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Bathroom Exhaust Fan

Installed correctly, a bathroom exhaust fan can be used to remove moisture and odors and also run continuously as a whole-house fresh air system for a healthier indoor environment.

Bathroom exhaust fans used only for spot exhaust shall have a mechanical exhaust capacity of at least 20 cfm for continuous operation, or at least 50 cfm for intermittent operation. To ensure these minimum flow rates are met, it is recommended that the bathroom exhaust fan selected has a rating of 50 cfm for continuous operation, or 70 cfm for intermittent operation.
Bathroom exhaust fans used for continuous whole-house ventilation shall have a mechanical exhaust capacity based on house size as follows: 50 cfm for up to 1,500 ft², 70 cfm for 1,501 to 2,500 ft², and 100 cfm over 2,500 ft².
The bathroom exhaust fan shall be ENERGY STAR certified.
The exhaust duct shall be made of rigid metal (e.g., galvanized steel, stainless steel, or copper), have a smooth interior surface, be equipped with a backdraft damper, meet the maximum length guidelines specified in the IRC (2015 IRC, Table M1506.2), and meet the minimum diameter or dimension guidelines specified in the fan manufacturer’s installation instructions.
The bathroom exhaust fan shall be installed to vent outdoors, not into an attic, crawlspace, or space between floors.
The exhaust duct shall be installed with the most direct route to the outside, with as few bends as possible, and with no bends for the first three feet of duct from the fan.
The exhaust duct outlet vent shall be located on the exterior of the home at least 10 feet from any air inlet and where it does not direct air flow onto a walkway.
All exhaust duct seams and connections shall be sealed with mastic or UL 181 tape.
All ceiling and wall or roof penetrations shall be sealed with mastic, caulk, or spray foam on interior surfaces and flashed as needed on exterior surfaces for full weather protection.
In a vented attic, the exhaust fan housing should be covered with an airtight box made of rigid foam or another solid material. The box should be air sealed to the ceiling drywall with caulk, spray foam, or tape. The box and the duct shall be covered with attic insulation.
The wall cap shall include a damper that closes when the fan is not exhausting.
The wall cap damper shall be checked to ensure it is operating correctly.
The exhaust fan flow rate shall be measured using a flow hood, flow grid, or anemometer, in accordance with the test procedures listed in ANSI/RESNET/ICC 380-2016 and adjustments shall be made to ensure the fan is providing the minimum flow rates specified above.
All operation and maintenance procedures shall be reviewed with the homeowner (e.g., checking vent annually to ensure it is cleared of debris or insect nests) and the fan should be set up to operate as desired by the homeowner (e.g., only spot-ventilation or continuously for whole-house ventilation).
At the completion of the work, a radon test kit shall be provided to the homeowner to measure post-retrofit radon levels in the conditioned space of the home and remediate if radon levels exceed EPA limits.

BASC Guides

Guide describing how to install a bathroom fan to remove excess moisture, cleaning chemical fumes, etc.

Guide describing the bathroom exhaust fan ENERGY STAR rating requirements.

Guide describing how to air seal bathroom and kitchen exhaust fans.

Tips to Sell Quality Installed Home Improvements

Home Improvement Expert (HIE) is a valuable tool for organizations committed to quality installed work. The following tips help optimize the value of this tool when selling home improvements:

  • Be the Expert: Take advantage of Building America Solution Center comprehensive guidance on ‘Existing Home’ retrofits.
  • Earn Trust: Inform homeowners how your work conforms to U.S. Department of Energy (DOE) world-class expert guidance and recommend homeowners visit the DOE website as evidence these are indeed official best practices.
  • Clarity with Contrast: Tell prospective homeowner clients to compare your expert recommended best practices with other contractors.
  • Ensure Equivalent Pricing: Tell prospective homeowner clients to insist other bids also include DOE checklists to ensure equivalent quality work.
  • Translate Value: Note your company uses DOE HIE Checklists based on world-class expert recommendations for home improvements on all your public-facing communication including websites, advertising, and signage.
  • Create Emotional Experiences: Provide visual evidence contrasting the difference between poor and high quality work such as infrared images for good and bad insulation and air sealing; pre- and post-energy bills following quality installed work; short and long warranties for standard and high-efficiency equipment; and charts showing amounts of contaminants in homes that can be reduced with effective fresh air systems.

Bathroom Exhaust Fan Background

Preparing a bathroom exhaust duct

Washing and bathing release significant amounts of water vapor into the air. For instance, a shower produces a half pint of water vapor for every 5 minutes of run time. With no exhaust fan, this moisture can accumulate and increase the risk of mold and mildew on floors, walls, and ceilings. High-efficiency exhaust fans can quietly exhaust bathroom air to the outside. These bathroom exhaust fans can be operated manually or automatically with humidity sensors. In addition, they can be operated continuously as a whole-house fresh air system to exhaust stale air. Make-up fresh air supply relies on natural infiltration air flow through the holes and cracks in home construction.