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
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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.
Yeah, I’ve heard about people seeing canned foam issues over time, but I’ve never see. It myself. I did recently notice the canned foam didn’t seem to expand correctly when I used it in very cold (near freezing) temperatures — it was more bubbly and crumbly, similar to what I’ve heard people talk about, so maybe application temperature is the big issue there.
The “door and window” canned foam can tolerate small amounts of movement better than the normal stuff. The Loctite “Tite Foam” stuff seems to be more durable than any of the great stuff products. If you have a trouble spot where you want durable foam, try the TiteFoam stuff, which seems to be under the GE brand now (but it’s still made by Henkel according to the can). I like that stuff for foaming in blocks or rigid foam as one example, since it’s much denser than regular canned foam and more durable as a result. The tradeoff is higher cost, but it’s still much cheaper than a 2 part spray foam kit for smaller jobs.
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.
http://passivehousepa.blogspot.com/2012/11/using-spray-foam-to-seal-air-leaks-for.html
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.
Malcolm, canned foam is our friend! :-)
If you want a clean line, I find a fine tooth hacksaw blade used by hand (I.e. without a handle) can be made to bend by spring force and will cut a nice clean cut across overexpanded canned foam. After that, you can paint over the stuff, or put up a trim piece without the need to air seal the time piece itself. I’ve done this countless times, just be careful with the hacksaw blade — if it grabs some wood or other material like that, you can slide your hand over the blade. I have some of those one-end hacksaw blade holders that are great for tougher trimming jobs.
Bill
https://www.harborfreight.com/6-12-in-flush-cut-saw-71141.html?gQT=1
a great tool to keep on hand for trimming up spray foam. the blade is super flexible, cuts decently smooth. less risk of slicing hand :)
I just might have to go get myself one of those!
Thanks for posting that link.
Bill