Do I need mastic here?
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The furnace has not been installed yet. These images are looking inside the vents the furnace will connect to. Do I need to apply mastic where the red arrows are pointing?
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Did mastic not get applied to the seams on the exterior side? It's generally easier to apply on the outside, rather than crawling inside the ductwork.
This duct enters the attic. The portion that's extending out into the home doesn't have mastic on the outside anywhere. I'm assuming its not on the exterior on the attic side either.
If it's well taped, you're fine. Tape is an acceptable way to seal ductwork too, mastic just works better in some cases (especially old ducts). It's not usually necassary to seal corner seams in fabricated ductwork, but it doesn't hurt. If you can't get access to use mastic on the exterior, I'd use foil tape (Nashua 324A is my favorite for this). Normally you'd tape on the exterior. I would be reluctant to use mastic on the interior of the duct myself.
Bill
Thanks. Will this foil tape work?
Polyken 339
https://a.co/d/iN16BvW
And can you apply foil tape on top of mastic? Will it stick well?
I am not familiar with that tape. You can get the Nashua 324A for about a dollar more from Amazon, but I'd check your local Home Depot which will usually have it in stock.
I would not try taping over mastic, you're unlikely to get a good seal between the tape and the irregular surface of the mastic. When using tape, you want to go right against clean ductwork (and be sure there is no oil film if it's brand new, you can clean off oil films with isopropyl alcohol). If you need to seal over an irregular surface, mastic is the better choice. If you apply mastic, remember the "nickel thick" rule, which gives you an idea of how thick the mastic layer should be. Most people apply mastic too thin.
Bill
Do you think I just need to apply the mastic on thicker? I had a hard time covering the area because of the metal tabs. Maybe I just need to use a putty knife to spread it on thick?
Yeah, you want it thicker. I'd try for a layer thick enough to completely cover and conceal the tabs.
BTW, I usually just wear some cheap gloves and smear the mastic on with my gloved hands. This usually works better than tools, especially in confined spaces.
Bill
Any reason you are reluctant to use mastic on the interior?