How to choose a bathroom exhaust fan

Good bathroom exhaust fans protect your home from excess moisture, which can lead to mold and mildew. (Illustration by Steven Shepard)

Good bathroom exhaust fans protect your home from excess moisture, which can lead to mold and mildew. (Illustration by Steven Shepard)

A good bathroom exhaust fan reduces moisture in the dampest room of the house, preventing mold, improving air quality and protecting walls and fixtures — not to mention removing stinky odors from the air! Choose the best bathroom fan for your needs with these tips.

1. Find the right airflow rating for room size

All ventilation fans have an airflow rating measured in cubic feet per minute (CFM). To find the CFM rating you need, measure the room to determine the square footage.

The Home Ventilating Institute recommends one CFM per square foot of bathroom area, with a minimum of 50 CFM. Bathrooms larger than 100 square feet need higher CFM ratings: add 50 CFM for each toilet, shower, bathtub and jetted tub. Higher-rated fans may also be needed for rooms with ceilings more than 8 feet tall.

2. Choose an acceptable noise level

Noise level is measured in sones. One sone or less is about as loud as a refrigerator running. Three sones is slightly louder than typical office noise. Noise levels for fans range from below 1 sone to as many as 4 sones.

Choose a sound level that you can tolerate for at least 15 minutes — about the time it takes to remove moisture. Expect to pay a premium for the quietest fans, though quieter isn't always better. Louder fans in powder rooms can dampen noises from the sink and toilet.

3. Explore fans with added features

Fans come with many extra features, such as lighting, heaters and automated timers. Some come with a humidistat that will turn the bath fan on and off as the humidity level in the bathroom changes.

Avoid fans that turn on and off with the light switch — they are often turned off before the fan has time to work.

4. Determine the best ventilation method

The type of fan you choose will most likely be determined by available space and proximity to existing duct systems. Bathroom fans come in a few main types:

Ceiling-mounted fans are installed into the ceiling and vent to the outside through the roof or ducts. This is the most common type of bathroom fan and is best for removing hot, damp air as it rises. The exhaust is almost undetectable from the outside.

Wall-mounted fans are installed in exterior walls and pull air directly outside. These fans do not need ductwork but might have a visible register.

Inline fans are installed in the ceiling or wall, but the motor sits between ducts, remotely pulling the air outside. Because the motor is not attached directly to the ceiling, this is a very quiet bathroom fan. These are ideal when there's not enough space in the ceiling.

Never vent an exhaust fan to the attic. This keeps moist air indoors, often in a place where mold can grow undetected. Always vent bathroom fans to the outside.

Can I install a ventilation fan myself?

Installing a bathroom exhaust fan is sometimes a simple do-it-yourself project, but can get complicated as the fan connects to the house's ductwork. A skilled HVAC expert can assess your needs and install a new fan quickly.

Editor's note: This is an updated version of a story originally posted on Sept. 18, 2012.

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