The Importance of Ventilation
The importance of
ventilation cannot be overstated. 
Exhaust
fans are crucial to proper ventilation in a bathroom. Their particular
value is in drawing moist, warm air out of the bathroom before it causes
damage. Problems associated with poor ventilation include mold, rust,
swelling doors, peeling paint and loose wallpaper. Even with a window
that opens, bathrooms need a ventilation system. Operable skylights will
vent a bathroom when open, but too often people either forget to open
them or close them upon stepping out of the shower, which negates the
whole purpose.
Standard bathroom ventilation calls for fans to move 50 cubic feet of air per minute (cfm) at a minimum. Check the fan's cfm rating before ordering. What size you select will vary widely, depending on the size of the bathroom and whether it has a large shower or whirlpool bathtub, both of which create lots of humidity. Such bathrooms may require up to 400-cfm rated fans.
You may also need a larger fan depending on the length of the duct run from the bathroom to the home exterior — never into the attic or crawl space. The duct diameter and material also affects air movement through friction. For any runs over a few feet, use plastic pipe or sheet metal ducts rather than flexible plastic pipe, which restricts airflow and traps moisture.
Exhaust fan installation instructions usually include specifications on the maximum length of the duct run, the duct diameter and material to be used.
In selecting a bathroom exhaust fan, consider whether you want just an exhaust fan, one with a light, or one with both a light and a heat lamp. These can be wired in different configurations, depending on their use. For instance, in a bathroom used primarily by children, wire the vent to go on with the light, which ensures that moisture is vented. Some units can be programmed to stay on for several minutes after the light is turned off to rid the room of all moisture. Each exhaust unit element, whether fan, light or heat lamp, also can be wired to operate independently.
Noise is another key consideration: If a fan is too noisy people may just opt not to use it. Fan noise is measured in sones, and 1 sone is about the sound level of a quiet refrigerator in a quiet kitchen. Not all fans are noise-rated by the Home Ventilation Institute, but for those that are rated, 6.5 sones is the highest rating allowed. A quiet fan will be rated at 3 sones or less.