Duct mastic

[rk05]

Yes you are looking at the top of the return but those gaps are where the joist bring the returns from different rooms to the main return trunk. You can see down into the return trunk through those gaps so that is allowing air to be pulled in from the basement as well.

[energy_rater_La]

I'd use the Hardcast brand 1402 mastic tape. once you
position it correctly you'll be done.
this is the one tape I've found that makes all kinds of
transitions, metal to wood, wood to sheetrock, foam board
to ductboard (not that you'd use foam & ductboard together).
as long as surfaces are clean & dry the mastic tape
will seal well, use a credit card or small squeegee to press
air bubbles out.

best of luck.

[rk05]

Where would I find this stuff? I have a Grainger and a Fergusion supply house near by. I would only need about 10' of it if that.

[jtrammel]

Where would I find this stuff? I have a Grainger and a Fergusion supply house near by. I would only need about 10' of it if that.

It comes in 100ft rolls and ferguson should have it.

[energy_rater_La]

not to worry...you'll use the other 90'.
this tape has more uses than....
duct tape & bubble gum.

[jakeesspoo]

your making this way too complicated. put a very thin bead of expansion foam in there. Knowone is going so see it so it wont look like hackery. It will work fine. dont use too much though or get minimal expansion foam so it does not actually push down and bend metal ductwork when it expands. I have seen people spray spray foam into walls and actually blow out the drywall as it expanded and cured.

[energy_rater_La]

don't do that. there is a reason why foam isn't used in ductwork or
near heat sources, and that it isn't an approved duct sealant by
MEC, ACCA or other entities.

just because no one sees it doesn't mean it is the right thing to do.

foam will shrink, mastic will stay where it is applied.

[jtrammel]

don't do that. there is a reason why foam isn't used in ductwork or
near heat sources, and that it isn't an approved duct sealant by
MEC, ACCA or other entities.

just because no one sees it doesn't mean it is the right thing to do.

foam will shrink, mastic will stay where it is applied.

X2

[hearthman]

I agree w/ no foam where to seal ducts. Would need to be listed to UL 181 a/b- fx
With larger gaps use fiberglass mesh tape bedded in mastic then mastic over that. You can get caulking tubes of most mastics. Drop the big bucks for a roll of Hardcast tape. Whatever you don't use on ducts can be used a zillion other places. If you think rubber cement duct tape is handy wait until you try Hardcast. You can tape a blob of Jello to a greased pole---we'll almost. Incredibly sticky, strong and resilient.