Building a Staw Bale House
(Page 4 of 10)
December/January 1995
By Athena and Bill Steen
Recent testing (monitoring the humidity in the wall of a straw bale house in Nova Scotia), demonstrated that the level of humidity remained below the point where deterioration of straw would occur due to microbial activity. Further testing is planned to determine if in-wall condensation does occur. The Burritt Mansion, built in 1937 in Huntsville, Alabama, which is a warm, humid climate, has exterior siding and appears to be in good condition today. The key in all these cases may be that the wall surfaces and finishes have retained sufficient "breatheability" to allow moisture to pass freely through the walls. Differences in wall finish permeability need to be considered carefully. The exterior wall surface should be more permeable than the interior wall in cold climates like the Northeast and Midwest. The reverse would be true in warm, humid climates using air-conditioning, like the South and Gulf Coast states.
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Bale Sizes and Characteristics
Straw bales typically used for building come in two different sizes and are generally referred to as two-string and threestring bales. Typically, different geographic regions will tend to have primarily one type or the other. For example, threestring bales are common in the state of Arizona, while two-string bales are common in New Mexico. Prices will vary depending on how close you are to where the crop of straw has been grown. They can run as low as $.50 if no transportation is involved, or as much as $3.75 to $4.00 if they need to be moved any distance. The best value will typically be from a wholesaler or grower rather than a retail feed outlet.
Bales come with either polypropylene twine or wire. For us, the poly twine is much easier to work with when it comes to modifying bales both in terms of size and shape. Another important factor, especially when bales are used in applications where they are expected to carry loads, is that wire can rust and break, jeopardizing the integrity of a wall.
Bales have three different linear measures: height, length, and width. When the wider side, or the width is laid parallel to the ground, it is commonly said that the bale has been laid flat. When the narrower of the two sides is laid parallel to the ground, the bale has been laid on edge. Three-string bales offer the greatest amount of stability because of their width and density especially when laid flat, while both types of bales are less stable when laid on edge.
If space is a major consideration it could be advantageous to lay the bales on edge, creating a slightly narrower wall. When laid in this manner, the twine or wire is exposed and can be a convenient attachment point for stucco netting or furring strips. In this position, bales have slightly more insulation value per inch, but significantly less compressive strength and are better suited to applications where the bales will not be carrying heavy structural loads. In contrast, bales laid flat have more than four times the compressive strength than bales on edge, are more stable, offer an excellent surface for plaster adhesion without any stucco netting, and, since the bale wires or twine are not exposed, the bales can be more easily carved, notched. or modified.
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