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great stuff should not be used.
I've seen it pick up boxes off the attic floor 1 1/2"
and it shrinks. little control with foam.
off gassing & leakage are my concerns.

this is how I seal supply boxes.
hardcast #1402 mastic tape only
I buy 3" rolls & split width to 1 1/2"
it only needs to go from sheetrock
ceiling (within area covered by grill)
to metal lip inside supply box.
reinforce corners.
this is the only tape I've found that is
strong enough not to come lose when
you replace the grill.

caulk doesn't stick to the inside of the
sheetrock (dust) and if you put the
grill back up before caulk dries..it sticks
to the grill. I've taken many a supply
grill off that the caulk was on the grill
and not on the gap.
trying to seal from inside the attic..
what about the side of the box attached to
the rafter??

just my experience.
heres my pic.
best of luck.
Attachment 382461

Great stuff has its place... I guess, but ductwork is not the place.

it doesn't really matter wheather you seal it inside or outside just so long as it is sealed. IMO nothing beats a good old bucket of mastic and a 2" brush. It is messy but once up there and dry it will be there when the cows come home.LOL!!!

My concern is the off gassing

Thanks for the discussion, I've got some water based hvac mastic that I used on a couple to seal the flex line to the boot. I also have some in a caulk gun tube that I haven't tried yet. I also have great stuff cans. I thought about spraying great stuff in around the boots from the attic to seal the boot to the sheetrock. I also thought of using the caulk gun mastic from below and then using the metallic tape I have to go from the boot to the sheetrock on the bottom side. The register would then cover that up. Is Great Stuff a viable option here at least for the boot to sheetrock seal?

Originally Posted by jpbhvac

That does sound like the easy way, but I've never seen that approach last over the long haul. I recommend a tight seal that lasts the distance. My experience has shown mastic to fill the demand.

There is no weather exposure, so latex calk sealing the boot to the sheet rock will basically last forever if applied halfway properly.
They also package duct sealing mastic in calk tubes.

I'll seal the boots from the attic if I'm having to go around to all of the boots to replace the duct, etc. anyway, otherwise they get sealed from inside when I'm replacing the registers with better ones, and/or cleaning the popcorn texture out of them.
Sealing from inside is the only choice for most downstairs boots in 2 story homes.

Originally Posted by mark beiser

Impossible?
It is very easy to seal boots to the sheet rock from the inside with latex caulk...

That does sound like the easy way, but I've never seen that approach last over the long haul. I recommend a tight seal that lasts the distance. My experience has shown mastic to fill the demand.

Originally Posted by jpbhvac

Nothing but mastic sealant. But what about the joint between the boot and the drywall? There is a great amount of infiltration here and it's impossible to get a good seal from the inside. I would seal the ducts and the boots from the attic side.

Impossible?
It is very easy to seal boots to the sheet rock from the inside with latex caulk...

I like that product, but I've not had good luck on Sheetrock nor the mastic paste.

Originally Posted by jpbhvac

Nothing but mastic sealant. But what about the joint between the boot and the drywall? There is a great amount of infiltration here and it's impossible to get a good seal from the inside. I would seal the ducts and the boots from the attic side.

mastic tape like hard cast 1402, will stick to the sheet rock and boot and stay there.

Nothing but mastic sealant. But what about the joint between the boot and the drywall? There is a great amount of infiltration here and it's impossible to get a good seal from the inside. I would seal the ducts and the boots from the attic side.

What they said. The tape won't hold after a while. You can use some mastic to seal it this way and. Have done this at times to insure a seal in the connection in areas that were hard to access any other way.

I wouldn't use duct tape inside the duct. Mastic tape or brush on mastic works on the inside of duct.

A seal is a seal. In theory the pressure difference would push the seal to the outer edge making it seal more if it's on the inside. However the supply air at that point in A/C mode would have almost a 100% RH which could cause the tape to fail over time.

Sealing Duct Work From The Inside? Good Idea? Bad Idea?

I have 9 flex duct lines coming off a sheet metal trunk in my attic. I started sealing it last year, but quickly lost interest I was doing it the way I thought was right, pulling the duct tape off, putting HVAC mastic on the supply box cylinder, using a zip tie to hold that in place, and then using another zip tie to hold the outer sleeve in place. I'm just finishing up spraying Great Stuff into every nook and cranny I can get to and I'm not hip on spending any more time up there than I have to.

I have an idea motivated by laziness. However, I feel it's a good idea. What if I take the diffuser off from the ceiling and then just use some HVAC rated tape to seal the metal to the inside of the flex duct? It's 6 inches in diameter which doesn't give me a lot of room to maneuver, but if I can pull it off, would I really be losing out on a proper seal? I know I'll still need to seal the ductwork at the trunk from the outside, but will this work for the supplies?