2. Stains Around a Bath Fan?
The Family Handyman
What it means: Condensation is forming inside the duct.
The stain could be caused by a roof leak, but condensation inside the duct is the most likely cause.
If you live in a cold climate, there's a good chance that the warm, moist air from the bathroom is condensing inside the duct and the water is seeping back down into the fan housing.
It's soaking the drywall around the fan and may be ruining your fan motor or even the framing components in your attic.
What to do: Start by checking the damper inside the fan housing and the one on the vent outside.
Vents are usually on walls or roofs, but sometimes they're in the soffits.
A stuck damper can lead to heavy condensation.
A bath fan duct that's not insulated (or poorly insulated) gets really cold in the attic.
A cold duct filled with warm moist air is a recipe for condensation.
On exceptionally cold days, that condensed water freezes and then drips back down when the temperature rises.
Even insulated ducts get cold enough for condensation to form when the fan first starts up.
If a fan is run long enough, the duct will warm up and dry out.
Consider replacing the wall switch with a timer switch, which will run the fan for a set period of time.
Condensation forms in ducts (above left).
Warm air condenses on the inside of a cold duct and the water runs back down into the house. Insulate the duct (above right).
You could wrap the existing duct in insulation, but it's usually easier to replace it with a duct prewrapped in an insulated jacket.