Stop exhaust fans from dripping water in cold weather

07/19/2014 7:00 AM

07/14/2014 12:21 PM

During cold weather, water drips from the exhaust fans in the bathrooms. The ducts are metal and are connected to form one duct to go through the roof to vent outside. About three years ago I had a new roof with a ridge vent installed, and this problem has developed since then. Previously I had a roof exhaust fan. How can I stop this leak?

Moisture from the bath fans condenses inside the pipes in colder weather and drips back through the fans. I don’t think adding the ridge vent in place of a power-ventilated fan would make any difference.

The power fan generally does not operate in the winter. You might want to check to see if the roofers covered the opening where the bath fans vented through the roof. Bath fans and ventilated kitchen range hoods must vent by means of a metal pipe to the exterior either through the roof or through a sidewall to the exterior.

Bathroom vent fans cannot terminate in an attic or vent to the soffit area (overhang) where humidity buildup can cause mold and decay. Too many times I have seen damage to the roof’s decking where fans are improperly vented. The dampness will also soak into the attic’s insulation, providing an ideal environment for mold and ceiling stains to appear.

Each fan should have its own opening to the exterior. The separate pipes should not be joined together, because one fan could push the moisture-laden air through the vent pipe to the other bathroom’s fan. Most bathroom fans have a one-way gate valve so that air flows out but not in, but the gate valve can become stuck open because of the buildup of dirt and lint. The pipes should be insulated in the attic space to slow the condensation process.

C. Dwight Barnett is a certified master inspector with the American Society of Home Inspectors.

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