Frost-Protected Shallow Foundations

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:

Oikos: http://oikos.com/esb/43/foundations.html

NAHB: http://www.toolbase.org/Technology-Inventory/Foundations/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

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9 Comments

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9 responses to “Frost-Protected Shallow Foundations

  1. Darin

    I’m in the process of designing my next personal home, a super-insulated / passive solar house in Colorado. I’m strongly considering using a FPSF and was intrigued at the implementation here. First off, I’d like to request a copy of the 1st picture, as when enlarged it’s a bit hard to read and I want to confirm what I believe I see. Can that be sent to my email please?

    If the design is what I believe I see, it looks like the approach taken was to implement this as a FPSF for an “unheated building” (http://www.cs.arizona.edu/people/jcropper/desguide.pdf See figure 14). While I liked the idea of a FPSF, I was concerned at what appeared to be a design the used heat escaping from the house to basically warm the area under the foundation to prevent frost heave. I too was looking for a way to effectively implement this type of foundation, but keep as much of the heat inside the house as possible while still protecting the foundation. The article / diagram in my link above seemed to be the best way to do it, but was concerned about effectiveness for a lived in space.

    When I came across the diagram for the Unity College Passive House it appears to be designed to accomplish both the super insulation of the house while also keeping the house foundation warm using just the heat from the earth. I’ve not seen many other detailed examples of this type of approach, even in the links provided in the blog, only in the doc link I attached. How did Unity House finalize this design for effectiveness in this application?

    Thanks! Looking forward to seeing the end result!

  2. Keith

    I’d like to request a copy of the 1st picture, as when enlarged it’s a bit hard to read. Could you send me a diagram for the perimeter grade beam that uses insulated concrete forms? Can that be sent to my email please?
    Thank you

  3. I do not understand why you insulted beneath the floor of the house. You are losing the ambient heat of the earth that comes from below the frost line. I insulated the perimetre of my house with 2 inches of foam out 96 inches from all exterior walls and was delighted with the result: with no solar gain and no fuel consumption the house never freezes regardless how cold it has been outside. With the attached greenhouse on a sunny day even with -20C temperatures outside the entire house is very warm. I should mention the north, east and west walls are nearly five feet of cord wood (cheaper than roxul and with considerable thermal mass). Surplus solar gain is stored in a thermal water tank (2500L). The only time I need any fuel is on snowy days or very overcast days. More often I find I want a fire when none is needed.

    • To avoid insulating under the slab you need to insulate vertically to a depth deep enough to channel the earth temperature up and shield the cold out.
      In my area Denver Colorado the depth is 12′.
      I have put foam along the foundation and under the slab and on a out building insulated horizontally to protect the footing at a substantial savings in concrete and labor
      Richard Sims
      Sims Construction

  4. Pingback: Compacted Fill – Hayfield House

  5. Ken Gartner

    Which peel & stick covering did you apply to protect the vertical insulation from weather and hazards? Also which ‘panel screws’ … something from Tapcon? In the diagram, I did not see any indication of waterproofing/mastic applied to the external foundation (under the two inch vertical foam) …. did you actually apply a waterproofing coating when you built it? After these several years, are you satisfied with how the vertical insulation has performed? Thanks for writing about FPSFs.

    • I’ll check the “As Built” notebook on the foundation cover (a pre-painted metal flashing material). Our performance ($275 to heat the building in ’14-’15) and our infrared photos (2016) indicate no problems in the envelope.

  6. See http://www.iso-slab.com is easy and performing insulated slab-on-grade kit

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