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  1. #1
    Join Date
    Aug 2009
    Location
    St Louis, MO
    Posts
    2

    Default Exhaust Fan Damper Question

    I plan to vent a bathroom (1/2 bath; no shower or tub) exhaust fan (Broan 744) through a soffit (only available option). I plan to use insulated duct from the fan to the soffit--is that necessary? The fan has a damper in its housing, but do I need another damper just inside the exhaust vent at the soffit to prevent condensation and icing problems in the duct during the winter? I live in the St Louis area and it gets below freezing in the winter.

  2. #2
    Join Date
    May 2008
    Location
    Pacific Northwet
    Posts
    1,361

    Default Re: Exhaust Fan Damper Question

    It would be a good idea.

    Where the duct goes through unheated areas, insulating the duct will prevent condensation inside the duct -- a damper on the outside won't help here, because the moist air being vented out will condense on the inside of a cold, uninsulated pipe. In heated areas, insulation is unnecessary.

    Placing a damper on the outside vent may not be necessary for preventing air infiltration since there's an inside damper, but it IS necessary to prevent insects, birds, and vermin from entering the duct and building their nests in that nice, warm spot.

    P.S. -- you said that there's no tub/shower, but the heated air in your house will contain enough moisture that condensation in a cold duct could be a problem.
    The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.

  3. #3
    Join Date
    Aug 2009
    Location
    St Louis, MO
    Posts
    2

    Default Re: Exhaust Fan Damper Question

    Fencepost-
    Exactly what I needed to know; I'm ready to get started now. Thanks!

  4. #4
    Join Date
    Dec 2007
    Location
    MN
    Posts
    455

    Default Re: Exhaust Fan Damper Question

    Personally I wouldn't exhaust out the soffit. One would think some of the moist air would end up back into the attic.

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