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Low expansion foam or airtight tape for windows?

Griffin728 | Posted in Green Building Techniques on

Hi all. One more air sealing question. Is there a preference here between using low expansion foam to seal the gap between a window frame and rough opening vs. tape? I’ve heard both methods referenced here. Should I consider doing both, or is foam alone sufficient?

This is a retrofit full frame replacement, with most drywall and siding left in place. Since we are eliminating the 2″ window weight pocket, I have asked to reduce the 6″ metal trim on the outside to a smaller size to allow for more glass in the opening. I’m also cutting back drywall to the rough opening to get the windows as close to rough opening size as possible.

Is low expansion foam the best way to get the new windows airtight, or is there a better method such as high performance tape or a combination of the two? Builder is only familiar with the foam method, but I want to make sure my .13 U value windows don’t leak air at the frames.


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  1. GBA Editor
    Martin Holladay | | #1

    Almost all sources agree that a high-quality tape will do a better job of air sealing than canned spray foam. The best tapes for this purpose are European tapes sold by Small Planet Workshop and 475 High-Performance Building Materials.

    Of course, the tapes are expensive. Whether or not the investment makes sense depends on your airtightness goal.

    If you plan to use tape, an argument can be made that you should also use canned spray foam (especially if the gap is large), because the spray foam adds R-value to the gap.

  2. Expert Member
    Dana Dorsett | | #2

    Wedging in backer-rod (flexible closed cell foam rods) is a good idea too, with or without the tape /spray foam.

  3. STEPHEN SHEEHY | | #3

    Ryan. The foam insulates, but the tape air seals. I've used a lot of tape air sealing my new house. The European tapes ( I've used Siga) cost about double what 3M 8067 flashing tape (which I've also used) costs. Both stick tenaciously.

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