What is straw bale construction?
Straw bale construction uses the renewable material of straw, in bale form to act as an infill material to timber frame structures or as a load bearing structure to buildings of no more than two storeys. The construction method is considered to have a low environmental impact and provide good thermal insulation properties. Although the use of straw is widespread in roof construction in the form of thatching, straw bales as a modern construction method are gaining in popularity.
The straw bale construction method is popular with self builders and the environmentally conscious. The straw bales produce a building with deep reveals and a quirky character.
Straw bale construction methods
Load bearing wall material built on site
Infill wall material within frame and faced walls
Prefabricated cassettes packed with straw
Load bearing straw bale construction
Load bearing straw bale construction uses a simple assembly method of laying the bales in courses, with each bale centred on the bale joint beneath – stretcher bond.
The bales are raised from ground level by a brick plinth, to avoid contact with moisture. They are fixed to a base plate that has a series of wooden spikes to secure the bales in place and prevent the wall from slipping. The wall is often spiked or pinned as it is constructed to maintain the stiffness of the structure.
These straw bale walls can be up to 500mm thick, depending on the size of the bales sourced. Sizes vary from country to country, but in the UK the general size tends to be in the region of 990 x 500 x 375mm. Bales can be made from a variety of crops including wheat, oats or barley.
Compression of the straw bales, to improve the structural integrity is achieved with ratchet straps which are then removed before the roof goes on. When the bales are compressed, it also makes it easier to carry out the next stage.
Once the wall has been constructed, additional strength is added in the form of lime render to both the interior and exterior of the wall. Render must be vapour permeable, with the head of the wall being protected by roof overhang to prevent over saturating the wall. The render acts as an additional structural layer, which stiffens the wall and prevents decay, while also acting as additional fire resistance.
Openings are also susceptible to water damage and must be carefully considered. Windows and doors are often installed close to the external face of the wall. Thermal bridges can cause a build up of moisture in the straw that would damage the structural integrity, therefore metal framings and fixings should be avoided or wrapped in an appropriate protection to avoid contact with the straw.
It is preferable not to cut the bales, so openings and dimensions should be sized according to the bales to reduce cutting where possible. Cutting the bales can be time consuming and reduce compressive strength.
Infill straw bale construction
Advantages and Limitations of straw bale construction
- Good insulation
- Sustainable material
- Low cost material
- Lightweight material
- Usually possible to source locally
- Adaptable to both in situ construction or prefabrication
- Good availability
- Can be used for Passivhaus projects
- Relatively low skilled construction
- Susceptible to moisture ingress, and decay, requires good detailing and planning during construction
- Wall thickness is deep
- Requires protection during construction
- Must be installed above damp proof course level
- Still relatively new to general market, so can be difficult to obtain mortgages or insurance
- Restricted design due to wall thickness, need to protect from water ingress, etc.
Examples of Straw Bale Construction Projects
SCL Straw Bale House – Jimmi Pianezzola Architetto
Wood and Straw Housing – NZI Architects