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I'm finishing my basement and want to seal up the mechanicals before ceiling gets closed up with drywall. What is better for sealing the connections: heavy duty foil tape or mastic?

My current situation involves rectangular ducts that have a metal band at the seams as well as tubular ducts leading to floor registers on first floor. Since I can access most of it right now, deliberating which approach makes most sense, some of the top seams might be hard to reach since they are directly beneath the floor, tape seems potentially easier to “floss” up there but I’ve heard mastic would be stronger. Mastic application doesn’t seem that intimidating, as long as it’s just a matter of wrapping in fiberglass tape and slathering on some mastic to cover the seam (wouldn’t be taking anything apart just dealing with what’s in place). Hence my question, if it totally subjective and it doesn’t matter which I choose as a DIYer, my apologies.

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    Voting to close as opinion-based/product recommendation. This is a matter of product quality, duct type, installation preference, local codes, etc.
    – isherwood
    Feb 17, 2022 at 13:51
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    Any question asking about different approaches to a problem is tangentially dependent on the quality of the products and labor chosen with each approach. That alone doesn't make the question off topic. If the answer depends on circumstances, the answer should describe the circumstances where one or another approach is best. I think this is a good question. If OP described their own situation better, it would still be a good question, with narrower scope and narrower answers.
    – jay613
    Feb 17, 2022 at 16:56
  • I would agree if the question was worded differently and had more information. Simply asking what's "best" with no detail or qualifiers is certainly a subjective thing.
    – isherwood
    Feb 17, 2022 at 17:56
  • Tape goes on the unit to facilitate repairs. Do w/e you want anywhere else.
    – Mazura
    Feb 18, 2022 at 8:55

2 Answers 2

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If you don't have experience with duct sealant, and just doing a once-in-a-lifetime installation in your own house, and especially if you are working overhead, from all accounts I would say a high quality HVAC foil tape is the best approach.

Not because it's better, but because you'll get better results, and it's easier to learn and fix your mistakes if you've never worked with either before.

There are situations where one or the other is better .... long straight flat well fitting seams are where tape does great. Tight bends, compound bends, round flanges, big gaps ... are where mastic and fiberglass tape do better but again ... not necessarily to a DIYer doing it for the first and last time.

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Most foil tapes are absolute garbage and the adhesion doesn't don't last much longer than one season if you're lucky.

I use Nashua Foilmastic tape for straight runs such as seams along straight pipe and where one pipe inserts into another.

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For elbow bends I get a pair of cheap-o latex gloves and lather on some mastic. Make sure you're wearing clothes which you don't care about because mastic does not wash off. If it gets on your clothes then just let it dry and maybe you'll be able to peel it later.

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    This hasn't been my experience. Both the tape I happen to have on hand and the one my HVAC pros have used have shown no sign of release after many vigorous heating and cooling seasons. Maybe we were lucky, but this question boils down to a product recommendation, which is off topic.
    – isherwood
    Feb 17, 2022 at 13:51
  • @isherwood My experience has been more along this road: diy.stackexchange.com/q/42755/42053
    – MonkeyZeus
    Feb 17, 2022 at 13:56
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    No doubt that's the case for some. My greater point was that this question has no objective answer. Caulk isn't a silver bullet either.
    – isherwood
    Feb 17, 2022 at 13:59
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    @isherwood I read the question as akin to "Should I use latex caulk or bathroom silicone in my bathroom?" and the objective answer is "Use caulk for surfaces to be painted like the transition from baseboard to wall and bathroom silicone for sealing the tub." Yes, my answer does include a specific product recommendation.
    – MonkeyZeus
    Feb 17, 2022 at 14:00
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    @isherwood latex/silicone was just an analogy
    – MonkeyZeus
    Feb 17, 2022 at 14:04

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