Building a Staw Bale House
(Page 3 of 10)
December/January 1995
By Athena and Bill Steen
Bales are also susceptible to moisture damage during the construction process. The tops of the walls and the window sills are vulnerable points where rain can penetrate to the interior of the bale. Once that happens it is very likely mold or rot will commence before drying takes place. You should keep coverings for these points handy if rain threatens. Flakes of bales or loose straw can suffice if the rain is light. These points are typically given an additional measure of protection from moisture, such as black felt paper, before the final finishing of the walls. Generous roof overhangs will help to keep rain away from the base of the walls and all soils surrounding the building should be graded to ensure good drainage.
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The point where the bales rest on the foundation is equally vulnerable. The top of the foundation should be elevated at least a minimum of six inches above grade and be moisture-proofed to protect the bottom of the first course of bales. A combination of asphalt emulsion and black felt has been the most common way of moisture-proofing foundations for straw bale buildings. Polymer-modified asphalt is more durable over time and has greater ability to span cracks that could arise due to settling or shrinkage. One other product that comes highly recommended is Safecoat DynoSeal. This neoprene-modified asphalt emulsion is nontoxic and can be used by chemically sensitive people. Cementitious coatings like Thoroseal are durable and breathe, which could help minimize any possible condensation problems, but don't stretch and will tend to crack where foundations do.
If the floor and the top of the foundation are the same height, as occurs with a monolithic concrete slab, there is always the danger that rain or water from minor flooding could puddle on the slab and seep between the first course of bales and the moisture-proofing. To alleviate this problem, many have started elevating the portion of the slab where the bales are placed a minimum of four inches above floor level. Placing polystyrene board on top of the moisture-proofed foundation would also elevate the bales as well as provide a thermal break between the first course of bales and the cold concrete, an extra insurance against potential condensation problems.
Grade beams that float on rubbled trenches (a variation of the "floating slab") have been a popular foundation option. The rubble trench extends below the frost or clay line, affording excellent drainage and reducing the amount of concrete needed for the extra wide bale walls.
Moisture damage could also happen if condensation oc curred within the bale wall cavity. Less is known about the behav ior of humid air within bale wall cavities than al most any other area. Warm air holds more moisture than cold air, and the differences in hu midity and tem perature cause it to move toward cooler surfaces. When warm hu mid air meets its dew point it be gins to condense. Many different variables make it difficult to know where the dew point occurs within a wall cavity, however the more humid the air, the higher the po tential for water accumulation. A level of 35 percent humidity is generally consid ered safe for the interior of a building.
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