What's the Point of the Bathroom Exhaust Fan?

taking a shower
Bathroom fans are essential for removing moisture from bathrooms and preventing mold growth. Manuel Breva Colmeiro/Getty Images

If you've ever had to deal with a bathroom that lacks a working fan or window, you know exactly how unpleasant the experience can be. Odors, normally whisked away in just a few minutes, may linger for much longer, whether from poop or powerful bathroom cleaning products.

But moisture is the real enemy. Humidity from showers fogs up the mirror and leaks into walls. As the warm air cools, it turns into water, and gets trapped in the walls.


"Indoor condensation can lead to mold growth," says Dave Bayne, CEO and owner of A1 Mold Testing & Remediation. "Drywall is porous and paint is breathable, so the moisture can seep into the drywall and create fungal growth," often in places where you can't see it.

If you're exposed to mold long enough (or if your body is allergic or sensitive to its spores), the Centers for Disease Control and Prevention indicates that you may suffer a laundry list of ill effects, from allergies to coughing to skin irritation.

Excessive moisture may also cause peeling paint and wallpaper and even contribute to warped doors and rusted fixtures. Dampness also fuels bacterial growth and other nasties.

By simply turning on the bathroom fan, you minimize a lot of those problems. "You should turn the fan on each time you run water in the bathroom and leave it running for about 20 minutes after you open the [bathroom] door," says Bayne. The most important time to do this is after you shower. If you're continually forgetting to turn it off, install a fan timer. And though it's seldom done, the fan should be cleaned at least yearly.

If you're a homeowner, be sure that your fan is properly vented directly to the home's exterior via an insulated duct. "It was common in the '70s and '80s for it to be vented into the attic space," says Bayne. "Building codes today require that the exhaust is vented outside through insulated ducting to a roof vent that goes directly to the exterior." Otherwise, you may be contributing to moisture and mold problems in your attic or wall spaces.

If you don't have a bathroom fan, most building codes in the U.S. specify that there must be an operable window to allow for better airflow.


Bathroom Exhaust Fan FAQ

Why do you need a bathroom fan?
A bathroom fan is essential because it helps expel moisture from the bathroom. It also eliminates odor and maintains a clean environment in restrooms. A bathroom fan ensures a comfortable ambiance by eliminating undesired smells.
How long should the fan be running?
You should turn on the fan as soon as you turn on the shower or bath water. Additionally, leave the fan on for about 20 minutes after you’re finished bathing. This will help keep your bathroom free of moisture and mold.
Is it hard to replace a bathroom exhaust fan?
Replacing a bathroom exhaust fan is fairly simple and doesn’t involve much work. In fact, you can use the existing connections and wires to attach a new fan in place. Just buy a fan of the same size as the fan you are replacing to avoid ceiling hole adjustments. Remember to use switch wires and connect the new wires by hand to avoid mistakes.
How do you select the right size bathroom fan?
Bathroom fans are rated per the airflow in CFM (cubic feet per minute). The rule of thumb is that you need at least 1 CFM per square foot of room area, if your bathroom is less than 100 square feet. So, a bathroom of 60 square feet would need a 60 CFM fan. For bathrooms larger than 100 square feet, add up the CFMs associated with each fixture you have to determine what size fan to buy.