In a room that may have had some kind of condensation on the past, how should one go about sealing the area around the vent hole in the ceiling, specifically to reduce the likelihood of returning condensation on the future?

This room is on the second of two floors in a residential building. The second floor HVAC system runs through insulated rigid ducting in the attic space.
The room in question seems to have had condensation issues in the past (dark spots were on the windows, and see attached image of ceiling vent to be replaced).
It looks like someone previously tried to seal around the edges of the drywall of a ceiling vent (see ceiling image attached).
According to the owners, it was unoccupied for likely several months, but less than a year at one point several years ago. During the time the building was unoccupied, it's not known if the HVAC was functional or not, but the concern is high temperature fluctuation.
This building is in Southeastern North America, in an area that is regularly humid.
The vents in this room are already being replaced.

How should one go about sealing the area around the vent hole in the ceiling, specifically to reduce the likelihood of returning condensation on the future?

**Edit to fix unuploaded images.

Ceiling hole

enter image description here

  • Are you sure that is not mold on the vent? You say it has insulation of the rigid duct. Is the vent boot also insulated? There should be no condensate on an insulated duct.
    – peinal
    Nov 9 '19 at 22:20
  • @peinal Not mold, popcorn ceiling original on the residence. Overspray on the duct, I believe. The mold doors were on the window frame and vents, not ducts. Nov 9 '19 at 22:32
  • @peinal I just reread your comment. To clarify my response: That might have been mold on the vent itself, but the inside of the duct was popcorn overspray. Apr 12 '20 at 11:25
  • Bro... That's mold... I hope you used a HEPA filter on that thing.
    – Itsme101
    Jun 17 '21 at 5:31

All my registers have a black foam strip around them, like the foam weather strips you can get at your home store. The strips are about 1" x 1/4" by perimeter of register.


There are really two problems here:

  1. Yes the registers should be sealed. The manner doesn't matter too much. The purpose should be air leakage and insulation. The conditioned air should not be leaking into the unconditioned space (your ceiling interior). You could use some spray foam around the edges of the registers to the drywall. I prefer the DAP Latex foam as it's easier to deal with than the urethane Great Stuff. The foam also provides insulation. You indicate that the duct work is insulated. That is good, you may want to cover any bare metal at the joints, boots, and transitions as well, if you have access to them. Also if the joints of the duct work leak, duct mastic can be used to seal them.

  2. Moisture condenses on the cold side of things. Think of a glass of ice water. So this part baffles me. Maybe someone else can comment. If you are seeing signs of moisture and corrosion or rust, that means the other side is colder than your interior. Is this a winter time or summer time thing? If the duct space is not well insulated it's possible you are seeing condensation like you would see on a poorly insulated exterior window. More insulation would help minimize this.

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