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Community and Q&A

Sealing Ductwork With Expanding Foam

christian_330 | Posted in General Questions on

I’ve got some tricky duct work near the registers that I’d like to try to seal up from the basement/garage down below. I’m planning on just filling large gaps with expanding foam and trying to cover everything with mastic to make it all one surface similar to how I use the Proseco R-guard liquid flashing in my window openings. Is there any better or different way to do this?

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  1. Expert Member
    BILL WICHERS | | #1

    You may find caulk works better than spray foam in some of those areas. Caulk is better for small gaps. What I would do with that floor register is to run a small bead of urethane caulk around the perimeter of the subfloor cutout, then screw the duct flange to subfloor to compress the caulk. For belt and suspenders, you could run some tape over the top of the subfloor to tie the duct in. Make sure you seal any gaps around where the ductwork has been cut, those are classic leaky spots.

    I would use primarily tape in those wall openings.

    I would be reluctant to use liquid flashing since it might smell when heated by hot air in the duct.


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