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How do you air seal a thermally-broken floating slab?

qbrt | Posted in Green Building Techniques on

Looking at (ignoring the optional R-15 insulation, which I don’t have room for), I can’t figure out where the air barrier is. Obviously the concrete itself is an air barrier, but it doesn’t extend the entire way, or seal against the wall. If you taped the EPS at the slab perimeter to the concrete, you’d get one step closer. Do you then tape the EPS to the poly, and the poly to the wall?

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  1. 5Stud | | #1

    6 mil poly is the air barrier. It is taped 18 inches up the wall. With very expensive tape.
    You certainly could tape the EPS to the concrete with more of that expensive tape.
    I don't understand why or how you would tape the poly to EPS.

    1. qbrt | | #4

      As I understood it, the poly was a vapor barrier and the concrete was the air barrier. The poly is very easy to rip and poke holes in while pouring the concrete. With the poly as the air barrier, it seems like one little hole underneath the slab now ruins your entire continuous air control later, because good luck getting at that hole to patch it.

      1. 5Stud | | #6

        My thinking is the horizontal ~8 inches above the footing and the 18 inches up the wall are the only crucial areas to protect the poly. Small holes anywhere else under slab are not a big deal.

      2. Expert Member
        MALCOLM TAYLOR | | #7


        Small holes in air-barriers are a problem when they lead to cavities or voids, but not when they are sandwiched against solid materials. So nail holes into studs, under slabs, etc. simply aren't worth worrying about unless there is a path for that air - and a large pressure differential to move it. As long as the perimeter of the poly is sealed it's fine.

  2. Expert Member
    Michael Maines | | #2

    The tape can seem expensive--I spec Siga Fentrim--but it's very effective. The alternatives include extending the poly sheeting all the way up the wall, which isn't cheap, and you would still need to seal it somehow at the top; or you could use a stinky, sticky product such as acoustical sealant but its, well, stinky.

    1. 5Stud | | #3

      For sure, Fentrim is very effective for sealing to concrete.
      My point was why use more than you need to.

    2. qbrt | | #5

      Is Fentrim still the right product to seal poly to a liquid air barrier like Visconn applied on bricks?

      1. Expert Member
        Michael Maines | | #8

        I believe that would work but I'm not sure. I would ask a Siga rep.

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