Bathroom exhaust fans must be vented to the outdoors, either directly through an exterior wall or into ducting that leads to a wall or roof vent. This allows the fan to remove moisture-laden air from the room that could cause mold and mildew buildup if left to condense.
Roof vents are a common method of venting bathroom exhaust fans, but they require care during installation to minimize leaks. Since the vent must pass through the roof of the home, the area around the vent must be properly sealed and insulated to prevent water from entering the attic. Wall venting can avoid this problem, but may require complex ductwork to vent interior bathrooms.