FAN SPECIFICATIONS

Energy Star compliant, low-sone (quite) fans should be installed to save energy and reduce excess noise. In order to function properly, bathroom fans should be sized for the room they are exhausting. Ventilation standards are shifting, and the most recent standard by ASHRAE, the national engineering society leading the field in setting standards, has lowered recommended ventilation rates. Bathroom fans should draw 50 cfm, which you can test by holding toilet paper up to the fan while running -- if it holds up two pieces, it is drawing 50 cfm of air.

According to the Home Ventilating Institute Bathroom Ventilation Guidelines, fans should be sized as follows:

Location & Performance

Exhausting directly to the outdoors, fans can vent through the ceiling or the wall. Vents passing through attic spaces should be insulated to prevent condensation and fans should never vent into the attic itself. In both wall and ceiling vents, seal around the penetration to reduce air leakage to unconditioned spaces. Install dampers at the exterior side of the vent to prevent cold outside air from entering the house. Backdrafting is caused when fans draw so much air that they do not permit combustion appliances like hot water heaters or boilers to draft through chimneys, which can result in the build-up of carbon monoxide, a very dangerous toxic gas. If you add ventilation to the home, you can affect whole-house ventilation. It is recommended you hire someone to test and verify that use of fans does not create backdrafting conditions.