Building a Staw Bale House
(Page 2 of 10)
December/January 1995
By Athena and Bill Steen
To build with straw bales requires a good understanding of the basic characteristics of straw and how it behaves as a building material. In general, straw and other natural fibers have low compressive strength but when twisted, interwoven, bundled, baled or combined with other materials like clay, their compressive strength improves dramatically and they can then be used for a variety of structural and nonstructural building applications. However, it is important to remember that despite the improvement in strength, bales do compress under loads. The more compact the bale, the less it will compress. Bales you use for building should be solid, compact, and keep their shape when you handle them, especially if they will be asked to support the kind of bearing walls that we will ask them to.
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And yes, you can fire up the wood, stove in a bale home. Densely baled straw limits the amount of oxygen needed for combustion to take place, and is therefore very resistant to fire. Fire testing, conducted in New Mexico through The Straw Bale Association, demonstrated that an 18-inch straw bale wall survived fire penetration more than two hours, while an unfinished wall survived for just 34 minutes. Loose straw, of course, is very flammable and can be a hazard. During construction, we have seen sparks from a grinder or welder ignite piles of loose straw that had collected at the base of the walls. If you use loose straw for roof insulation, treat it with a suitable fire retardant. We have often used borax and a product called Nochar Fire Preventer to do the job.
Keeping Bales Dry
Understanding the relationship between baled straw and moisture is critical to the successful construction of a durable straw bale building. When natural fibers are bound into bales they become more susceptible to moisture damage because it is more difficult for air to reach the interior of the bale and allow drying to take place. The interior of bales most often becomes wet when they are either baled wet or rain falls directly on them from above. Eventually they can dry, but usually not until mold or rot has affected them. Dry healthy bales will be bright and evenly colored, without signs of discoloration or deterioration. The sides of the bales are much more resilient and will dry out fairly quickly after a rain. One straw bale house in eastern Washington has remained unplastered for more than 10 years. Problems can occur, however, if the sides become wet and something is placed against them that does not allow sufficient air circulation.
Bales stored in stacks are even more susceptible to moisture problems due to lack of air circulation, particularly where the bales rest on the ground. We have seen hundreds of good bales ruined from poor storage. Experience has shown us that large and tall stacks are very unmanageable for most people. Smaller multiple stacks, that are not to tall or wide, are easier to cover and uncover. Crowning the stacks helps to shed rain quickly. Wood pallets are a convenient way to elevate stacks of bales above the ground, and provide a convenient location to tie down tarps. We use only good quality waterproof tarps to cover bales in storage and check them regularly. The blue poly tarps commonly found in many retail outlets don't last (especially in the sun), and are only moisture resistant, not waterproof. If bales will be stored for only a short period of time, sixmil plastic can be used underneath poly tarps for additional protection. Waterproof industrial PVC tarps are much more durable and should be used if the bales are to remain in storage for any length of time. Silver-coated tarps are a reasonable priced and durable alternative.
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