Smooth ducts with sealed joints will offer less resistance to air movement and provide quieter, more efficient operation. Any duct that must go through spaces that are not heated or cooled should be insulated to help prevent moisture condensation. The ducts should not terminate in the attic, but continue to the outside. Warm, moist air being exhausted into the attic can lead to condensation and eventual structural problems.
A back draft damper on the exhaust vent is important to the fan system. This prevents outside air from leaking back into the home when the fan is not operating. Also, the flap can prevent insects, birds, and other animals from getting into the fan duct.
A fan that is installed on rubber gaskets or similar cushioning material is less likely to vibrate and will operate more quietly.
When a bathroom fan is operating, it is exhausting or removing air from the room. This creates a negative pressure in the room—and replacement air must come from somewhere. If replacement air is not provided, the effectiveness of the fan is reduced. A bathroom door should be undercut slightly to allow air to flow into the room, even with the door closed. A louvered door is sometimes used, although this may be unsatisfactory from a privacy concern.
The bathroom fan is not just exhausting air from the bathroom, but from the whole house. Some of the replacement air may come from open windows, or from people moving in and out of doors. Some replacement air may come from leaks and cracks in the building envelope. However, in well-constructed, energy-efficient homes, there are few places for replacement air to leak into the home.
If replacement air is not provided, negative pressure can become an issue. Problems can occur with the operation of appliances that need to exhaust to the outside, such as gas furnaces or water heaters. This situation is referred to as back drafting. When back drafting occurs, dangerous combustion pollutants, such as carbon monoxide, as well as excess moisture and radon can be pulled into the home.
A simple solution to back drafting problems, and to providing replacement air, can be opening a window when operating exhaust fans. However, this is not always practical. Other solutions include passive fresh air intake vents in the home and a whole-house mechanical ventilation system that balances airflow.