Sealing around ducts

[JackOfDiamonds]

This is what the drywall looks like around my ducts. I think it should be better, both for looks, and to keep leaks out, but I don't know what to use. Caulk? Duct sealant? Drywall mud?

The ducts are odd shape, so a round trim plate won't really fit.

 

[karoc]

Your going have flex or hard pipe connect to that, so for sake of help keep things from rattling I would make 1/8 gap all way around that sheetmetal box then caulk it.
If that’s going have cold air going through it, you may need insulate around it

 

[Dumber than lumber]

No to caulk.
Mastic and fiberglass tape.
Flamesuit on.

 

[Gozo]

I’m gonna second the no to caulk. Ducting tends to shift, vibrate, expand/contract, etc. Caulk doesn’t have a lot of movement forgiveness; it tends to crack over time. Go with something with some flex. I’d use some kind of gasketing, like the spongy stuff that you’d find where windows or doors close on.

 

[PoorUB]

This is what the drywall looks like around my ducts. I think it should be better, both for looks, and to keep leaks out, but I don't know what to use. Caulk? Duct sealant? Drywall mud?

The ducts are odd shape, so a round trim plate won't really fit.

Since nobody else asked, where are these ducts, and how pretty do they need to be?

 

[Dig Doug]

This is what the drywall looks like around my ducts. I think it should be better, both for looks, and to keep leaks out, but I don't know what to use. Caulk? Duct sealant? Drywall mud?

The ducts are odd shape, so a round trim plate won't really fit.

What exactly are you showing?

is that exposed in a room?
above a drop ceiling and won’t be seen ?
are you wanting to Install an AC grill on that round stub ?

 

[Dig Doug]

That looks like this style of fitting. It doesn’t appear to have any ductwork attached to it …

So I’m not sure what your trying to to accomplish
We need more information

 

[PoorUB]

No to caulk.
Mastic and fiberglass tape.
Flamesuit on.

I’m gonna second the no to caulk. Ducting tends to shift, vibrate, expand/contract, etc. Caulk doesn’t have a lot of movement forgiveness; it tends to crack over time. Go with something with some flex. I’d use some kind of gasketing, like the spongy stuff that you’d find where windows or doors close on.

Looking closer at the picture, it looks like the back side of a duct boot. The boot is securely screwed into the studs so it isn't going very far, so close to zero movement. I would take drywall mud and mud right up to the duct, short of 1/8"-1/4" all around, then fill the gap with paintable caulk. You could mud right up to the duct, then carefully cut out the gap using a sawzall blade or something similar.

We did it this way hundreds of times when I was doing HVAC work. Never had a complaint.