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.

 

[JackOfDiamonds]

That's a duct boot. What you are looking at is the side inside the AC closet, where the AC air handler will be. There will be flex duct connected between the air handler and the duct boot. The other side of the wall is the living space where the air will blow out, and it will have a grill/register on it. The boot is screwed to the studs inside the wall.

This side (on the inside of the AC closet) doesn't have to look great, but by default I think all drywall penetrations should be sealed somehow, just to keep drafts out, and keep air from streaming back into the AC closet.

The penetrations in the drywall of my existing garage are sealed with some kind of reddish, flexible, rubbery goop that's still not quite fully set even after 30 years. All the penetrations like where the garage door opener wires go through the drywall, or where the pipes for the softener system come through the drywall, are sealed with it. It's not caulk, it's not drywall mud, and it's not foam. I've been trying to figure out exactly what it is.

 

[welder4956]

That's a duct boot. What you are looking at is the side inside the AC closet, where the AC air handler will be. There will be flex duct connected between the air handler and the duct boot. The other side of the wall is the living space where the air will blow out, and it will have a grill/register on it. The boot is screwed to the studs inside the wall.

This side (on the inside of the AC closet) doesn't have to look great, but by default I think all drywall penetrations should be sealed somehow, just to keep drafts out, and keep air from streaming back into the AC closet.

The penetrations in the drywall of my existing garage are sealed with some kind of reddish, flexible, rubbery goop that's still not quite fully set even after 30 years. All the penetrations like where the garage door opener wires go through the drywall, or where the pipes for the softener system come through the drywall, are sealed with it. It's not caulk, it's not drywall mud, and it's not foam. I've been trying to figure out exactly what it is.

The red stuff is probably this: https://www.amazon.com/3M-Fire-Barrier-Moldable-Putty/dp/B002FYAMPM/?tag=atomicindus08-20

You might try this: https://www.amazon.com/RectorSeal-8...plication/dp/B008A3UG94/?tag=atomicindus08-20

 

[Fav Onefour]

Are you opposed to using fireblock foam? It can be trimmed after curing.

The opening already looks better than about 99.8% of the stuff I've seen inside closets.

Get some aluminum duct tape and seal up the flange before finishing the install. That will help keep air in your ducts.

 

[PCustoms]

Are you opposed to using fireblock foam? It can be trimmed after curing.

I was about to say this.

Gently spray it in, trim with knife once cured. Can sand, drywall compound etc. to really pretty it up if needed.

Inside a utility room I just left a little bead.

 

[PoorUB]

The penetrations in the drywall of my existing garage are sealed with some kind of reddish, flexible, rubbery goop that's still not quite fully set even after 30 years. All the penetrations like where the garage door opener wires go through the drywall, or where the pipes for the softener system come through the drywall, are sealed with it. It's not caulk, it's not drywall mud, and it's not foam. I've been trying to figure out exactly what it is.

If this is in the wall between the garage and house, you are dealing with a fire wall and the red stuff is fire caulk. When heated from a fire it expands to block off any penetration better to prevent the spread of the fire.
I used to do a lot of fire caulk in commercial buildings, but residential is more lenient. I am not certain fire caulk is required in a residence, but the drywall needs to be mudded up to the duct. Different areas of the country have different requirements. Here we would just have the guys finishing the drywall mud it up, but that probably has changed as I have been out of it for a few years.