How to spot ventilation problems in your home
Do you have fog or condensation in your bathroom windows and mirrors, even when the fan is running?
It may be sign that the air in the room is cold, or you don't have enough airflow in the bathroom.
If the air in the room is cold enough, then condensation can still form on windows and mirror surfaces.
If your bathroom is sealed tightly, replacement air may not be entering the room fast enough to displace the moist air. It may help to cut a few inches off the bottom of the bathroom door let more fresh air into the room.
Fan placement can also be a factor. Your fan should be located far from where the air will flow in from to ensure the moist air is drawn out first.
Finally, too many twists and turns in the ducting will significantly reduce the ability of the blower to remove moist air. Make sure your ductwork is as short and straight as it can be, with gradual turns rather than tight corners where necessary.
If you have water dripping from the grille of the fan it's usually due to condensation (usually due to cold ductwork or improper duct installation), or a problem with the seal on a wall or roof vent. Insulated ductwork can help solve condensation problems, and running the fan longer will ensure moisture is completely removed from the duct.
Ductwork should slope down toward a wall vent to direct condensation out of the exterior vent opening rather than back into the fan housing.