How to Choose a Bathroom Exhaust Fan

Bathroom exhaust fans maintain indoor air quality and prevent moisture-related issues. In the video above, This Old House expert Heath Eastman demonstrates various exhaust fan options, features, and installation methods.

This guide describes essential factors to consider when selecting the right bathroom exhaust fan for your home.

Types of Bathroom Exhaust Fans

The most common bathroom exhaust fans include:

• Ceiling-mounted fans: These are the most common and are installed directly in the bathroom ceiling.
• Combination fans: This fan type combines an exhaust fan with other features like lighting or heating.
• Inline fans: These fans are mounted in the attic or another remote location and connected to the bathroom via ductwork.
• Wall-mounted fans: These fans are ideal for bathrooms without attic access or where ceiling installation is not feasible.

Choosing the Right Size Exhaust Fan

A fan’s capacity is measured in cubic feet per minute (CFM), which indicates how much air the fan can move in one minute. To determine the right CFM for your bathroom, follow these guidelines:

1. Measure your bathroom’s cubic footage by multiplying the room’s length, width, and height.
2. For bathrooms up to 100 square feet, you will need 1 CFM per cubic foot.
3. For larger bathrooms, add 50 CFM for each toilet, shower, or bathtub and 100 CFM for a whirlpool tub.

Pro Tip: The 7.5-Minute Rule: Another way to calculate the minimum CFM for your bathroom fan is to divide the room’s total volume (length × width × height) by 7.5. This ensures the fan can pull all the air out of your bathroom in 7½ minutes. If your duct run exceeds 20 feet or has more than one elbow, go with a higher CFM to compensate for the added resistance.

Bathroom Exhaust Fan Noise Levels and Sones

The noise level of an exhaust fan is measured in sones. Lower sone ratings indicate quieter operation. For a peaceful bathroom environment, consider these guidelines:

• 1. 0 sones or less: Very quiet, ideal for master bathrooms
• 1. 0 to 2. 0 sones: Quiet, suitable for guest bathrooms
• 2. 0 to 4. 0 sones: Moderate, acceptable for most applications
• 4. 0 sones or higher: Loud, may be disruptive

Bathroom Venting Considerations

Eastman emphasizes the importance of venting to the outdoors and avoiding common mistakes. When installing a bathroom exhaust fan, consider these best practices:

1. Always vent to the outdoors, never into an attic or soffit.
2. Install a roof cap or wall cap to prevent backdrafts and keep pests out.
3. Minimize the number of bends in the ductwork to improve airflow.
4. Use insulated flexible ducting to prevent condensation and reduce noise.

Eastman recommends venting out a side wall whenever possible. If that’s not an option, venting through the roof is acceptable. However, he strongly advises against venting into an attic or through a soffit, as this can lead to moisture problems and potential mold growth.

Pro Tip: Tom Silva, This Old House general contractor, stresses that exhaust fans must always be vented outside the house, not into the attic: “Otherwise you’re dumping moisture on the underside of the roof.” In his own house, he installed timer switches to allow bathroom fans to run for a full hour after use.

Remote Motor Options

For bathrooms where standard venting is challenging, Eastman showcases a remote motor option. This system allows for greater flexibility in installation, as the motor can be located in an attic or basement and connected to the bathroom via ductwork. This option is particularly useful for:

• Bathrooms located far from exterior walls
• Reducing noise in the bathroom itself
• Situations requiring longer duct runs

Pro Tip: In a recent This Old House project, plumber Heath Eastman walked homeowner Jericho through exactly this scenario. With a small ceiling area already occupied by a light fixture, a standard fan would have been too bulky. Heath proposed a remote motor solution: “We can do a remote motor, a remote fan where the motor goes up here. We just put a small duct in the ceiling downstairs, draws a lot of air up here, and then pushes it out the side of the gable. A lot quieter, cleaner, smaller footprint down there.” The homeowner’s response? “I love the idea of a quiet fan.”

Energy Efficiency and Additional Fan Features

Some modern exhaust fans come equipped with humidity sensors that automatically turn the fan on when moisture levels rise and off when they return to normal. This feature ensures proper ventilation without relying on manual operation.

Pro Tip: In a This Old House project, a homeowner hired an electrician to install a bathroom vent fan in their only shower. The electrician vented it directly into the attic—and just a few months later, the homeowners discovered moisture damage on their master bedroom ceiling. As one of the experts on the project explained, all that warm, moist air was rising into the attic and freezing on the underside of the roof. To make matters worse, the installer had removed attic insulation to place the fan but never put it back, allowing warm air to seep into the attic even when the fan was off.

Integrated Lighting and Heating

Many exhaust fans now include built-in lighting or heating elements, offering additional functionality in a single unit. These combination units can save space and provide a sleek, integrated look in your bathroom.

Pro Tip: Models that combine the fan with a light, a heat lamp, or a light and night-light are widely available. Some even turn on automatically when they sense motion or a sudden increase in humidity. If you’re replacing a ceiling-mounted light fixture with a vent fan or combination fan/light, you can connect to the existing electrical cable and wall switch. However, note that vent fans that are UL-listed for installation directly over a tub or shower must be connected to a GFCI-protected circuit.

Timer Switches

Timer switches are an excellent option for ensuring the fan runs long enough to remove excess moisture. Heath shows two types of timer switches:

1. Mechanical wind-up timer: A simple, reliable option that allows you to set the fan to run for a specific duration.
2. Digital timer: Offers more precise control, typically with preset time intervals.

Advanced Control Modules

For the most flexible control options, Heath presents a modular fan system that allows for various control methods:

• Constant low-speed operation with boost mode during high humidity
• Customizable settings to suit your specific needs
• Plug-and-play modules for different functions (timer, humidity sensor, motion detection)

Expert assistance with this segment was provided by Eaton and Eastman Electric.