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Frost-Protected Shallow Foundations

Frost-protected shallow foundations consist of a monolithic thicken-edge slab wrapped with vertical and horizontal rigid-foam insulation

The footings of most foundations are placed below the frost depth. In colder areas of the United States, this can mean excavating and pouring concrete 4 ft. or more below grade. If you include enough rigid-foam insulation around a foundation, however, you can keep the soil under the house warm enough to permit shallow excavations, which can be 12 in. or 16 in. deep, even in northern areas.

[To read more of Martin Holladay’s article from the December 2010/January 2011 issue of Fine Hombuilding, click the link below.]

Frost-Protected Shallow Foundations.pdf

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  1. Hugh Weisman | | #1

    Wow.! an eight year old article with no comments? I'm curious about how much leeway inspectors are likely to give regarding the typical detail shown in the ICC 2018 code? The detail shows a slab on grade with insulation to top of slab on the edge. We have a basement that's on a hill where it's essentially walk out at the bottom. Instead of a slab on grade, we would have footings and a standard concrete basement wall, I'd like to wrap the footings on the outside with insulation and extend it up the wall to the level, or a little above the level, of the basement floor.....the rest of the basement wall would be insulated with a stud wall on the interior and left bare on the exterior (I know that'a a green building heresy, but we need a finish on the inside and want to see concrete on exterior for architectural reasons). Chances are that the bottom of the footings would be deeper than the 12" allowed by code. Also, are there any issues regarding insulating the bottom and edge of basement slab per code requirements and good practice.....obviously, less heat into the earth but we're relying on the 50 degree earth anyway.

    1. GBA Editor
      Martin Holladay | | #2

      The only code interpretation that matters in your case is the interpretation of your local code official; my opinion is irrelevant. So call up your local building department and ask.

      For more details along this line, see this article: "Insulating a slab on grade."

  2. Jamie Jewett | | #3

    Is there any issue with FPSF when used for additions?

    My project has an existing dirt floor crawl space with stem walls (mix of block and poured) and I would like to use a FPSF for a 12x22' 2 story addition...

    Can I just epoxy rebar stems into the existing and continue with the FPSF program?
    if so, should I put a vertical piece of insulation to separate the existing foundation from the new FPSF?

    Thank you so much for any insight on this!

  3. fablefarmfermentory | | #4

    when you say you want the horizontal insulation a minimum 12" below grade, is this the measurement to the top of the wing insulation or to the bottom of it (I reckon the former)? I'm using recycled glass aggregate for winged insulation for a footer, wall and independent slab walk-out basement that sits on ledge (and crushed stone) and can't really get that 12" in all areas. as mentioned footers are on ledge/crushed stone, drainage is bomber all the way around (no fines, curtain drain, etc), site is high and dry. around 2500 air degree index. one of the main reasons I can't get 12" is because the Revised Builder's Guide to Frost Protected Shallow Foundations (2004) says for a stem wall and independent slab for the horizontal insulation to be place above top of footer. thus my issue with 12". would it be prudent to get the 12" and place horizontal insulation so that it comes to top of footer or splits it, or adhere to the guide referenced above and put it all above the footer?

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