Bathrooms generate large amounts of humidity. In a poorly ventilated or unventilated bathroom, moisture can build up and penetrate walls, ceilings, floors, and countertops. Mold and mildew thrive, wall coverings curl, paint peels, fixtures rust, and wood (possibly including structural framing) rots. Because you can't always open a window, your bath should have a ventilating fan that expels moisture, odors, aerosol sprays, and household cleaners. It's important to find a fan that's up to the job.
Fan Capacity
A fan's ability to move air is measured in cubic feet per minute (cfm). For bathrooms up to 100 square feet, the Home Ventilating Institute recommends that an exhaust fan provide 1 cfm per square foot of floor space. So for a bathroom measuring 8x6 feet, the minimum recommendation is a fan rated at 48 cfm.
For bathrooms above 100 square feet, the institute recommends a ventilation rate based on the number and type of fixtures present: 50 cfm for each toilet, shower, and bathtub, and 100 cfm for each jetted tub. So a 140-square-foot bathroom with a tub, a separate shower, and a toilet would need a fan rated at 150 cfm.
When shopping for a fan, also consider how loud it is. Bathroom fans are noise-rated based on units called sones. The lower the sone rating, the quieter the fan will run.
Optional Features
Some bathroom ventilation fans include a built-in light or heater. Top-of-the-line models may have humidity or motion sensors that turn the fan on and off. Most bathroom fans have only one speed, although multiple-speed models are available.
Installation
Your local building code may specify where the fan must be placed, so check the code before making your final plans. Generally, a fan should be close to the shower or tub, or high on an exterior wall opposite the bathroom door. Try to keep the ductwork as short and straight as possible. If the exhaust duct will have to twist and turn its way over a long distance, buy a fan with a larger capacity. It's also a good idea to wrap the duct with some insulation to reduce condensation inside it.
Most experts agree that the fan should be vented directly to the outdoors, not into the attic. The best options are to go straight through an exterior wall or straight up through a cap on the roof.
