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Air sealing ability of canned spray foam (minimal expanding) when cut????

toothman2020 | Posted in General Questions on

Minimal expansion spray foam was used to install my windows. The foam expanded towards the interior and was then cut flush with the windows. I was planning to seal the windows to the wood framing with Fentrim IS 20 from Siga Swiss. To my demise a dry wall return was riveted to the window thus preventing the use of tape on the interior. 

1-Is the air sealing ability of minimally expanding spray foam reduced when it is cut and that hard finished layer is removed? 
2- Would caulking this exposed spray foam be warranted? Roughly 1-2″ wide need to be covered.


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  1. Expert Member
    Akos | | #1

    Canned foam provides a decent initial seal. Since buildings move with seasons and wind, even flex foam will separate and will start to leak. Not a large leak, but enough to be felt.

    The best is always to back up the canned foam with either flex caulk or tape. If you use one of the thinner tapes, I'm pretty sure you can tape into the drywall return. Fussy but doable.

    Cutting the foam surface doesn't effect the air seal at all.

  2. Expert Member
    BILL WICHERS | | #2

    The manufacturer actually claims that the "skin" on the exterior of the cured foam helps with the seal, but my own experience trimming canned foam flush is that it doesn't seem to make any difference as long as you have a reasonably thick layer ("reasonably thick" here is a normal application that is usually around an inch or so deep or more).

    I wouldn't bother with caulk. If you have a foam gun for the canned foam, you can use one of the snap-on plastic mini tips as a sort of needle to inject a little more foam in any thin or questionable areas. I've done that before, I call it "injecting" foam. I inject enough for it to ooze out somewhere that makes me think I've probably filled the void, then I trim it flush after it cures.

    Tape is good for extra insurance too, and I completely agree with Akos that backing the foam up with tape is a good idea if you have that option available.


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