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  1. #1
    Join Date
    Jun 2008
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    Default bathroom fan drips when 2 feet of snow on roof - what to do?

    We have a house in the Northeast Kingdom of Vermont, where we get a lot of snow. Last winter we twice had 2 feet of snow built up on the roof before we shoveled it off. This completely covers the roof vent for our bathroom fan, and consequently the moisture condenses and drips when you turn the fan on (and sometimes even when it is off). The fan is a typical corrugated flexible hose with a small hood vent on the roof. We are uncertain what to do about this. Do we build some sort of ugly extension on the roof with pvc or stovepipe (and how do you then attach it to the roof so it doesn't sway in the wind?), or do we vent through the soffit, and somehow try to seal off the soffit for a couple of feet on either side of the vent in order that the moisture doesn't re-enter the attic via the soffit? The gable end is at least 10-12 feet away so I think venting through the wall at the gable end would not be possible. Any advice would be welcome!

  2. #2
    Join Date
    Aug 2007
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    The Great White North
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    4,045

    Default Re: bathroom fan drips when 2 feet of snow on roof - what to do?

    Last winter we twice had 2 feet of snow built up on the roof before we shoveled it off. This completely covers the roof vent for our bathroom fan, and consequently the moisture condenses and drips when you turn the fan on (and sometimes even when it is off).
    This is why I don't recommend roof venting bathroom fans in areas that have more than light to moderate amount of snow on roofs.
    Other problems than can occur from this would be the vent being covered with snow .... the heavily moisture laden warm air will melt the snow from the underside causing an issue similar to ice damning. This will eventually have ice working itself under the shingles and creating moisture problems with the roof as well this will wear off the granular coating of the shingles.



    a typical corrugated flexible hose
    Major no no ... first thing to get rid of for 2 reasons.
    1. Those white plastic flex hoses have a rough surface inside the hose which causes restrictions and turbulence to the air flow. This impedance reduces the performance of the amount of air flow the fan is trying to move.
    2. They are not insulated which greatly increases condensation to form. You are filling this tube with very warm moisture laden air moving through a cold attic causing a large amount of condensation to form when it suddenly cools.
    Replacing it with an insulated flexible duct is only marginally better. Even though it's insulated to prevent condensation the inside is similar that it's inner surface is rough having the same impedance as the cheap white plastic flex ducts.

    You are better off using the metal ducting since it's smooth inner surface is ideal for air flow. Seal all joints with the proper foil duct tape ( not the cloth tape) then cover with an insulating sleeve for ducts.

    Another idea is to use either PVC or ABS 4 inch piping used for plumbing ... with the glued connections will form an air tight seal and won't rust .... covered with an insulted sleeve.


    do we vent through the soffit, and somehow try to seal off the soffit for a couple of feet on either side of the vent in order that the moisture doesn't re-enter the attic via the soffit?
    Personally I wouldn't do this .... it will be very difficult to ensure the moisture won't be routed back into the attic from winds.
    Besides .... you would be closing off the soffit vents and negating their intended purpose of venting the attic space.


    The gable end is at least 10-12 feet away so I think venting through the wall at the gable end would not be possible.
    This is the best option.

    Use the rigid ducting of at least 4 inch diameter and minimize sharp 90 degree elbows. For every sharp 90 degree transition will create a restriction equivalent to adding roughly 10 feet of straight run.

    Also have the ducting sloped down toward the exhaust vent .... this will allow the moisture that will accumulate to run down to the vent and not toward the bath fan.

    You should also consider having a bath fan rated at least 100 cubic feet per minute (CFM) or more.


    Just some thoughts and hope this helps.
    "" an ounce of perception -- a pound of obscure "
    - Rush

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