A bathroom exhaust fan pulls in airborne moisture before it can cause harm. If not removed promptly, the moisture from showering or bathing can condense on bathroom floors and make them dangerously slippery. Airborne moisture condensing on the walls encourages the growth of bacteria, mold and mildew that can cause nasty bathroom odors and pose a potential health risk.
Bathroom fans are installed in the ceiling or a wall of the bathroom to suck warm, muggy air out of the bathroom so fresh air can enter. A properly installed electric bathroom fan will connect to an insulated metal duct leading directly outside. The duct should be as straight and as short as possible. In many older homes, though, bathroom exhaust fans just vented the humid air into the attic or the joist space above the bathroom ceiling. This practice now is prohibited under most current building codes.
An effective bathroom vent fan should move 1 cubic foot of air per minute for each square foot of floor area. To find the fan size you need for a small bathroom under 100 square feet, measure the length and the width in feet. Round up inches to the next whole foot. Multiply the two measurements. The result is the minimum number of cubic feet of air the fan must be able to pull out of the bathroom each minute. For instance, a bathroom 9 feet by 8 feet would need a fan that moves at least 72 cubic feet of air per minute.
For bathrooms larger than 100 square feet, count 50 cubic feet per minute for each standard toilet, shower or tub in the bathroom and add them together. If there's a whirlpool bath, count another 100 square feet. That means a fully equipped large bathroom with a toilet, shower, bathtub and whirlpool bath would need a fan that moves 250 cubic feet of air per minute.
Using the Fan
Don't turn off the bathroom exhaust fan as you exit the bathroom after a shower. Effective removal of muggy air takes time. Allow at least 30 minutes after you leave for the fan to completely change the air in the bathroom. For convenience, you can replace the standard on-off fan switch with a timer switch that turns the fan off after a half hour. Leaving the fan running won't be an annoyance if you install a quiet fan. Fan noise is rated in units called sones. The lower the number of sones, the quieter the fan will be. A whisper-quiet bathroom fan will have a noise rating of around 1.2 sones. A fan rated at 4 sones will have a low hum.