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Community and Q&A

Under slab vapor barrier and insulation taped?

dfvellone | Posted in Green Building Techniques on

Do I need to tape seams in the vapor barrier and xps that will be under my slab-on-grade house foundation? I read Joseph Lstiburek’s article on concrete floor problems and his assertions regarding air movement and vapor diffusion lead me to assume that overlapping the barrier would be effective since there won’t be much air movement at all under the slab.

Also, tape the xps? I haven’t found much in the way of tape that sticks well to it, particularly if it’s cool and damp out – which is the dominant weather here now.

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  1. GBA Editor
    Martin Holladay | | #1

    I agree with Joe on this one -- there is no need to tape the vapor barrier or the rigid insulation panels under a concrete slab.

  2. Jesse Thompson | | #2

    We've seen surprising amounts of air movement coming up from under concrete floor slabs during blower door tests, we now mandate taping all vapor barrier seams, and then crucially bringing the sheet plastic up from under the slab and taped to the wall air barrier to ensure under-slab air never gets brought into the inside of the house.

    Concrete sure doesn't let air move through it, but concrete slabs have a surprising amount of penetrations in them and around their edges, especially with foam thermal breaks at slab edges.

    It seems to be moving through the crushed stone and up around slab edges and penetrations, and feels like taping the vapor barrier sheet will at least help close down potential radon pathways.

    Dow makes a Weathermate tape that they say is meant to be used with their foam boards.

  3. BobHr | | #3


    make sure the concrete is poured on the barrier.


    It sounds like a typical air barrier problems such as drywall. It is the edges and holes poked through the drywall that leak. The concrete poured on the poly will adhere to it. If the seam is at the middle of the slab it is really not going to make any difference. But the gap along the edges or where anything penetrates the floor are the problem areas.

    I go to Joe L report of CA builders putting poly then sand then concrete. Doing this allows any rips or seams in the poly to pass air or water to the sand . The air or water than passes though the sand and can make its way through the concrete or cracks in the concrete. When I say through concrete I mean the capilary action of the concrete to transport moisture to the other side.

    Its been a while since I read the report but it came down to the concrete should be poured on top of the poly. Small punctures were not a problem and dont use "fines" ie sand due to the capilary action of fines.

  4. wjrobinson | | #4

    Do just a Joe L. poly job with no tape or wrapping up the sides... then....thinking out of the box or slab...

    Pull from under slab radon and leaking interior air to the HRV as in the slab leakage as talked about above. Also if some of the under slab air is from other than the home interior it would set the home up for slight pressurization possibly and be a good thing?

    An experiment worth looking into

    Another benefit, th HRV replaces the radon fan and the energy it uses. Sucking interior air out past the slab is taking the coldest interior air out, warming the slab... all beneficial.

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