Sealing air duct that will be permanently hidden

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KevP
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Sealing air duct that will be permanently hidden

Post by KevP » Tue Aug 06, 2019 6:17 pm

Hey Shannon,
Juat wanted to see your preference on duct sealing methods between foil tape vs mastic. Most HVAc folks love mastic as it has better lifespan. I see some butyl mastic tapes such as the masticfoil from Nashua which claima permanent bond and an alternative to mastic.
I have a duct that goes through a joist and right next to the rim joist. Im framing that wall see a lot of open and unsealed gaps. Not sure if tape holds long term or not. Only issue on mastic is that this area is tight amd hard to get a brush in. Thoughts?
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Aaron
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Re: Sealing air duct that will be permanently hidden

Post by Aaron » Tue Aug 06, 2019 7:49 pm

Boy I dunno. I'm not sure there's code either way you go. The mastic is probably overkill in most cases, or maybe should be used in ideal circumstances when you have easy access to apply it.

That foil tape is very sticky stuff and I think it works pretty darn well at sealing ducts. If you can temporarily remove those sheet-metal straps across the structure and pull the duct forward JUST enough to slip some of that tape around it, it should be just FINE.

KevP
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Re: Sealing air duct that will be permanently hidden

Post by KevP » Tue Aug 06, 2019 8:45 pm

Thanks. I thought about that too. Another thing i thought of is spray foam. Touch N Foam has a 15 ft kit for small applications and the nozzle is pretty small.

http://www.touch-n-foam.com/system-15.html

KevP
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Re: Sealing air duct that will be permanently hidden

Post by KevP » Tue Aug 06, 2019 8:48 pm

I should be able to do it with tape with no issue. Mastic gets a bit tricky since i have to apply the top with gloves.
The butyle tape is much stickier and stronger than regular foil tape

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Shannon
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Re: Sealing air duct that will be permanently hidden

Post by Shannon » Tue Aug 06, 2019 9:06 pm

I generally use foil tape. wipe the dust off the duct work and do a good job pressing it on and it sticks well. Watch for sharp edges ,the tape will split right open and so will your skin!
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A. Spruce
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Re: Sealing air duct that will be permanently hidden

Post by A. Spruce » Tue Aug 06, 2019 11:05 pm

I am another advocate of metal tape. The only time I've seen it fail is when it was misused or the ducting was abused (major traffic area of an attic or crawl space, etc.) As Shannon points out, sharp edges are the death of metal tape, however, a couple of layers that are applied offset to each other, will go a long way to taming those sharp edges.

What I mean by offset is that if you stack the tape one layer atop another, sure, you buffer the sharp edge, but you do not gain any holding power, if you offset your layers so that each has some purchase on the original base, you will not only build up your cushion, you will also increase your holding power.

You can usually soften sharp edges a bit with some ingenuity, how you go about it depends on where the sharp edge is, access to it, and surrounding purchase area for the tape.
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KevP
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Re: Sealing air duct that will be permanently hidden

Post by KevP » Wed Aug 07, 2019 9:50 am

Thanks guys. Im going to stick to the tape and maybe spray some great stuff around the opening.

Another thing, do you guys think it would be fine to leave the cavity uninsulated? So this rim joist and joist setup runs for 50 ft. The basement is divided into two rooms. Im framing the one which would be a family room. On the other side, the joist is doubled close to the foundation edge so there is literally 1/4 gap left, so no way to insulate that area. Once im done framing this wall the gap on this side is closed up with foam. I will still have a cavity that isnot filled up. I guess that would be ok? I mean that gap existed for 70 years.

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A. Spruce
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Re: Sealing air duct that will be permanently hidden

Post by A. Spruce » Wed Aug 07, 2019 10:21 pm

When working in confined spaces, use spray foam extremely sparingly! It expands so much and so quickly that the leading edge can dry/cure before the central mass stops expanding, which can crush or distort things pretty severely. If you do foam that tiny gap, stick the straw as deep as you can and give it just enough of a squeeze to get the product out the end of the tube and leave a small bead behind in your path. Let this sit and cure for the minimum recommended time, then come back and repeat the process until the gap is filled sufficiently.

Reality is, you are not likely to see any dividends from trying to insulate such a tiny gap. A 1/4" of foam really isn't much, and the mess it makes during the application or down the road when repairs or modifications to the duct need to be made, it just isn't worth it.
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