Straw bale construction is a sustainable construction technique that uses bales or bundles of straw as the main construction element. Straw bales can be used as a load-bearing or a non-load bearing component in a building.
Straw bales can be designed as a load-bearing structure to withstand lateral and shear resistance imposed by wind and seismic loads. As a non-load bearing component, straw bales are used for their insulation properties and are often opted as in-fill in different load-bearing frames.
The bales of straw are usually bound together with two or three strings. Two string bales are best suited for non-load bearing applications while three string bales are more appropriate for load-bearing applications. The bales are tied using polypropylene twine or baling wire.
In the 19th century, when cement wasn’t a very popular construction material, houses were mostly built with straw bales. The bales were usually piled up one over the other, without giving any consideration to design, to build a square or rectangular dwelling with a sloped roof, just good enough to act as a temporary abode.
Today, we are seeing a resurgence in the popularity of straw bale construction with the rising cost and decreasing availability of lumber. The difference between then and now is the evolved methods of designing and the understanding of the advantages Straw bale construction provides.
Straw Bale Construction Procedure
- Straw, waste stalks from wheat, rye etc. harvest is first collected together and turned into bales.
- To build a load bearing wall, the bales are tied with three strings using polypropylene wire twine or baling wire.
- The foundation is prepared with 12mm threaded rods to make the first layer and the next layer is pushed on top.
- More rods are pushed in and each successive layer is added.
- Openings for windows and doors are cut into the bales.
- Bales are arranged in a running bond i.e. the second layer or course of bale are staggered atop the first so that the middle of each bale in the next higher layer is located over the beam between two bales on the lower layer.
- This (stacking) using a running bond produces a wall that is stronger and more stable.
- A wooden box beam is used as the top plate to support the roof.
- The threaded rod protruding through the box beam fastens the entire wall together.
- The conduit work, wiring and plumbing are placed in the grooves between the bales.
- After the straw bale is laid to beam level and conducting work is completed, plastering work commences using cement plaster ratio 1:6 or 1:8.
Advantages of Straw Bale Construction
- Straw bale construction is a sustainable construction technique, with straw as a renewable material that is readily available.
- Straw bales have good insulation properties and keep the building warm in winter and cool in summer, with R-values between 40 and 60 when built correctly.
- It is relatively cheap and easily obtained from agricultural by-products.
- Straw bale construction is that it is a relatively easy method for new builders, meaning that it can be utilized by owner-builders without extensive experience or volunteer crews. This also reduces the labor costs involved in building which can be substantial in many parts of the developed world.
- Straw bales construction comes with favourable aesthetic qualities, creating thick walls which allow for window seats and shelves, as well as lending itself to numerous architectural styles.
Disadvantages of Straw Bale Construction
- Straw bales are susceptible to rot and mould if they are not kept dry.
- Building with straw bales can also be challenging when it comes to obtaining building permits or insurance coverage, as they are not a conventional building material or part of all local building codes.
- Care must be taken to keep rodents and other small animals from infiltrating straw bales during construction.
- Wall thicknesses mean that more of the building’s overall floor space is unusable.
- 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.