very durable.
- Straw bales are 100% biodegradable.
- Properly-constructed straw bale walls can offer better fire resistance than timber frame construction.
Some of the disadvantages of straw bale construction include:
- Moisture and mould are significant risks. Bales must be kept dry to avoid compressed straw expanding due to moisture absorption, which can lead to cracking.
- Fire is also a risk, however, because of the density of bales, fire tends to smoulder rather than spread when an ignition source is removed.
- As it is not a conventional building material, familiarity among builders, and building regulations approval can present obstacles to use.
- Wall thicknesses mean that more of the building’s overall floor space is unusable.
- Unless straw bales can be sourced from near the construction site, the cost of transporting them can be high.
- Care must be taken to keep rodents and other small animals from infiltrating straw bales during construction.
- Straw dust can cause breathing difficulties for people with allergies.
In 2014-15, the University of Bath undertook research into straw bale construction. They focused on ’ModCell’ panels, which are prefabricated panels consisting of a timber frame infilled with bales, with a breathable lime-based render. They found that straw bale construction provided healthier living through higher levels of thermal insulation and humidity regulation.
As a result of this, the first straw houses offered on the open market went on sale in 2015. The seven houses, located in Bristol, were built-clad but with timber-framed straw bale walls. The purchase price was below average, and it was estimated that fuel bills would be up to 90% cheaper than an equivalent brick-built house.
Peter Walker, from the University of Bath, said:
”In terms of durability, we have undertaken laboratory tests and undertaken monitoring of existing buildings and we have also done accelerated weather tests. The results of all these tests suggest that straw is a very durable construction solution.”
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