GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Wrapping subslab poly over perimeter foam

ethant | Posted in Green Building Techniques on

I am trying to figure out how to connect my subslab poly to Intello air Barrier. As you can see on the attached detail, I am proposing taping the poly to the foam… and the Intello is taped to the sill plate and perhaps the top of the foam. I’m not sure how best to bridge this gap. I know that the foam isn’t an air barrier, but I’m not sure how to bridge this gap… perhaps I could use some sort of faced foam at this location?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. ethant | | #1

    Perhaps if I use Johns Manville AP Foil-Faced Polyiso Continuous Insulation at the slab perimeter, I can tape the top of the polyiso to the foundation wall top, assuming I can find a tape compatible with the Polyiso foil face and the Henry CM100.

    I am drawn to the Polyiso because, per JM literature, "AP Foil-Faced Polyiso Continuous Insulation is produced with an EPA-compliant hydrocarbon-based blowing agent that has zero Ozone Depletion Potential (ODP) and virtually no Global Warming Potential (GWP); it also meets both CFC- and HCFC-free specification requirement."

    It also seems that in this location the Polyiso is less likely to suffer R Value degradation, moisture problems, or termite problems.

  2. user-2310254 | | #2


    Aren't you worried about the polyiso absorbing moisture? Reclaimed XPS or EPS would be safer and greener.

  3. Expert Member
    Dana Dorsett | | #3

    Taped foam pretty much IS an air barrier.

    But lapping the sub-slab vapor barrier up and over the top of the slab-edge foam then taping it to the ProClima where it is supported by the sill plate is better. That would also provide a capillary break for the cut edge of the (hygroscopic) polyiso on the framed wall.

    Some manufacturers' marketing sheets notwithstanding, it's risky to use polyiso as the slab edge foam.

  4. ethant | | #4

    Thanks DANA, that is very helpful. 475 also recommended just lapping the sub slab barrier... another case of me overthinking... better than underthinking (in most cases)!

  5. ethant | | #5

    Steve, I am fine with reclaimed XPS or EPS, my question is how to wrap the air barrier up and over. Our specified liquid applied membrane can't go over foam, and I am not confident in folding the poly sheet up and over, so I am trying to find the right way to connect my subslab air barrier and wall cavity air barrier, bridging the gap which is the foam. My question is whether there will really be much moisture in the location shown, but I am fine avoiding Polyiso, I just was thinking about it because it comes with a foil face which could act as the air barrier.

  6. DAVID GOODYEAR | | #6

    I had a similar detail with foam at the interior of a frost wall. The sub slab barrier wraps up and under the wall plate. Once the walls a framed you'll have a couple of inches of sub slab barrier exposed at the interior around the perimeter. Tape and acoustical sealant can be used to connect the interior intello barrier to the sub slab barrier.

  7. Expert Member


    This isn't directly helpful to your question, but I'd suggest introducing a little wiggle-room into that foundation detail so everything doesn't have to be bang-on to work.

    If the foundation ends up a little out of square, or had even the slightest bow, you could have trouble covering the slab edge with your baseboard trim, or have to spend a lot of time trying to get the foam coplanar.

    A few suggestions would be to:

    - Taper the top of the interior foam.
    - Reduce the foundation width under the plate to 5 1/2"
    - Break the continuity of the exterior foam at the flashing

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |