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Green Building With Straw Bale
Straw is a waste material and is usually burned once the grain is harvested. By using straw bale walls as insulation, no pollution is added to the air through burning. Once plastered with mud, these two-string 18" bales will insulate the walls to around R-35.
The straw bales are stacked like bricks in this post-and-beam structure. Where needed, some bales are notched around posts and cut in half for a better fit. During an all-volunteer straw bale workshop, twenty-five workers were able to place approximately 80% of the bales in just two days of work. A straw bale workshop is similar to an old-fashioned barn raising. House building becomes a community effort, with the volunteers learning a new skill and the homeowner receiving valuable assistance at little or no cost.
The diagonal braces are temporary and are removed once the roof sheeting is installed. Their purpose is to keep the walls square in the early stages of construction.
Post and Beam Framing
Building code in New Mexico, as in many other parts of the country, does not permit the straw bales to carry the load of the roof. Therefore, the roof is supported by a post-and-beam structure, and the straw bales are used as insulation infill only. One advantage to this is that the roof can be constructed before the bales are positioned. This offers some protection from rain during construction.
The posts are 4 x 4's with a 4" x 8" perimeter beam on top. The posts are spaced up to 8' apart. Note the closer spacing for framing around windows and doors. The roof trusses, sitting on top of the perimeter beam, are deep enough to allow for R-55 cellulose insulation (R-30 is code). Please refer to the "Building With Awareness" DVD to learn the step-by-step process of post-and-beam construction.
Two inches of CFC-free foam insulation is used as the outter form to hold the concrete. This foam remains in place to insulate the foundation. The interior forms of straw bale are temporary and are removed after the concrete has set.
A diagram of a rubble trench foundation appears on page 30 of the "Building With Awareness" guidebook.
Why use a rubble trench foundation? Because the production of portland cement accounts for around 5% of all greenhouse gasses. Because of this, it is important to reduce the amount of concrete while building a green home. The need to reduce the amount of concrete is made even more important because of the wide straw bale walls which require a wide foundations.
A rubble trench foundation reduced the amount of foundation concrete by over 50%. To create a rubble trench, a trench is dug below the frost line (about 18" deep in this area) and filled with 1 1/2" stone to just below grade level (see picture). The remaining 8" void is then filled with concrete. Pollution is reduced, money is saved, and the foundation was as structurally sound as a conventional foundation.
Stone or brick will work just as well as adobe and can be just as beautiful. When nature makes the material, less fosil fuels are burned and less air pollution is created. In addition, natural materials tend to look great with little or no additional work.
Thermal mass is often overlooked when designing an energy-efficient building. Using straw bale walls for insulation does not in itself guarantee stable indoor termperatures. Thick thermal mass walls (of at least 10" in thickness) will store and release excess heat. In the summer, heat is absorbed by the walls to help cool the home. The adobe thermal mass, combined with the highly insulated exterior straw bale walls, made it possible to eliminate the mechanical air conditioning system in this home. In the winter, the thermal mass walls release heat into the room at night.
Copyright © 2008, Ted Owens. All rights reserved.
Building With Awareness is a registered trademark of Ted Owens.