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Rigid foam seams on roof deck insulation

Joe Norm | Posted in General Questions on

I have access to affordable foam, but the edges of some of the pieces are slightly deformed or cracked, some look worse than others. None look terrible.

My concern is using one single layer of this foam for an unvented assembly, on top of roof deck, how do I address some of the edge issues. Climate 4C.

Is this a product to stay away from or will it work fine? Price is less than $20 per sheet.

I will try to attach photo.

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  1. Expert Member
    Zephyr7 | | #1

    I was about to refer you to a recent question about this and then noticed you asked that earlier question and thus one is a little different :-)

    Imperfect edges are a fact of life when using discount foam. Your sheets look pretty good with reasonably clean cuts (not smashed gouges). I’d butt them tightly together and then inject some canned foam in the gap to seal as good as you can, then tape. The idea with the canned foam is just fill the worst of the void, the tape will provide an air seal. Try not to let the foam expand out of the gap where it will interfere with the tape.

    You can also try trimming the edges. A saw will make a mess (staticky dust that will go everywhere and cling to everything), but a sharpened putty knife can run along a long metal straight edge and make short work of it. This will be a bit more labor, but I’d probably try this before using canned foam.

    You can use a 10’ stick of strut (find it in the electrical department of your local box store) as a cheap straight edge. Use the narrow 7/8” stuff. This is cheaper than a “real” straight edge and will go the entire 8’ length of a sheet of insulation.


  2. Joe Norm | | #2

    Can you successfully tape un faced (no foil) Polyiso?

  3. Jon R | | #3

    Thinner foam could be cut with a: "CenterFire 10 in. Circular Foam Blade"

    One could also air seal with a fully adhered underlayment over the foam.

    1. Joe Norm | | #4

      My question remains, do tapes stick to polyiso board without foil?

      1. Expert Member
        Zephyr7 | | #5

        Yes, but not as well, and not all tapes. You need more aggressive adhesives to stick to non-foil faced polyiso. Unfortunately, I can’t recommend a good tape for this, but there are others on here who can so you should get a recommendation from someone.


  4. Expert Member
    Akos | | #6


    I've tried Grace detail flashing and it sticks, but if you pull the paper fibers come off quite easily. I would not rely on taped fiber faced foam as an air barrier, probably better to handle the air sealing bellow the foam.

  5. Expert Member
    Peter Yost | | #7

    Hi Joe -

    I would not use any PSA tape on unfaced PI rigid insulation without a compatible primer. It can be hard to match primers IF you choose a PSA tape that is not manufactured and designed for the tape. I have tried to use "water-based" primers with solvent-based tapes and vice versa with bad results. If you are unsure about the compatibility of your primer and your tape, then just test them--you will know right away if they are compatible. If compatible, you will destroy the rigid foam before the adhesive of the tape lets go.



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