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  1. #14
    Join Date
    Oct 2010
    Anderson, South Carolina, United States
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    Great stuff has its place... I guess, but ductwork is not the place.

  2. #15
    Join Date
    May 2004
    south louisiana
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    great stuff should not be used.
    I've seen it pick up boxes off the attic floor 1 1/2"
    and it shrinks. little control with foam.
    off gassing & leakage are my concerns.

    this is how I seal supply boxes.
    hardcast #1402 mastic tape only
    I buy 3" rolls & split width to 1 1/2"
    it only needs to go from sheetrock
    ceiling (within area covered by grill)
    to metal lip inside supply box.
    reinforce corners.
    this is the only tape I've found that is
    strong enough not to come lose when
    you replace the grill.

    caulk doesn't stick to the inside of the
    sheetrock (dust) and if you put the
    grill back up before caulk sticks
    to the grill. I've taken many a supply
    grill off that the caulk was on the grill
    and not on the gap.
    trying to seal from inside the attic..
    what about the side of the box attached to
    the rafter??

    just my experience.
    heres my pic.
    best of luck.
    Name:  how to seal supply box from inside with MASTIC tape only.jpg
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    The cure of the part should not be attempted without the cure of the whole. ~Plato

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