Bathroom Exhaust Fans

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Bathrooms should be equipped with an exhaust fan that vents directly outdoors
Bathrooms should be equipped with an exhaust fan that vents directly outdoors

Install an exhaust fan in the bathroom to exhaust steam to the outdoors.

  • Install the fan to vent outdoors, not into an attic, crawlspace, or space between floors.
  • Choose a duct with the diameter specified by the fan manufacturer.
  • Install the duct with the most direct route to the outside with as few bends as possible.
  • Seal all seams and around ceiling and wall or roof penetrations with mastic or spray foam. Flash exterior surfaces as needed.
  • Install a fan that meets Rater-measured airflow codes and standards for local exhaust; meet whole-house ventilation requirements if the bath fan is used for whole-house mechanical ventilation.

See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.


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. Operable windows are a nice feature but they 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. To fulfill the local exhaust airflow requirements of ASHRAE 62.2 and the International Residential Code (IRC 2015, 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, and have 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 Bathroom Fan RatingsIntermittent Supply/Exhaust Fan Ratings, and Continuous Supply/Exhaust Fan Ratings.

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 Guide Whole House Ventilation Strategies for Existing Homes.

How to Install Bathroom Exhaust Fans

  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 70 cfm. Choose ENERGY STAR-rated fans for energy efficiency and low noise level.
  2. Install the fan in the bathroom ceiling in accordance with the manufacturer’s instructions. The fan’s exhaust port should point in the direction of the termination point. Cut openings in the ceiling for the fan no bigger than needed to fit the fan. After installation, air seal between the fan housing and the drywall with caulk.
  3. Connect the exhaust duct to the fan port with mechanical fasteners and/or mastic. 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.
  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. At a minimum, the first three feet of duct extending from the fan exhaust port should lay straight; an installation with a 90-degree elbow immediately adjacent to the fan exhaust port can cause air to flow back into the fan. If bends are necessary, gradual bends are preferred to 90-degree elbows for optimum flow and less airflow noise. 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 fan housing to direct any condensation away from the fan. Seal seams mastic or metal tape. To minimize condensation, insulate the duct.
    For better performance, make duct runs as straight as possible

    Figure 1 - For better performance, make duct runs as straight as possible. If a bend is necessary, allow two to three feet of straight duct run from the fan exhaust port to the first elbow in the duct run. 

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

    Figure 2 - Elbows in exhaust duct should be gradual rather than 90 degree turns if possible. Smooth rigid duct is preferred over ridged flex duct. 

  5. Locate the exhaust duct outlet vent at the exterior of the home at least 10 ft from any air inlet. The wall cap should include a damper that closes when the fan is not exhausting; this can be a motorized damper or a gravity-operate, 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.
    Bathroom exhaust fan can vent out through the wall or up through the roof
    Figure 3 - Bathroom exhaust fan can vent out through the wall or up through the roof. 
  6. Cover the fan housing with an insulated, airtight box. Make the box from rigid foam. Tape 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.
    Air seal and insulate around the exhaust fan with a rigid foam box
    Figure 4 - Create a box from rigid foam to cover the exhaust fan housing. Tape the seams and caulk the edges of the box to the ceiling drywall to make the box airtight. Cover the box and the duct with attic insulation. 
  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.

Ensuring Success

Visually inspect the fan for proper installation as follows:

  1. Ensure that the fan exhausts outside, not into the attic; that it is set to exhaust, not recirculate; that any dampers on the outside termination are able to open freely; and that packing tape is removed.
  2. Confirm that any openings made in the ceiling for the fan or exhaust duct are properly air sealed and that the exhaust duct is sealed to the fan housing with both mechanical fasteners and mastic for flex duct and mechanical fasteners and mastic or spray foam for rigid duct.
  3. Check the sone rating; ASHRAE 62.2 requires 3 sones or less for intermittent (occupant-controlled) kitchen or bath exhaust fans or 1.0 sone or less for continuous fans.
  4. See the “compliance” tab for calculating exhaust rates to meet ASHRAE 62.2 requirements and ENERGY STAR guidelines for intermittent and continuous operation, as well as IRC requirements on fan size. If the bathroom fan will be used as the primary means for meeting code-required ventilation, calculate the ventilation rate required based on the size of the home as described in ASHRAE 62.2 and ensure that the fan’s tested flow rate will meet this requirement.

A certified energy rater will test the fan’s operation to determine the flow rate using a powered flow hood, an airflow resistance device, or a passive flow hood using test procedures described in ANSI/RESNET/ICC 380-2016 Section 5.1, or those established by the Associated Air Barrier Council, the National Environmental Balancing Bureau, ASHRAE, or another equivalent method. Bathroom fans are typically rated by how many cubic feet per minute the fan will exhaust in a factory setting. Duct work, termination choices, and installation may decrease the measured cubic feet per minute below the factory-rated value. To ensure the installed fan exhausts the correct amount of cubic feet per minute, EPA recommends the HVAC contractor install a fan with a rating higher than the required measured amount.


No climate specific information applies.



The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

ENERGY STAR Certified Homes, Version 3/3.1 (Rev. 09)

National Rater Field Checklist

HVAC System.
7. Whole-House Mechanical Ventilation System.
7.4 System fan rated ≤ 3 sones if intermittent and ≤ 1 sone if continuous, or exempted.44

8. Local Mechanical Exhaust - In each kitchen and bathroom, a system is installed that exhausts directly to the outdoors and meets one of the following Rater-measured airflow and manufacturer-rated sound level standards:42, 48

National Rater Field Checklist: Local Mechanical Exhaust
National Rater Field Checklist: Local Mechanical Exhaust  (Source: ENERGY STAR Certified Homes, Version (Rev. 09))

Footnote 42) The whole-house ventilation air flow and local exhaust air flows shall be determined and documented by a Rater using the same version of ANSI / RESNET / ICC Std. 380 that is utilized by RESNET for HERS ratings.

Footnote 44) Whole-house mechanical ventilation fans shall be rated for sound at no less than the airflow rate in Item 2.3 of the National HVAC Design Report. Fans exempted from this requirement include HVAC air handler fans, remote-mounted fans, and intermittent fans rated ≥ 400 CFM. To be considered for this exemption, a remote-mounted fan must be mounted outside the habitable spaces, bathrooms, toilets, and hallways and there shall be ≥ 4 ft. ductwork between the fan and intake grill. Per ASHRAE 62.2-2010, habitable spaces are intended for continual human occupancy; such space generally includes areas used for living, sleeping, dining, and cooking but does not generally include bathrooms, toilets, hallways, storage areas, closets, or utility rooms.

Footnote 48) Continuous bathroom local mechanical exhaust fans shall be rated for sound at no less than the airflow rate in Item 8.2. Intermittent bathroom and both intermittent and continuous kitchen local mechanical exhaust fans are recommended, but not required, to be rated for sound at no less than the airflow rate in Items 8.1 and 8.2. Per ASHRAE 62.2-2010, an exhaust system is one or more fans that remove air from the building, causing outdoor air to enter by ventilation inlets or normal leakage paths through the building envelope (e.g., bath exhaust fans, range hoods, clothes dryers). Per ASHRAE 62.2-2010, a bathroom is any room containing a bathtub, shower, spa, or similar source of moisture.

Footnote 49) An intermittent mechanical exhaust system, where provided, shall be designed to operate as needed by the occupant. Control devices shall not impede occupant control in intermittent systems.

Please see the ENERGY STAR Certified Homes Implementation Timeline for the program version and revision currently applicable in in your state.

DOE Zero Energy Ready Home (Revision 07)

Exhibit 1 Mandatory Requirements.
Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.
Exhibit 1, Item 5) All installed bathroom ventilation and ceiling fans are ENERGY STAR qualified.
Exhibit 1, Item 6) Certified under EPA Indoor airPLUS.

American Society of Heating, Air-Conditioning and Refrigeration Engineers (ASHRAE)

ASHRAE Standard 62.2-2010 Ventilation for Acceptable Indoor Air Quality in Low-Rise Residential Buildings (available for purchase at link above). The standard provides minimum requirements for mechanical and natural ventilation systems and the building envelope intended to provide acceptable indoor air quality in low-rise residential buildings.

American National Standards Institute (ANSI)/Residential Energy Services Network (RESNET)/International Code Council (ICC) 380-2016

Standard for Testing Airtightness of Building Enclosures, Airtightness of Heating and Cooling Air Distribution Systems, and Airflow of Mechanical Ventilation Systems. Section 5.1 defines procedures for measuring the airflow of a mechanical ventilation system including a powered flow hood, an airflow resistance device, or a passive flow hood.

2009 International Energy Conservation Code (IECC)

Section 403.5 Mechanical ventilation (Mandatory). Automatic or gravity dampers are installed on all outdoor air intakes and exhausts.

2012, 2015, and 2018 IECC

Section R403.5 (R403.6 in 2015 and 2018 IECC) Mechanical ventilation shall meet the requirements of the International Residential Code or the International Mechanical Code. Automatic or gravity dampers are installed on all outdoor air intakes and exhausts.

Table R403.5.1 (R403.6.1 in 2015 and 2018 IECC) Mechanical Ventilation System Fan Efficacy.

  • Bath/utility room with rated airflow rate of ≥ 10 cfm and < 90 cfm should have a minimum efficacy of 1.4 cfm/watt.
  • Bath/utility room with  rated airflow rate of ≥ 90 cfm should have a minimum efficacy of 2.8 cfm/watt.


2009, 2012, 2015, 2018, and 2021 IECC

Section R101.4.3 (Section R501.1.1 in 2015, 2018, and 2021 IECC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

2009 International Residential Code (IRC)

Section M1507.3 Ventilation rate. Bathrooms - toilet rooms should have mechanical exhaust capacity of at least 50 cfm intermittent or 20 cfm continuous.

2012, 2015, and 2018 IRC

Section M1507.2 (M1505.2 in 2018 IRC) Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or to another dwelling unit. It should be exhausted directly outdoors; it should not discharge into an attic, crawl space, or other areas within the building.

Section M1507.3.1 (M1505.4.1 in 2018 IRC) The whole-house mechanical ventilation system shall consist of one or more supply or exhaust fans, or a combination of such and associated ducts and controls. Local exhaust or supply fans are permitted to serve as such a system. Outdoor air ducts connected to the return side of an air handler shall be considered as providing supply ventilation. The whole-house mechanical ventilation system shall be provided with controls that enable manual override.

The whole house mechanical ventilation system shall provide outdoor air at a rate equivalent to the minimum rates shown in Tables M1507.3.3(1) and M1507.3.3(2) (M1505.4.3(1) and M1505.4.3(2) in 2018 IRC), which are based on the home's floor area.

Section M1507.4 (M1505.4.4 in 2018 IRC) Local exhaust rates. Bathrooms and toilet rooms should have mechanical exhaust capacity of at least 50 cfm intermittent or 20 cfm continuous.


2009, 2012, 20152018, and 2021 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018, N1109.1 in 2021 IRC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.

This Retrofit tab provides information that helps installers apply this “new home” guide to improvement projects for existing homes. This tab is organized with headings that mirror the new home tabs, such as “Scope,” “Description,” “Success,” etc. If there is no retrofit-specific information for a section, that heading is not included.

Existing Homes


Install or replace the bathroom exhaust fan, if the bathroom currently lacks an exhaust fan or the existing fan has insufficient draw, as determined by occupant experience or through fan testing by a home energy rater.

If the current bathroom fan exhausts into the attic, crawlspace, or between floors, replace the duct with one that exhausts to the outside.

Verify that the exhaust fan duct has an operating damper and that the exterior end of the exhaust duct is protected with a hooded wall cap and that the opening is covered with screening to keep out birds and pests.

See the assessment guide, Pre-Retrofit Assessment of Attics, Ceilings, and Roofs.

For more on exhaust fan ventilation, see the U.S. Department of Energy’s Standard Work Specifications.

See the Scope tab for additional job specifications.


Installation of a new bathroom exhaust fan is warranted if the bathroom currently has no exhaust fan. Other reasons to replace the fan include noise or moisture complaints by the homeowner or fan testing as part of a home energy audit that reveals that the fan has insufficient draw. Follow the installation instructions provided in the Description tab.


See Compliance tab. 

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Case Studies
References and Resources*
Building Science Corporation
Publication Date
Fact sheet providing detailed information about air sealing attics.
*For non-dated media, such as websites, the date listed is the date accessed.
Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Building Science Measures
Building Science-to-Sales Translator

High-Efficiency Bathroom Fans = High-Efficiency Bath Fan

Technical Description

Bathroom exhaust fans remove moisture from the home to help keep relative humidity levels down. This is important for discouraging condensation and the growth of mold, mildew, and dust mites. Continuous or intermittently operating exhaust fans can also be part of a whole-house ventilation strategy. ENERGY STAR certified ventilation fans provide better efficiency and comfort with less noise, and in some cases, have controls for spot exhaust and continuous ventilation. They feature high-performance motors and improved blade design, providing more effective exhaust and longer life.

High-Efficiency Bath Fan
Sales Message

High-efficiency bath fans minimize the wasted energy exhausting air while being virtually silent. What this means to you is odors and humidity are efficiently removed without the noise. Isn’t it time homes used advanced technology components?

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