What is a straw bale construction?
If you are looking for alternative, energy efficient ways to build your home that may also be cheaper than traditional housing, a straw bale construction is the answer.
Most straw goes to waste as it is no longer used in traditional livestock breeding. It is plowed back into the fields or even burnt. However, people can take advantage of its natural qualities and use it to build highly insulated and highly fire proof buildings. Straw bale hasn’t gone under any manufacturing process; it is natural and can serve as an effective insulation AND building material. It is also unique, because even though it is an insulating material, it can carry the weight of a whole house.
The two basic types of straw bale construction are load bearing (or Nebraska style) and non-load bearing (or infill). In load bearing construction straw bales are used as large bricks that support all of the building loads, whereas in non-load bearing construction bales fill in between and don’t carry any of the building loads.
Straw bail buildings and fire
According to Canadian, U.S and UK materials laboratories, the straw bale structure wall has proven to be exceptionally resistant to fire. During these tests, standard construction offered only a 30 minute burn resistance. On the other hand, the flames took more than two hours to penetrate the plastered bale walls. It may seem weird, but if the straw bales are tightly compacted, they contain very little oxygen and thus resist combustion. However, loose straw and walls without plaster are at risk for fire.
Straw bail buildings and moisture
Choosing the appropriate straw bale is very important. It should be well-dried when you receive it and until you use it. Therefore, it is better to find a supplier who specialises in straw for straw bale houses and will provide you with dry, consistent bales which minimise labour and secure that no rotting will occur. Straw bales must contain 14% moisture or less when they are installed and have to be properly sealed within the plaster to be protected from water infiltration.
Proper design and construction methods prevent airborne vapour from getting trapped in walls. Therefore, vapour barriers are eliminated and special finishes are selected in order to create walls that "breathe" (for example, earth plasters, lime plasters, natural paints, etc.). Bales allow only moisture to move out of the walls, not air.This fact improves indoor air quality and provides insulation while keeping the bales dry. There are specific international standards (International Residential Code, Appendix M “Straw Bale Structures”) to facilitate this design and construction.
Straw bail buildings and pests
Straw is the stem of any grain plant, such as oat, wheat, rice and barley and is high in cellulose, which is similar to wood, and is therefore not digestible by animals. So straw shouldn’t be confused with hay, which is a food source for many animals. At the same time, no termites or rodents will get into densely packed bales. Pests cannot penetrate into properly plastered walls.
Straw bale buildings and durability
There are straw bale houses in Nebraska and Europe since the 1800’s. Straw bales are also considered to be ideal for building “seismic-resistant” constructions. Furthermore, straw bale homes can withstand severe weather and wind. Based on wind tests, straw bale structures see no movement in a sustained 75 mph gale and only 1/16 inch movement with 100 mph gusts.
Finally, straw bale construction can lower your energy costs by 75% and offers great sound proofing. By using this local, agricultural by-product as a building material we also contribute to the protection of the environment. We minimise the amount of straw burned, keeping our air clean, and we reduce the use of fossil fuels needed for material transportation. The basics of straw bale construction are simple to learn and require no expensive tools.
For more information I recommend having a look at http://www.strawworks.co.uk/
Image Credit (CC 2.0) Sandra Cohen-Rose and Colin Rose