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  1. #1
    Join Date
    Jul 2007
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    Question regarding the method of sealing fiberglass duct to the supply register floor

    About two years ago the duct work was replaced due to mold. Reason given for the mold was the fiberglass supply duct exterior coating had failed allowing moisture to saturate the duct over time. The home is located in coastal North Carolina, has a crawl space sized for a very small person. The Trane gas pack is new about three years old. All the work has been done by a local HVAC firm that has been in business over thirty years.

    Recently while doing some work in the crawl space I inspected one of the supply duct connections and found air leakage at the termination point of the fiberglass duct to the floor box. The leak was also causing condensation to form, which was running into the duct. The connection is made with a Panduit strap. The metal floor box has insulation wrapped tightly around it. Inspection of several other connections also revealed the same issue, supply air leakage resulting in condensation running into the duct creating mold.
    What is the proper sealing method to prevent this problem?

  2. #2
    Join Date
    Aug 2002
    manitowoc wisconsin
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    Have all the connections sealed with mastic.I prefer brush grade for the type of duct & connections you have.
    Take your time & do it right!

  3. #3
    Join Date
    Nov 2000
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    Your discription is a little vague, but one of the things to look at is whether the crawl space has a high humidity level. The air passing through the duct is the same temp as the air leaking from it, so the metal floor box you describe would be cold even if there were no leak. High humidity levels caused be missing or improperly installed vapor barriers or open crawl space vents could be the source of the condensation.
    "You do not merely want to be considered just the best of the best. You want to be considered the only ones who do what you do.".... Jerry Garcia
    "You just have to decide if you're a Tigger or an Eeyore,..I think you know where I stand on the great Tigger/Eeyore debate." Prof. Randy Pausch

  4. #4
    Join Date
    Jun 2003
    Hell Hole Swamp
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    Agree on the crawlspace humidity issue, its a big problem in your area (and mine), dewpoints have been in the upper 70's lately, if your crawlspace is ventilated your ductwork is very likely going to condensate, air leaks or not

    however to answer your question, the proper sealing method is a UL 181B-FX rated tape, assuming this is a flexible duct, or, preferably mastic which was mentioned above.

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