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Potential Issues Due to Air Gap

verm | Posted in General Questions on

Hi, if I fill 2×4 attic wall studs with 3 inch of closed cell spray foam and then install 1 inch of continuous super tuff-R rigid foam across the studs to address thermal bridging, will the 1/2in air gap between the closed cell and rigid foam lead to any moisture issues?
Also would the rigid foam create a double vapor barrier here with the closed cell? The contractor said it would not. Im in NY, zone 5.

Thank you for your help!

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  1. Expert Member
    BILL WICHERS | | #1

    Ideally, you don't want any air gap between layers of insulation. Normally you'd put your continuous rigid foam on the exterior, then apply spray foam directly over it from the inside for the type of wall assembly you're suggesting.

    The Super Tuff-R says it has a foil facer, which means it's a vapor barrier. The 3" layer of closed cell spray foam would be a second vapor barrier on the opposite side, so you would have a double vapor barrier here.

    If you can install the Tuff-R on the outside of the wall, you could use mineral wool batts between the studs and have overall wall performance not much different from what it would be using closed cell spray foam. If you really want to use spray foam and Tuff-R on the interior, then I'd use open cell spray foam, which will fill the stud bays and then be trimmed flush, which will eliminate the air gap, and allow more drying potential since open cell spray foam is more vapor open than closed cell. Note that even with the open cell spray foam, if you have anything acting as a vapor barrier on the exterior (like a layer of foil faced polyiso, for example), you still have the double vapor barrier issue.


    1. verm | | #2

      Hi Bill, thank you for the response. Im insulating a walk up attic to be conditioned, unvented. I dont plan on removing siding to insulate from the outside and Id like to get to recommended R vlaue of 20 for zone 5 walls. If I fill walls with open cell 3.5 inches thatll give me R 13.3 plus 1 in of rigid foam would be 19.8. I think thats close enough so your recommendation to use open cell would work. But would this be cheaper then completely filling with closed cell and grinding it down? The contractor is doing this for the 2x6 ceiling already . Ill have to find out... ty!

      1. Expert Member
        BILL WICHERS | | #3

        Your contractor will not like you if you want to trim that much closed cell. You have to trim closed cell with a saw. It's not worth the effort, and you'll never get a clean job of it. Just use open cell and don't worry about the slight difference in R value -- your overall whole-wall performance will be essentially the same with a full fill of open cell as it would with a partial fill of closed cell anyway.


      2. Expert Member
        Dana Dorsett | | #4

        >"If I fill walls with open cell 3.5 inches thatll give me R 13.3 plus 1 in of rigid foam would be 19.8. I think thats close enough so your recommendation to use open cell would work."

        Not only "...close enough...." It would fully meet IRC 2018 code on a U-factor basis (=< U0.060, which is R16.7 "whole wall"), since the inch of rigid foam would at least double the R-value of the framing fraction, cutting the thermal bridging framing losses by half. The R-value of 3.5" of 2x4 is roughly R4.2, and with an inch of EPS that becomes R8.4. The R value of the 5.5" of a 2x6 stud on which the R20 requirement was based is roughly R6.6.

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