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 together. The use of straw bales is typically found in natural building projects, or earthen construction, and while traditionally a common form of building material in many parts of the world, it has been received increased attention due to its sustainable properties.
Two different types of straw bale are common:
- 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 are used to tie bale walls together, or surface wire mesh is used.
The bale wall is then stuccoed or plastered using either a lime-based formulation or earth/clay render, although these should be considered with regard to the local climate conditions. For example, in wet climates, a vapour-permeable finish will be required rather than a cement-based stucco.
Straw bales can be designed for load-bearing structural support of a building, as well as to provide lateral and shear support for wind and seismic loads. They can also be designed to serve as an insulation substrate with a load-bearing structural frame using alternative materials, typically timber frame. By building a skeletal framework first, a basic roof structure can be provided which protects the bale wall during construction, which is when it may be at risk of water damage.
 Advantages and disadvantages
Some of the advantages of straw bale construction include:
- Straw bales are made from a waste product.
- Insulation values of R-30 - R-35 can be achieved using straw bales.
- The thickness of straw bale walls helps reflect sunlight through a room, and provides windows with a window seat/shelf.
- Relatively easy and cost-efficient to install.
- Straw bales have a low-embodied energy (i.e. minimal energy was used in the manufacturing process).
- 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 conventional timber frame construction.
Some of the disadvantages of straw bale construction include:
- Moisture and mould are two significant risks that face straw bale construction. Must be kept dry to avoid compressed straw expanding due to moisture absorption, which leads to cracking.
- Fire is also a considerable risk, however, because of the density of bales, they tend to smoulder rather than spread when an ignition source is removed.
- As it is not a conventional building material, building codes can present obstacles to its use.
- Wall thicknesses mean that more of the building’s overall floor space will be 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 to straw or hay.
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 and with a breathable lime-based render.
As a result of this, the first straw houses to be 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. As well as the purchase price being less than average, it was estimated that fuel bills would be up to 90% cheaper than an equivalent brick-built house.
Peter Walker, 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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