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Community and Q&A

Air sealing retrofit and how to protect rigid insulation past foundation?

welivehonestly | Posted in Green Building Techniques on

Hi all,

We are building an addition and replacing siding, roofing, and windows on the existing structure, so we have an opportunity to retrofit.

Now that we have some siding off, I can see the original wall is something like this:

1. 2×6 with R-21 batts
2. 1/2″ plywood sheathing
3. 1/2″ fiber-backed polyiso
4. Wrap/WRB
5. Cedar Siding

#3 is flush with the outside edge of the foundation wall, which is ICF with stucco. We will be replacing from #4 onward.

The addition/retrofit (steps 6 onward) will be:
1. Drywall
2. Certainteed MemBrain taped with Siga
3. 2×6 with dense-packed fiberglass or cellulose
4. Sheathing (OSB or plywood) – 1/2″ to match existing?
5. 1/2″ fiber-backed polyiso (to match existing)
6. Wrap/WRB
7. 2″ GPS rigid insulation
8. 3/4″ plywood furring strips diagonal
9. EDCO board and batten vertical steel siding

First problem: air sealing. I had planned on using Siga Wigluv tape on the sheathing seams, but if we preserve the polyiso on the existing structure, we won’t be able to get to the sheathing there. Is it viable to use the polyiso layer as an air tight layer, taping its seams with Siga Wigluv?

Next: Because the polyiso is essentially flush with the foundation wall, I am less worried about protecting the bottom because we could seal the sill down to the top of the ICF foundation wall. But I am concerned about protecting the bottom edge 2″ GPS from pests. I have seen perforated J-channel but not in a 2″ depth. The furring layer will be protected with Cor-A-Vent SV-5 top and bottom.

What do you suggest to protect the 2″ bottom edge of the GPS insulation?


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  1. Expert Member
    Michael Maines | | #1

    Patrick, I'm not sure if 1/2" fiber-backed polyiso meets airtightness standards. If it does, then sealing the seams with Siga Wigluv should work as an airtight layer. The polyiso will shrink a bit over time but the Wigluv can stretch. You might want to prime the areas where the tape will adhere to be safe.

    As for protecting the bottom of the GPS, you can get perforated metal J-channel custom-bent. Malcolm Taylor, a frequent contributor here, often recommends that approach.

    1. welivehonestly | | #2

      thanks for your reply, Michael.

      We haven't disassembled the wall enough to know how the polyiso is fastened, but I guess the other solution would be to remove it, seal the original plywood sheathing with the Siga Wigluv, then reattach it if it looks OK.

      I had looked at the perforated J and didn't realize they could be custom made - we will do that, thanks!

      1. Expert Member
        Michael Maines | | #3

        Patrick, you could also just go over the polyiso with an airtight WRB. In this case either a self-adhered type (Henry Blueskin is the best known; Siga Majvest SA and Pro Clima Adhero are others) or not self-adhered (Siga Majvest or Pro Clima Mento) would work. It's also important to understand what's happening at transitions to other planes and materials.

        1. welivehonestly | | #4

          OK - sure - on the original building they had the window flanges and house wrap outside the polyiso.
          If we used an airtight WRB, then went over that with the GPS, furring layer, and siding, would we be in good shape?
          I will need to research the self-adhered vs. not.

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