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Thread: condensate on air ducts - normal?

  1. #1
    Scientist jeremytl's Avatar
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    Jan 2007
    durham, nc

    Default condensate on air ducts - normal?

    Under the house in the crawl space, there is quite a bit of condensate dripping off the old metal air ducts. I was thinking about insulating these ducts to improve the heat in winter but after seeing all the condensate I think I'll hold off. I think that insulation would just be wet all the time in the summer. Am I right?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    It depends on how well you insulate the ducts and how you do it. The goal is to move the dew point higher so it doesn't condense. If you don't do something, you'll rust out the ductwork. the moisture in the area could provide the means for mold and other stuff to grow, too. Make sure that you seal the joints so the ducts don't leak, too.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Member Lightwave's Avatar
    Join Date
    Aug 2007
    Vancouver, BC


    Any air-permiable insulation you add will get soaked unless you install an air/moisture barrier over the insulation.

    The concepts involved aren't that much different from insulation and vapor barrier concepts relevant to house walls.

  4. #4
    Retired prof. engr. gator37's Avatar
    Join Date
    Nov 2009


    From what you have explained the dew point temperature in the basement is lower than the surface temperature of your duct causing condensate to collect on the cold duct surface unless you have a condensate drain problem that is somehow spilling over into the duct.
    First thing I would do is check the duct for rust (even galv duct will rust where it has been cut, replace the duct if you have to) then I would check for air leaks (all joints and lockseams) and seal them with a approved mastic (see IMC). To save yourself some grief, I would make sure you have at least 3 to 4, 3/8"self tapping hex head screws around the circumference at each joint (use a drill to install). You can insulate it with a foil type vapor barrier backed insulation. (I would use 2" thick insulation but that is your call) The way we used to install it was to wrap the duct with the insulation (even between the hangers and duct), staple the seams and joints with staples (with the use of a staple gun made for insulation), then apply mastic to all the joints. (We used to use foil back tape for joints but even when you apply heat or rub them with a piece of cardboard till the tape gets warm the joints always seem to come apart sooner or later.)

    hope this helps

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