Shell Busey Blog - Drip From Bathroom FanApril 4th 2018
The cause of your dripping bathroom fan is because you are exhausting warm moist air up into a duct that passes through a cold cavity like your ceiling cavity or roof cavity. Two things you should do to eliminate the problem is...
#1 – insulate the duct from the back of the exhaust fan to the roof jack vent exit using reflective foil insulation such as Reflectix available at most building supply stores. Insure the seams are tight and use foil tape.
#2 – Before having a bath or a shower turn on the fan for at least 15 minutes before. The warm air passing up through the duct will warm the ducting (metal) and cut down the chances of condensation forming (called cutting off chances of arriving at dew point).