Frost-protected shallow foundations reduce excavation and materials costs. This foundation, originally developed in Scandinavia, relies upon rigid foam perimeter insulation to keep footings from freezing and becoming unstable, in contrast to the conventional method of extending the foundation below frost line. Consequently, these footings extend only 12" below grade, rather than 60" as is standard in this part of Montana. As an additional benefit, this foundation system is easily modified to provide the width of footing needed for the strawbale walls, without greatly increasing the cost or materials requirements of the foundation. The foundation was designed and constructed for energy-efficiency in this northwest climate.
Natural resources use was also avoided through the use of fly ash, a byproduct of coal combustion at Montana power plants, and crushed post-consumer glass. The concrete used in both the foundations and floor slabs contains 18% industrial waste fly ash as a substitute for cement. This mixture produces a strong and exceptionally smooth concrete. The foundation backfill included crushed post-consumer glass which provides a fine drainage medium.
Glass is currently not recyclable in the Missoula area and the project had some difficulty in obtaining glass that was adequately processed to the required aggregate size and was free of contaminants such as lids and labels.
Savings realized during construction of the foundation helped this home to be an affordable home for low-income families.
Through efforts with the Women's Opportunity & Resource Development home, residential construction applications using glass aggregate show promise as a significant future market for used glass containers. Learn more about frost-protected shallow foundations by visiting the Toolbase Services PATH Technology factsheet.
Goals of Innovation:
Strawbales are an annually renewable resource, are produced in abundance locally and provide an energy efficient wall system. The first-floor walls of the home are strawbale, with bales acting as insulative infill between the post and beam structural framing. Posts in the frame are composed of multiple pieces of small-dimension lumber, to reduce the homes' demand for large trees. The bales are pinned together with bamboo stakes - a renewable and natural alternative to metal pins. The bale walls are finished with a low-maintenance cementitious stucco. With the stucco finish, the bales provide approximately R-40 walls. The post and beam walls with strawbale infill allowed the inclusion of unique features like window seats and built-in shelves that would typically be found only in custom homes.
The floor of the second story is framed with TrusJoist MacMillan Silent Floor I-joists that reduce wood fiber use by more than 30% compared to standard solid-sawn joists. The second floor of the home is wood framed, with walls insulated to R-19 with fiberglass batts, and the roof to R-30 with blown-in fiberglass.
Together the two homes used 509 2-string bales, at a cost of $2 per bale.
Contrary to popular belief, it was not the use of strawbales for the walls that made the project affordable. Rather, the entire design and material package, and in particular the land trust, made the project affordable. However, the use of bales made it possible to produce an exceptionally energy-efficient home with a comfortable and distinctly custom feel for the same cost that a run-of-the-mill stick-framed home could have been built. The strawbale construction increased value more than it reduced costs. More information on strawbale construction is provided in a factsheet produced by Toolbase Services PATH Technology.