Penetration suggestion for air sealing
Mshr1
| Posted in General Questions on
in my old 1930 house I have this penetration where my ductwork goes from the furnace plenum into the unconditioned attic. I’m not sure the best way to air seal this penetration. Any suggestions would help. Thank you
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
That looks like a perfect job for canned foam. Stick the straw in there, inject some foam, let it expand out of the gap a bit, done. Remember that the cans are pretty much a one-use deal, regardless of their claims of reusable stoppers and tips, so plan your project so that you can seal ALL the gaps in that same general area with one can. I highly recommend wearing gloves while doing this, and something to cover your hair if you'll be working overhead.
Bill
I would trim the fiberglass duct wrap back to just a inch or so up in into the drywall. Just by cutting it with a knife at an angle. Then spray foam insulation, just a canned spray foam should be fine, or the red fire rated foam to be safe?
Spray foam and canned foam aren't really the same in common usage. Most people mean two part foam when they say "spray foam". That's why I always say "canned foam" when I mean something like Great Stuff. I make the distinction because the issues that people worry about with "spray foam" have to do with mixing of the two components, one-part canned foam does not have those issues.
The orange foam is fire rated, but that has to do with smoke sealing, not temperature rating of the foam. In the OP's case, that won't matter unless that drywall is part of a fire rated assembly, such as the wall of an attached garage.
Bill
spray foam for the large gaps + brush on mastic after the foam cures imo.
https://www.amazon.com/Red-Devil-0841DS-Acrylic-Sealant/dp/B00R6GMYYG
is dirt cheap for how far it will go and it lasts for months/years when properly stored. might be worth buying a bucket if you haven't done much air sealing in your house. it is applied with chip brushes -- or whatever brush, but no need to waste a $20 brush.
can be used for sealing top plates, hvac ducts, bathroom ducts, etc.
Why mastic over the foam? The foam alone should be sufficient to seal the penetration. I see no benefit to layering mastic over the foam.
Bill
belt and suspenders type thing.
i've had a couple of instances in my house where spray foam was initially airtight, then no longer was after some time. of course its definitely not a huge amount. after looking more into the issue, it seems like its not an uncommon issue.
spray foam doesn't handle movement very well, which is basically the source of the problem.
Mshr1,
It's no doubt a personality flaw, but I just can't air-seal things that are going to be visible with spray foam or mastic. I would use galvanized L-flashing attached to the duct, and bedded in high heat caulking. Once that was in place I'd be okay with foaming from above.