Barriers

Why Build without a Vapor Barrier? In conventional building practice, moisture is prevented from migrating into the wall cavity through the use of continuous plastic vapor barriers. This addresses the very real concern of air leakage in stud-framed homes, but it is unnecessary and a structural compromise with bale walls.

Attaching a vapor barrier to a straw wall offers many complications. The barrier can only be attached to the top and bottom of the wall,

Inspection of mold in straw bale walls has shown that in most cases the moisture entered the wall during construction with the straw not being replaced prior to plastering. It can take up to two months for a perimeter wall to dry out, which is ample time to allow molds to form. Unplastered bales can withstand some surface wetting that will dry with air exposure. If during construction the walls are exposed to moisture then they should be inspected to see if the moisture has penetrated the core of the bales. A bale should be replaced if the moisture has penetrated more than three inches into the bale and above the 20 percent moisture content. Moisture meters with long probes are usually available from local farm supply stores. When handling any bales that have become moldy, wear personal protection in the form of respirators with HEPA filter cartridges, gloves, and goggles. Strong air blowers, used in the flood and fire industry, can be rented to dry surface moisture. Moisture can also enter bale walls from floods, a leaky roof, a large crack in the plaster, poor window and door placement, or plumbing leaks as is the case with any type of wall construction. All walls exposed to moisture regardless of the type of materials, can support mold growth and should be inspected and dealt with in a timely and appropriate manner.

This book is about best building practices of straw bale construction and following the recommendations on roofs, overhangs, and foundations that will ensure a healthy, mold-free structure. People who are considering straw bale building but who have allergies to dust and the general spores found in straw bales should realize that during construction these are present and should wear personal protection. After plastering, mold spores and dust are encapsulated and will not influence health or indoor air quality (IAQ). Straw bale walls actually improve the IAQ in homes due to the slow air diffusion of the walls.

Paul Battle is a certified Bau-Biologie Environmental Inspector in Ottawa Ontario. He specializes in the detection of home health hazards and consults on ecological home construction. Contact <www.thehousedoc.ca> (613) 297-2996 ■

making it hard to maintain a taut surface unless wooden attachment points are added to the wall. A vapor barrier prevents the plaster coating from attaching itself directly to the straw. This not only makes plastering much more difficult — and will likely require more metal reinforcement for the plaster — but eliminates the substantial structural benefits of bonding the plaster to the straw. Remember,bonded together, the two materials create a stressed skin panel far stronger than the sum of their independent elements. A straw wall without a vapor barrier is less time-consuming to build and eliminates the use of a manufactured product with a high embodied energy. Finally, straw, metal stucco mesh, and plastering tools all increase the risk of introducing punctures to the vapor barrier, rendering it less effective.

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