AC Plenum, distribution box and vent. Sealed?

[RICHARD TAN]

Hi Jerry,

AC Plenum, distribution box and vent. Do the inside of the duct board required to be sealed. In the old day the metal finger is tape and sealed with mastic. Is vent attachement to the drywall required to be sealed. I have found vent air leakage on my thermal imaging.

[Jerry Peck - Codeman]

Hi Richard,

That's a multi-part question which I need to break down into sections.

AC Plenum, distribution box and vent. Do the inside of the duct board required to be sealed. In the old day the metal finger is tape and sealed with mastic.

No, because it is sealed on the outside and the opening for the metal collar and the attachment of the duct should be such that there is no air leakage and the insulation fits around the metal collar.

Also, the metal color mechanical attachment to the duct board distribution box is by those metal fingers, thus all of those metal fingers need to be bent over and tightly holding the metal collar in place, i.e., the metal fingers should really be bent over the duct board at 90 degrees, not just lazily pushed outward.

Is vent attachement to the drywall required to be sealed.

The supply boot is sealed to the duct, the supply register should be sealed (typically a foam gasket of limited use) to the supply boot and no air leaks out ... at least theoretically anyway.

In practice, whatever needs to be sealed to keep it from leaking needs to be sealed, and many times that is the boot to the gypsum board with mastic and then the supply register installed, which is sealed to the mastic covered edge of the gypsum board.

It is hard to say what is to be sealed, only that it is not supposed to leak above or below the gypsum board.

If you see cold air leaking across the ceiling (and know for sure that it is leaking "at" the ceiling/supply register interface), then "something" needs to be sealed "somewhere". I would simply address it as "air leakage" which needs to be sealed and let the contractor determine what, and how, things are to be sealed.

That cold air leaking behind the supply register can cause condensation to collect on the top side of the gypsum board and/or on the room side of the gypsum board and/or within the gypsum board gypsum core.