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.

 

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

 

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.

 

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.

 

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

 

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?

 

My concern is the off gassing

 

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!!!

 

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

 

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.