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Community and Q&A

Duct Condensation Issue

djm883 | Posted in Mechanicals on

We moved into our home last year in the middle of summer. At the end of summer, we noticed condensation along one of our HVAC ductwork causing drywall damage in our basement. This ductwork is located in the ceiling in our basement and extends to the register in the bump out in the 1st level of our home.

The bump out is open below and has about a 6 inch gap between bottom of the bump out and ground outside. The bump out has a piece of plywood on the bottom with what appears to be some insulation.

I plan on fixing this before the AC is needed again this year and want to make sure I repair this correctly. I think I need to wrap the ductwork along the length of the ductwork in the bump out and extend back into my home approx. 4-5 feet. I also want to replace the insulation between the plywood and ducts then install a vapor barrier before reinstalling the plywood.

Would this be the correct fix? Any advice would be helpful.

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  1. GBA Editor
    Martin Holladay | | #1

    There are four possible elements to this problem:

    1. The duct seams may be leaking. (This can make the ducts colder.)

    2. The duct insulation may be insufficient or defective. (This can also make various surfaces colder than they should be.)

    3. The duct jacket may be ripped, damaged, or missing. (This can allow humid air to contact the cold surfaces of the duct.)

    4. Most important, the ducts aren't entirely inside your home's conditioned space. They are partially outdoors. That's why humid outdoor air is contacting your cold ducts. To fix this problem, you need to make sure that (a) there is a layer of R-20 or better insulation between the insulated duct and the soffit, and (b) the soffit is airtight.

    I'll bet that you have a leaky soffit. With no air barrier at the soffit, warm exterior air has free access to cold surfaces in the overhang.

    -- Martin Holladay

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