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xquercus 06-30-2007 09:40 PM

Condensation in Wall: Can I Seal a Dryer Duct With Caulk?
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I'm in the process of resolving a moisture problem in our walls. I posted about this earlier here:

The source of the moisture is a dryer ventilation duct that runs through a concrete floor. Vapor is leaking out through a joint in the duct. Then it moves up between the duct and the concrete into an adjacent wall. It caused a bit of a mold problem that can be seen in the thread linked above.

At some point, I'm going to re-route the vent so that it doesn't go through the concrete, as I believe the concrete route is problematic, and it will allow us the use of a corner in the room that is currently occupied by the duct. Due to the location of the dryer, running new duct is quite a project (there is concrete in the way and it's very difficult to get duct to a crawlspace or exterior wall).

In the meantime, I'm looking for a solution to seal a joint in the duct. The joint I need to seal is pointed out on one of the attached images. The arrows point right to it.

If I had access to the exterior of the duct, I'd use foil tape. The exterior of the duct isn't accessible, however, as it's embedded in concrete. If I clean the metal well, will a caulk properly seal this joint from the inside? If so, what kind of caulk? Silicone? Latex? Is there another approach I should take to sealing this joint?

Thanks All!

xquercus 07-01-2007 12:45 AM

I've been doing some more reading and perhaps duct mastic would work better than a silicone or acrylic caulk. Thoughts?

I suspect this is something I can pick up at Home Depot or the like. Any thoughts regarding if I need to use fiber glass cloth along with the mastic to properly seal the duct joint?

bigMikeB 07-01-2007 07:11 AM

I would ask myself why there is so much back pressure that the moisture becomes a problem? If the vent wasn't blocked in some way (too many elbows/ animal nest) it shouldn't have so much back pressure. Even though it's difficult, maybe now is the best time to do it right.

Clutchcargo 07-01-2007 07:20 AM

63 Attachment(s)
Which direction is the airflow?

xquercus 07-01-2007 01:19 PM

Looking at the image above and to the right the air flows up out of the concrete.

I've removed the duct that directs the air up out of the concrete to the duct in the wall on the right (the 2x4 frame wall) which vents to the outside.

Dan101 07-01-2007 02:33 PM

Are you sure the moisture is coming from concrete. Based on what you have photograghed, I would think it is more likely that water is forming condensation on the pipe after it exits the concrete. This would leak back down on the joint and appear to be coming from the joint.

The air is one temp as it travels thru the concrete foundation. When it exits the concrete and reaches the short piece of ductwork running thru the wall or living space it rapidly changes temperature. This will cause condensation. If you use well insulated duct or wrap the short piece of duct and elbow with a good insulating cover it will prevent the rapid temp change and should eliminate moisture. This would be my first step. Good luck with your project!

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