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Bath fan venting through roof

brianandson | Posted in General Questions on

I have two full bathrooms in my second floor and I just realized that the bath fans are venting into the attic and I have some mold in the attic.  I live in Michigan cold winters.  I want to use insulated flex ductwork to run both of the bath fans through my roof.  I am an amateur and hoping I can get some advice.  I have a couple questions.
– one of the fans is about 4ft from the edge of the roofline and the attic space above the fan has only about 18” of clearance before exiting the roof.  Can I run insulated flex duct only 18” up from the fan housing to the roof exhaust cap or is that too short of a run?  
– can the insulated flex duct go vertical from the bath fan unit, or does it need to run horizontal for a couple feet before going vertical and out of the roof?
– Is it a bad idea to have the two fan ducts connect together in the attic and then through one exhaust cap out the roof?
– in the cold wintertime should I be concerned with snow melting and ice damming at the bath vent exhaust caps?  It’s a low slope roof.  Maybe 4/12 pitch.  Or I’ve also read NOT to use the bath fans in the cold winter weather 
thank you for any advice you can give me

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  1. Expert Member


    - Yes you can have a short run of insulated duct. Man sure to air-seal well.
    - The duct can go straight up. You don't want horizontal runs to gather moisture.
    - If you are going to connect the two fans together you need to include back-damper or one fan will exhaust into the other bathroom. My own preference would be to keep them separate.
    - Yes the roof caps can freeze. If your roof has a gable end it would be better to run them out there than through the roof. If you go though the roof, you may want to route the exhausts diagonally a bit to get them away from the eaves.

    1. brianandson | | #2

      Awesome! Thank you Malcolm! I really appreciate your expert advice. Have a great Christmas

      1. Expert Member
        MALCOLM TAYLOR | | #3

        You too. We've even got an inch of snow on the ground, which nothing for you but unusual here on Vancouver Island, and adds to the holiday feel.

  2. brianandson | | #4

    Wow I bet it’s beautiful where you are at. We barely have any snow here. We love the snow, just don’t like driving on it. Haha. Thanks again for the info

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