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How to Air-Seal Ductwork in an Attic

This video is only available to GBA prime members

It’s best to keep all ducts in conditioned space, but if you can’t, you should at least stop them from pulling dirty air from your attic into your living space

Thermal Bypass Checklist package (50+ details).


Video Transcript:

Here we have housing for a ceiling light, a fan assembly in the bathroom below, and the discharge duct for the warm most air being exhausted from the bathroom. The trouble is, when this was installed, someone used regular duct tape. And over time, duct tape doesn’t seal well to the pipe. The adhesive gives up, deteriorates. Our goal is to make sure we don’t get any air leakage through these joints. We must remove this tape and reseal those joints, eliminating the potential problem of mold or condensation that can lead to rot in the roof rafters.

You can see when I pull back the tape, it easily comes off. And you can see the adhesive is sticking to the pipe and not bonding to the tape any more. I’ll pull all of the tape off at each of the joints. Because of the residue left by the old duct tape, I’m going to have to use a less-than-preferable method. If this were a new installation, I’d use aluminum tape that has an acrylic adhesive on the back side. It’s a lot more durable and long lasting than ordinary duct tape. Because there is contamination from the old duct tape residue on the joints, I’m not going to be able to get a good bond between the aluminum tape and the ducts themselves.

I’m going to use duct mastic, which is the messier method. It’s usually used with a HVAC system, but it’s also good to seal up ducts for exhausts venting…


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