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Thread: sealing of vent pipe at ceiling

  1. #1
    DIY Member ankhseeker's Avatar
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    Default sealing of vent pipe at ceiling

    I have a 6" vent from my over stove microwave that was installed by someone else. My wife noticed that there is cold air coming in the cabinet from the space between the vent pipe and the top of the cabinet. What should i seal this space with? Just insulation or is there a better product?
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    In the Trades Tom Sawyer's Avatar
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    Silicone, spray foam, insulation, take your pick.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Member ankhseeker's Avatar
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    Thanks! I appreciate your recommendation.

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    In the trades Dana's Avatar
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    +1 on the expanding can-foam for sealing around the duct penetration.

    But seal the ducts too- they look as leaky as a sieve. Get rid of all that gummy backed "duct" tape, and seal the seams & joints with duct-mastic. Leaking warm moist air into a cooler damp space is a recipe for mold risk. Duct tape might be good for some things, but sealing ducts isn't one of them, name notwithstanding.

    Check and make sure there is a backdraft preventer in line with the exhaust duct too- the space could be cold simply from the convection of cool outdoor air down the pipe, displacing the more buoyant warmer air.

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    DIY Member ankhseeker's Avatar
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    That duct passes through the attic and I can actually feel the cold breezy air coming through the opening. Great information. I will look into that duct-mastic stuff. Is that a paint on sealer? Thanks!

  6. #6
    In the trades Dana's Avatar
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    Duct mastic is a pasty goop that you can apply with a putty knife or paintbrush- just be sure to apply it at least 1/8-1/4" thick or it can crack and lose seal. Box stores every where carry it in half-gallon tubs priced at about a sawbuck, or smaller amounts in caulk-gun tubes.

    If this is the only duct you need to seal you don't need much, but most houses have multiple places where it might be appropriate. It comes with different temperature/fire ratings, so pay attention to what you buy and where you apply it. (I went through a handful of the half-gallon tubs sealing the 90 year old heating ducts in my house, which made the ducts much quieter, and the air handler zone on the heating system more efficient.)

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