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Community and Q&A

Sub slab air sealing

user-4885540 | Posted in General Questions on

I’m within a month of pouring the footings and basement walls for our house in SW MT (climate zone 6).  Plan is for six inches of EPS, stego wrap (or the equivalent) taped and brought up the exterior basement walls and eventually sealed to the wall with closed cell foam.  Then a 4 inch slab with embedded radiant tubes.  

What I’m not clear on is how to air seal under any interior posts or walls that are load bearing (Ie things in the basement that rest on footings inside the perimeter walls – stairwell footings, posts, walls).  Do you somehow bring and tape the poly up around the post or wall and tape it off?  Or do you run the poly over the foam (or footing) and the wall just bears on the slab directly, transmitting it’s load through the foam and to the footing?

I have been looking through the detail archive and can’t find a cross section, but methinks the order should be footing, aggregate, foam, poly, slab, and then post.  Or asked more simply, do load bearing points in a basement affect the sub slab insulation and air sealing?

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  1. silkwj | | #1

    I usually rely on the slab + VB combo to be my air barrier below grade. So I don't worry too much about bringing the VB up around posts etc, and just seal to the slab surface once it's set. Detail the VB for vapor control and easy sequencing around the pouring of the slab. If you've got footings that come up through the slab, the slab being poured around it will be a pretty good seal. If a gap forms, a simple sealant ought to take care of it.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Peter.

    This video shows a lot of the details you are asking about: Super-Insulated Slab.

  3. Expert Member
    Michael Maines | | #3

    Peter, the bulletproof approach would be to carefully tape the membrane around each penetration, but that's a lot of work and pouring concrete is messy work so the tape may be vulnerable. In most cases I think Cramer's approach is fine. One tip is to wrap columns with a couple layers of sill sealer; after the concrete cures, cut the sill sealer just below the surface and seal the space with a concrete joint filler.

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