Is foil tape or sealant better for sealing air ducts?

Question

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.

Answer 1

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.

Answer 2

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.

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.