Using both foil tape AND mastic?

Old 05-24-14, 10:01 AM
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Question Using both foil tape AND mastic?

Hey everyone. I'm about to start sealing up the seams and joints in some exposed ducts that run through my non-car (it's just for storage) garage.

A few months back I read somewhere that you could use both foil tape (not duct tape) and mastic along seams and joints -- as in tape first and then mastic over it -- but now for the life of me can't find that particular article. Everything I'm finding now says to use one or the other, and preferably the mastic vs. foil tape.

We're not talking about any giant gaps here, mostly just sealing up the straight seams where two pieces of duct meet, because I can see some of that black streaking from air leaking out. Apart from the added expense, is doing both overkill? Or assuming it's applied correctly is mastic on its own acceptable? (I will be insulating the ducts with rigid insulation after it's all finished.)


Last edited by Morgan19; 05-24-14 at 10:24 AM.
Old 05-24-14, 11:09 AM
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I'm not an authority on sealing ducts, but I have never read or seen both used, mastic over foil. IMO, overkill.
Personally, I recommend the easier of the two foil and especially any place where you might need to make changes. My personal opinion aside, I believe the pros prefer mastic, but maybe I just had a bad experience with it, messy.

Old 05-25-14, 07:29 PM
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I do know the article you are referring to where someone was saying to use both. I am a journeyman sheet metal worker and have NEVER used both. It would be pointless to use both since a true mastic seal already uses it's own mesh tape. (More below). Most of the time in residential you don't use anything on transverse seams on rectangular duct. But if you want to seal it up a little better there are a couple ways you can go about it each equally effective just giving a little different appearance.

My preferred way for residential is clear silicone, just run a small bead the whole way around the drive cleat on the sides (the one that is bent over top and bottom) and bend the tabs up a little ways and squirt some right underneath of it. If you don't go the whole way around the air will just come out further down. I like this method because it gives a good permanent seal with a great finished appearance.

The best seal is using a brush on duct sealer like G80-197 | Hardcast | DS-321-4 | Johnstone Supply. It looks like mastic and I think that is what is confusing people. Just goop it on thick at the corners and work it in at the sides of the cleats. You can improve the appearance of this by masking out the area to the sides of the joint to give you a nice clean line.

The easiest and fastest route is to just throw some foil tape over the seams. I'm not a fan of using foil tape, not because it doesn't offer a good seal, but just because the appearance always comes out looking like a scab job. I know residential installers that build their plenum transitions for furnace change-outs on site and scab them together with screws and foil tape. It does work just never seems professional to me.

Just an FYI that true mastic is a 2 part system as shown in this image [ATTACH=CONFIG]32238[/ATTACH]
It is rarely, if ever, used in residential. It takes more time to do and is very messy, though it does offer a good seal. The ONLY place I use this is for particulate exhaust like that used in dust collection systems or welding exhaust.
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