The Beauty of Bales

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3. Avoid the use of vapor barriers.

Vapor flows from warm to cold, and walls need the ability to dry. Since drying predominantly occurs through vapor diffusion, an impermeable barrier or coating can be detrimental. The vapor permeance or perm rating of both interior and exterior wall finishes needs to be chosen with due regard for each specific climate. Otherwise, vapor can be trapped within the wall, where it will condense. A general recommendation would be to make the finish on the warmer side of the wall semipermeable so that it slows the flow of vapor in the wall, while the finish on the cooler side of the wall should be permeable so that vapor can flow through.

Vapor permeability is typically rated in "perms":
Impermeable materials = one perm and less.
Cement stucco (one part cement, three parts sand) = 0.5 to 1.0 perms.
Semipermeable materials = one to six perms.
Cement lime stucco (one part cement, one part lime, six parts sand) = four to six perms.
Permeable materials = from six to more than 50 perms.
Lime plaster (one part lime, three parts sand) = eight to 12 perms.

4.Provide a "continuous" air barrier to block leaks.

Air flow or leakage occurs through openings (holes, cracks, etc.), and can carry 100 times more moisture than what might be generated by vapor diffusion through the wide expanse of wall and ceiling surfaces. Improperly sealed joints at penetrations in the ceiling, floor and wall junctions, as well as cracks around windows, doors and electrical boxes, are common problems. A continuous air barrier strategy relies on good detailing and a variety of materials to be effective. The stucco and plaster skins typically used on straw bales play an important part in establishing this continuous barrier.

5. Protect the bales from moisture at their connection with the foundation.

Keep the bales above floor level, with raised sills or toed-up foundations, to help prevent damage from plumbing leaks and minor floods. The tops of foundations also need to be moisture-proofed: install a capillary break between the foundation and wall structure to prevent wicking of moisture up into the bales.

6. Monitor the walls for moisture.

You can make your own moisture meters following instructions in issue 22 of The Last Straw Journal, page 114 [(505) 895-5400, www.strawhomes.com ] Moisture can be monitored either by checking moisture content or relative humidity. Straw that is under 15 percent in moisture content and lower than 75 percent in relative humidity can be considered sound and unaffected by moisture.

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