Mastic over mastic tape?

[b26440510]

What is the consensus about using mastic applied over mastic tape for metal ducts and plenums? I've been told that the moisture in the mastic will weaken the mastic tape and then both mastic tape & mastic will fail (or basically be useless).

Should it always be one or the other but not both?

[Kevin O'Neill]

Putting mastic over any tape is wasted effort. If the tape fails the mastic will come off with it. Period.

Put the mastic directly on the duct, embed mesh in it , then another coat of mastic.

[bmathews]

What is mastic tape? I've never seen it. There is hardcast and the mesh tape, like what they use for sheetrock repair.

[Rodeo]

Hello,
I would like to expound on what Kevin said. Never place MASTIC PASTE over foil tape. As the adhesion quality is only as good as what is in contact with the duct system.

In fact when installed on new duct system it would be best to put a small amount of mastic paste inside each duct union. This is the point where the slip & drive connect the rectangular trunk line. Essentially the same would apply for the supply lines. Our company has been using this method for several years and it has worked well enough as to where we are passing many of our duct blaster tests on the first go round.
Mastic Tape is ok-but it is costly. I prefer the paste.

Happy Trails Brother
Rodeo

[genduct]

Seems to be some confusion re the fabric used with mastic and mastic covering a previous use of some "mastic tape"
I think I understood the post as the later

[HVAC_Horn]

What is mastic tape? I've never seen it. There is hardcast and the mesh tape, like what they use for sheetrock repair.

Mastic tape is basically silver UL tape with a thin layer of gray mastic as the adhesive.

It is popular down here in Texas, some like to use both but as another poster said, go with one or the other. I prefer to use just mastic and save myself from blowing money on mastic tape as it is pretty expensive. Duct board is also more common here than sheet metal unfortunately. Ever since I moved to Texas I have hated working with it.

[b26440510]

Asked my contractor about their use of mastic over tape and mastic tape. They said that's the way they do it for airlines as well as residential. He said it "bulletproofs" the system.

[zzonko]

Asked my contractor about their use of mastic over tape and mastic tape. They said that's the way they do it for airlines as well as residential. He said it "bulletproofs" the system.

Mastic tape with mastic over it is wrong. Mastic with an embedded nylon mesh, and then another thin layer of mastic over the mesh is industry standard.

[Kevin O'Neill]

Asked my contractor about their use of mastic over tape and mastic tape. They said that's the way they do it for airlines as well as residential. He said it "bulletproofs" the system.

A lot of contractors do it that way. It is WRONG.
If the mastic tape comes unglued, the mastic on top of it is stuck to the tape, and it comes off too. Since the mastic is not stuck to the duct itself, it is just paint. It serves no function. It seals nothing and "bullet proofs" nothing. It is a waste of time and materials.

You need to get a new contractor.

[b26440510]

A lot of contractors do it that way. It is WRONG.
If the mastic tape comes unglued, the mastic on top of it is stuck to the tape, and it comes off too. Since the mastic is not stuck to the duct itself, it is just paint. It serves no function. It seals nothing and "bullet proofs" nothing. It is a waste of time and materials.

You need to get a new contractor.

Is this just a "best practice" thing or should it be caught by the code inspector?

As for getting a new one, its a little too late, even though there were trouble signs all over the place I stayed with this one. He wanted to schedule the code inspection for tomorrow but when I showed him some items on my punch list he said he'd wait on scheduling the inspection.

Doesn't switching cause a all kinds of nightmares (including legal -- who's owed what for what) much less the new contractor basically wants to rip all the previous contractors work out and start fresh. How do you balance it?

Rheem RGJF Furnace install

[Joker]

What is mastic tape? I've never seen it. There is hardcast and the mesh tape, like what they use for sheetrock repair.

Its tape sent from Heaven! LOL

It will stick to just about anything and has a slight thickness to it which helps when other tapes would tear.

If you have never used it ask your supply house they should have it.

[sui generis]

I use mastic all over the place. When I mastic over tape, I like to think the purpose is to make the tape job last longer. And it will, in my opinion. Sure, there's no point in putting mastic over only tape. But if you mastic where the edge of the tape meets the wrap, it joins and conforms them as one. And it seals any small gaps I may have between the tape and wrap, making the moisture barrier air tight. I don't see how it's "wrong." It's code here anyway.

edit: talking about foil tape, not mastic tape.

[budman21901]

First i have ever heard of it. Does this replace or equal too Mastic (duct seal) we put on with a brush?