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Bathroom vent duct install

mfleck | Posted in General Questions on

Hi all,

 As advised through several posts here about my plan to insulate my attic, I am making the space above the bathroom and adjacent to the main attic an unconditioned space.  Our contractor had used flexible, uninsulated ducting to vent the bathroom up through a penetration in the roof.    My plan is to replace this ducting with rigid metal that will be a more straight shot out of the roof vent.  I will also insulate this ductwork. 

 Here’s my question: I imagine that, even with insulation on the rigid metal duct, and even with running the fan for a few minutes before starting a shower,  there will still likely be some condensation on that duct pipe as the steamy air shoots up and out. Won’t that eventually drip back down, and into the joints of the ducting where Ill have the mastic. I’ve never worked with that material before. I see that it is water-based, does that mean that the water condensate will eventually compromise it seal?  Is it best practice to have the male parts of the duct pointing up so that the blowing air flows freely up and out? My worry if this is the case is that the dripping condensation on the way back down would work its way into the joints and compromise the mastic over time. 

Learning a lot here. Thanks for the advice.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Matthew.

    I have never heard of condensation compromising mastic. Maybe another GBA member has experience to share.

    To prevent condensation in your through-the-roof vent installation, run the fan for a while after showering to make sure all of the humid air is exhausted before the fan stops running. Timer switches are helpful for making sure that you are using your bath fan well.

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