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Community and Q&A

Protecting skirt insulation?

MikeSullivan72 | Posted in Green Building Techniques on

I am finishing up the exterior insulation on a monolithic slab detached garage project.  In addition to the sub slab and vertical perimeter insulation I am placing skirt insulation around the building.  I am using Formular 300 xps rigid foam for this purpose and will be backfilling with top soil that was excavated from the site.  Should I be putting anything on the skirt insulation to protect it prior to backfilling with soil?

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  1. Expert Member
    Michael Maines | | #1

    Mike, by "skirt insulation" do you mean vertical insulation at the slab, or a horizontal frost wing? Vertical insulation is not required to be protected, but it's a good idea to use backfill that won't damage the foam excessively. If it extends above grade, which it should, you would also want to cover the above-grade portion.

    Horizontal insulation less than 12" below grade is supposed to be protected, according to the IRC. See section 403.3.2 here: Your code may vary, and most builders ignore this detail without repercussion.

    1. MikeSullivan72 | | #2

      Thanks for the information Michael.

      The vertical xps board has been covered with a parging which is designed for use on rigid foam insulation. The horizontal frost wing which I have started to install isn’t covered with anything but the soil I’m backfilling with. Once all the back fill and grading is done the insulation will be around 12” deep. My concern was mainly that the foam could be exposed to wet/damp conditions more than I was concerned about physical or mechanical damage.

      Should I have put down some form of vapor barrier on the insulation before backfilling?

      1. Expert Member
        Dana Dorsett | | #3

        >" I am using Formular 300 xps rigid foam..."
        >"Should I have put down some form of vapor barrier on the insulation before backfilling?"

        Even at 1" Foamular 300 is rated at 0.87 perms ( which would meet the Canadian NBC definition of "vapour barrier"). It has zero capillarity and is hydrophobic with a max water absorption of 0.7% in an ASTM D2842 test. Given that, I'm not sure what it has to be protected from(?).

        If the property were under 25' of water for a month in a major flood I suppose it could be compromised, but I doubt you'd really care at that point.

        1. Expert Member
          Michael Maines | | #4

          I have assumed that the requirement is for protection against mechanical damage--i.e., digging up daisies.

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