Sometimes, particularly when a bathroom exhaust vent extends through an unheated section of your home, the possibility of condensation buildup can occur. When this is the case, warm, moist air from the bathroom can condense on the inside of the cold duct and can run back down into the bathroom. There are several steps that can be taken to prevent this from occurring.
Insulate Exhaust Duct
The best way to prevent this scenario from occurring will be to insulate the actual exhaust vent in order to normalize the air going through the vent with the walls of the duct itself. In order to do this, wrap the outside of the duct with duct insulation, following the directions on the insulation packaging. Do this along as much of the duct as possible.
Replace Duct Entirely
If your exhaust duct is currently made out of uninsulated plastic, consider replacing the entire duct with metal duct that is either pre-insulated or will be easy to insulate yourself. Wherever possible, make sure that ventilation ducts are sloping downhill away from the exhaust fan, so that any remaining condensation buildup will run away from the vent opening instead of running back into your bathroom through the vent.