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Short Frost Walls / Frost-Protected Shallow Foundation

Arnold K | Posted in General Questions on

Hi,

I was suggested to me that there would be significant saving by doing a short frost walls, about 16 inches below existing ground.
I presume they are referring to a frost protected shallow foundation but don’t these require foam on the exterior of the foundation plus a skirt about 12″ below ground around the perimeter?

Thank you,
Arnold

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Arnold,

    Any footings that don't extend down to whatever the frost depth is where you are building would require foam to insulate them against freezing.

    The other things the determine the depth of a foundation are: The thickness of the organic matter you have to remove to get down to good bearing soil, and the depth from grade to the footing to accommodate perimeter (French) drains, rock and separate pipes for downspouts.

    1. Arnold K | | #2

      So exterior foam would be require or can the foam be placed on the inside as shown in my drawing?

      Thanks,
      Arnold

      1. Expert Member
        Malcolm Taylor | | #3

        Arnold,

        Building where the frost depth is only a foot below grade, it's not something I've ever dug into very deeply. I believe the definitive guide is this one: https://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf

        A couple of things I noticed on the detail you posted:
        - You show the foundation width reduced at the top. That is usually done to keep the perimeter foam under the exterior walls. Your wall is so wide that wouldn't be necessary. The foam could run up the interior of the stem-wall continuously from the footing to the top.
        - With a wall that wide, are you sure you need exterior insulation? It adds a lot of complexity for very litttle gain. Double-stud walls and exterior insulation are usually an either / or choice.

        1. Arnold K | | #4

          Hi Malcolm,

          The frost line in Ottawa, Canada is around 3.5 feet below grade.

          I'll look into stream lining that interior vertical insulation on the stem wall. Doing it the way you mention would make it easier and more straight forward during construction.
          I did that thinking a shelve was required for the slab to sit on in order for the stem wall to support it.

          Thanks,
          Arnold

          1. Expert Member
            Malcolm Taylor | | #5

            Arnold,

            You are better off having the whole slab over the compacted fill so any differential settlement doesn't crack it.

            I'd have loved to have lived in a house insulated like that when I was in Ottawa. Most of the ones I ended up in were extremely cold and inefficient.

  2. Arnold K | | #6

    Thanks for the tip Malcolm, I will definitely look into having the whole slab over the compacted fill.

    My current house is subdivision, code minimum house and it shows in the winter. The temperature fluctuation varies a lot, even the kids bedroom which are next to each other have several degrees difference.

    Thanks,
    Arnold

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