See the Progress Log page for images of yesterday’s framing work. It’s exciting to see the TerraHaus enter the third dimension.
The TerraHaus design calls for an insulated slab foundation called a frost-protected shallow foundation (FPSF). Standard foundations use a frost wall that extends below the frost line, about four feet below grade. The idea is to prevent frost heaving which would damage the structure. The floor is then poured near the base for a basement design or the space is filled with gravel and poured at the top of the frost wall if the poured floor is to form the bottom floor of the building.
The FPSF uses insulation below and extending a couple feet out from the foundation to prevent frost formation under the building. This not only prevents frost heaves, but it also insulates the floor. Four feet of soil has an R value of about 1, so 6” of foam insulation can easily take care of the frost issue while keeping the floor toasty.
Two basic designs can be used for FPSFs: the Monolithic Slab (also known as the “Thickened Perimeter Slab”) and the Perimeter Grade Beam used for the TerraHaus. The monolithic slab, formed in one pour, has a 16-inch deep x 12-inch wide thickened slab edge poured over and a foot beyond tapered gravel edges. The perimeter grade beam uses insulated concrete forms for the thickened perimeter. Both use a five-inch slab on top of compacted gravel fill.
The diagram below shows a monolithic slab detail, and the two photos show the TerraHaus Perimeter Grade Beam in progress.
Due to the ease of installation and the reduced excavation and concrete costs, a FPSF may save $1500, $3000 or up to $20,000 over a standard full foundation in a residential setting. A standard carpentry crew can do most of the work rather than relying on foundation specialists. Builders like avoiding the deep trench adjacent to the frost wall that they constantly have to bridge over. In many cases, the result is a more thoroughly insulated foundation and floor.
FPSFs are recognized by the American Society of Civil Engineers (ASCE), the International Building Code, the National Association of Home Builders (NAHB), and the Council of American Building Officials (CABO). NAHB has been enthusiastically promoting the FPSF since about 1995.
For more information on Frost-protected Shallow Foundations:
Gibson, Alan. 2010. Super-insulated Slab Foundations. Journal of Light Construction. April 2010. 8 pp.
Doug Fox, Director, Center for Sustainability and Global Change, Unity College