|
Doesn’t straw rot?
Long term or repeated exposure to liquid water is the greatest danger that straw-bale walls face. Given enough liquid moisture and two to three weeks, the fungi that are always present in bales produce enzymes that break down straw cellulose. - Fortunately, straw moisture content must be above 20% (by weight) to support fungal growth.
Other than the success and survival of historic structures in diverse climates like Nebraska and Huntsville, Alabama, we know little about the actual long term effect of moisture on straw-bale bales. Straw-bale wall moisture monitoring is underway in climates as diverse as Portland, OR, Alberta, Canada, and Nova Scotia, Canada. Tests to date show rot problems only in sections of walls with leaks or direct bale-concrete foundation or bale-soil contact. Wise material use and good architectural detailing, building practices and regular maintenance generally keep the moisture content of straw below the decay threshold. - A watertight roof
- large overhanging eaves
- elevated foundations
- separation from wicking concrete
- adequate door and window flashing
- and a skin of good weathering plaster or siding all work together to protect bales from rain and ground water.
Special care should be taken to protect horizontal bale surfaces (window sills and the top of walls) where a leak can penetrate deeply into the center of the wall where drying is difficult. At window sills, consider using a self-sealing membrane to create a continuous sloping “diaper” or use a continuous pre-formed metal sill. Drip edges on windows and walls should lead water away from the wall below. As in conventional construction, exhaust moist air from kitchens and bathrooms to prevent excessive interior moisture, and many suggest using a low permeability latex paint on interior straw walls to inhibit moisture migration in these areas. Conventional wood-frame building codes generally require moisture barriers under exterior siding or stucco, but moisture barriers in straw bale walls can cause as many problems as they prevent by trapping moisture inside walls. Most practicing straw-bale professionals recommend against using sheet barriers on walls, except draped over the bottom two courses as a splash guard. A breathable barrier is best, tucked into the joint between bales at the top and extended over the edge of the foundation for several inches. Never tuck the bottom edge beneath the bale- it creates a “bathtub”. Moisture testing in plastered straw bale walls suggest that maintaining the breathability of straw-bale walls may be the best insurance against rot . Bale walls thoroughly soaked in rain storms during construction have survived without problems when allowed to dry out thoroughly before plastering, but there has been an instance in which a plastered garden wall with a tar paper moisture barrier and a leaky cap rotted because it was not able to dry out. |
