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Thread: Duct mastic

  1. #1
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    Duct mastic

    What size gap can duct mastic be used up to? Can I use it on a gap about a 1/2"?

    If I can't use mastic what should be used to fill that gap, silicon?

    It's to seal off a return where the top has sagged down from the joist.

  2. #2
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    I would try to push the duct back into place and fasten it first. If this is not possible, fill the gap with light gauge sheet metal then seal with mastic.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  3. #3
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    I will have to go get some light metal then because this is the top side of the return where it runs perpendicular to the joist so no way of pulling the top up.

  4. #4
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    Mastic tape the cover with mastic or sheet metal and mastic.

  5. #5
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    What about using that heavy metal foil tape (May be mastic tape?) with matic over that? I have some of that stuff when I built my attic box to block off my pull down stairs for insulating it.

  6. #6
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    If its butyl backed foil tape it will work well (putty like about 1/16" thick) regular foil tape would be ok with a thick layer of mastic as long as you can get it to stick. Mastic tape (butyl backed foil tape) will adhere to just about anything.

  7. #7
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    It will need insulating afterwords of course, to prevent sweating or heat gain/loss.

  8. #8
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    Quote Originally Posted by walterc View Post
    It will need insulating afterwords of course, to prevent sweating or heat gain/loss.
    This is in a finished basement.

    This stuff: http://www.homedepot.com/p/Nashua-Ta...cked+foil+tape

  9. #9
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    No, this stuff Removed link to direct purchase site.
    Last edited by beenthere; 04-27-2013 at 09:58 AM.

  10. #10
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    Sorry BT, hardcast 1402 is mastic tape.

  11. #11
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    that's my tape of choice 1402 mastic Hardcast brand!!

    the link is just foil with sticky backing...not mastic.

    you can use nylon webbing pushed into first layer
    of mastic & then mastic over it.
    but you should give it 24 hours to dry & recoat
    it with mastic.

    can you show us a picture?

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  12. #12
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    Here are two pics where it has a larger gap.




  13. #13
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    that is the top of the return & a joist above it?
    why seal return to joist??
    it looks like return is independent of joist...
    and not a part of the return.
    am I just seeing this wrong??
    The cure of the part should not be attempted without the cure of the whole. ~Plato

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