Regardless of what kind of ventilation system you have for the rest of the house, exhaust fans are recommended in the bathrooms to provide local exhaust to remove excess moisture, cleaning chemical fumes, etc. The fan should be ducted to exhaust outside of the home, not into the attic, crawlspace, garage, interstitial floor space, or wall cavities. Operable windows should not be relied on for consistent bathroom ventilation.

Bathroom fans can be run intermittently (occupant controlled) or continuously; some fan models have multiple speed settings and can be used for either. When operated intermittently, it is recommended to control the fan using a timer switch rather than a standard wall switch. It is recommended to leave the fan running for one hour after a shower for effective humidity control. A timer switch allows the user to set the runtime to 1 hour, after which the fan will automatically shut off.

To fulfill the local exhaust airflow requirements of ASHRAE 62.2 (2010) and the International Residential Code (IRC 2021, Section M1507), bathroom fans should have a mechanical exhaust capacity of ≥ 50 cfm for intermittent operation or ≥ 20 cfm of ventilation when operated continuously. Select fans that are ENERGY STAR rated with low sound ratings and low power draw. ENERGY STAR recommends that exhaust fans have sound ratings of ≤ 1 sone if set to run continuously or ≤ 3 sones if operating intermittently. For more on bathroom fan ratings and ASHRAE 62.2 requirements, see Intermittent Supply/Exhaust Fan Ratings and Continuous Supply/Exhaust Fan Ratings. For more on bathroom exhaust strategies in multifamily buildings, see Ventilation and Exhaust Strategies for Multifamily Housing. 

Although a single-point exhaust fan such as a bathroom fan could be used to provide code-required whole-house ventilation, this strategy is not recommended. For more on whole house ventilation strategies and requirements, see the Building America Solution Center Guides Whole House Ventilation Strategies for New Homes and Whole House Ventilation Strategies for Existing Homes.

How to Install Bathroom Exhaust Fans

Step 1: Determine the appropriate fan size for your application. For a continuous rate of ≥ 20 cfm, ENERGY STAR recommends selecting a fan that provides more than 50 cfm to pull the required amount. For an intermittent rate of ≥ 50 cfm, ENERGY STAR recommends selecting a fan with a rating of at least 70 cfm. Choose ENERGY STAR-rated fans to ensure energy efficiency and low noise level.

Step 2: Install the fan in the bathroom ceiling in accordance with the manufacturer’s instructions. The fan’s exhaust port should face in the direction of the termination point. Cut an opening in the ceiling just large enough to fit the fan. After installation, air seal between the fan housing and the drywall with caulk.

Step 3: Connect the exhaust duct to the fan port with mechanical fasteners or a pipe clamp and seal with mastic or UL-approved metal tape. Choose a smooth-surfaced duct such as galvanized sheet metal or PVC that is the size specified by the manufacturer. If aluminum flex duct is used, it should be stretched tight to avoid unnecessary sagging and to minimize friction. Ideally, the duct should be the same diameter as the exit opening on the fan housing.

Step 4: Vent the fan exhaust directly to the outside, not into an attic or crawlspace. The duct should be supported so that it hangs as straight as possible and positioned so that it has as few bends as possible (Figure 1). At a minimum, the first three feet of duct extending from the fan exhaust port should be straight; an installation with a 90-degree elbow immediately adjacent to the fan exhaust port can significantly reduce airflow. If bends are necessary, gradual bends are preferred to 90-degree elbows for optimum flow and less airflow noise (Figure 2). The duct should be routed so that it is out of the way of other ducts and equipment in the attic and care should be taken not to crush or kink the duct during or after installation. If possible, the duct terminal should be located to the side wall slightly below the fan, allowing the duct to slope down and away from the housing to direct any condensation away from the fan. Seal duct joints and seams with mastic or metal tape, including longitudinal seams in rigid metal duct. To minimize condensation, insulate the duct.
 

Allow two to three feet of straight duct run from the fan exhaust port to the first elbow

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Exhaust pipe should be made of smooth, rigid duct and any bends should be gradual, not sharp

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Step 5: Ensure the exhaust duct outlet vent is located at the exterior of the home at least 10 ft from any mechanical air inlet or at least 3 ft from any gravity air inlet per Section M1504.3 of the IRC (Figure 3). The wall cap should include a damper that closes when the fan is not exhausting; this can be a motorized damper or a gravity-operated, butterfly, or cape-style fabric sleeve damper. The wall cap may come with a screen or grille to keep out birds and animals. The exhaust air should not be directed onto a walkway. For more information, see the guide Ventilation Air Inlet Locations. 

Bathroom exhaust fan can vent out through the wall or up through the roof

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Step 6: If the fan is located in an attic, cover the fan housing with an insulated, airtight box made of rigid foam (Figure 4). Seal the seams with housewrap tape. Use caulk or spray foam to seal the box to the ceiling drywall and to seal around the exhaust pipe. Cover the box with attic insulation.

Step 7: For bathroom exhaust fans used as part of a whole-house ventilation system, make sure that the fan switch is clearly designated and install override controls that are clearly labeled and located in an accessible place, such as near the thermostat, on the electrical panel, on the switch plate, or on the air handler. See Obvious Ventilation Controls and Continuously Operating Ventilation and Exhaust Fans for more information.