The New Strawbale Home: Design Essentials

To live up to their promise, straw-bale building systems must be understood and optimized.

The number-one nemesis of straw is water, the universal solvent. If exposed to very high levels of moisture, bales (and wood) will support fungal growth and begin to decompose. Wet bales have also been linked to insect infestations, although these seem to disappear as the bales dry out. Conversely, if kept perfectly dry, straw can remain inert for centuries —even millennia. Since the vast majority of failures in houses of any type are directly related to moisture intrusion, it's not surprising that appropriate bale-building design is consistent with good design practice for homes in general.

Straw and wood are similar in composition, and both will rot under the right conditions. The danger point in straw bale walls is generally accepted to be around 20 percent moisture content (above 85 percent relative humidity), sustained for a week or two at warm temperatures. These are the necessary conditions to encourage the growth of fungus that will begin to decompose the straw. Occasional high moisture levels are acceptable, if the bales (and wood) can dry out again.

From Old England comes an expression that describes a well-built home: “Give her a hat, good shoes, and a coat that breathes, and she'll last forever.” The “hat” is a long-lasting roof, “shoes” refers to a sturdy damp-proof foundation, and the “coat that breathes” implies a thick wall system. Just as in centuries-old European buildings, following these traditional design guidelines will prevent most problems with moisture in bale structures.