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 Single-Family New 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, crawlspace, garage, interstitial floor space, or wall cavities. 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. 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, 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 Ratings, Intermittent 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
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
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.
Visually inspect the fan for proper installation as follows:
- 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.
- 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.
- 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.
- 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.
National Rater Field Checklist
7. Dwelling Unit Mechanical Ventilation Systems (“Vent System”) 45 & Inlets In Return Duct 46
7.4 System fan rated ≤ 3 sones if intermittent and ≤ 1 sone if continuous, or exempted. 51
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: 47, 57
Footnote 45) As defined by ANSI / RESNET / ICC Std. 301-2019, a Dwelling Unit Mechanical Ventilation System is a ventilation system consisting of powered ventilation equipment such as motor-driven fans and blowers and related mechanical components such as ducts, inlets, dampers, filters and associated control devices that provides dwelling-unit ventilation at a known or measured airflow rate.
Footnote 46) Item 7.3 applies to any outdoor air inlet connected to a ducted return of the dwelling unit HVAC system, regardless of its intended purpose (e.g., for ventilation air, make-up air, combustion air). This Item does not apply to HVAC systems without a ducted return.
Footnote 47) The Dwelling Unit Mechanical Ventilation System air flows and local exhaust air flows shall be determined and documented by a Rater using ANSI / RESNET / ICC Std. 380 including all Addenda and Normative Appendices, with new versions and Addenda implemented according to the schedule defined by the HCO that the home is being certified under.
Footnote 51) Dwelling Unit Mechanical Ventilation System 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 57) 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 58) 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.
Footnote 59) Kitchen volume shall be determined by drawing the smallest possible rectangle on the floor plan that encompasses all cabinets, pantries, islands, peninsulas, ranges / ovens, and the kitchen exhaust fan, and multiplying by the average ceiling height for this area. In addition, the continuous kitchen exhaust rate shall be ≥ 25 CFM, per 2009 IRC Table M1507.3, regardless of the rate calculated using the kitchen volume. Cabinet volume shall be included in the kitchen volume.
Footnote 60) Homes shall meet this Item. Alternatively, the prescriptive duct sizing requirements in Table 5.3 of ASHRAE 62.2-2010 / 2013 / 2016 are permitted to be used for kitchen exhaust fans based upon the rated airflow of the fan at 0.25 IWC. If the rated airflow is unknown, ≥ 6 in. smooth duct shall be used, with a rectangular to round duct transition as needed. Guidance to assist partners with these alternatives is available at http://www.energystar.gov/newhomesguidance. As an alternative to Item 8.1, homes are permitted to use a continuous kitchen exhaust rate of 25 CFM per 2009 IRC Table M1507.3, if they are either a) PHIUS+ or PHI certified, or b) provide both dwelling unit ventilation and local mechanical kitchen exhaust using a balanced system, and have a Rater-verified whole-building infiltration rate ≤ 1.0 ACH50 or ≤ 0.05 CFM50 per sq. ft. of Enclosure Area, and a Rater-verified dwelling unit compartmentalization rate ≤ 0.30 CFM50 per sq. ft. of Enclosure Area if multiple dwelling units are present in the building. ‘Enclosure Area’ is defined as the area of the surfaces that bound the volume being pressurized / depressurized during the test.
Footnote 61) All intermittent kitchen exhaust fans must be capable of exhausting at least 100 CFM. In addition, if the fan is not part of a vented range hood or appliance-range hood combination (i.e., if the fan is not integrated with the range), then it must also be capable of exhausting ≥ 5 ACH, based on the kitchen volume.
Please see the ENERGY STAR Single-Family New Homes Implementation Timeline for the program version and revision currently applicable in your state.
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.
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.
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.
Section 403.5 Mechanical ventilation (Mandatory). Automatic or gravity dampers are installed on all outdoor air intakes and exhausts.
Section R403.5 (R403.6 in 2015, 2018, and 2021 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 and R403.6.2 in 2021 IECC) Whole-dwelling 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.
Section R101.4.3 (in 2009 and 2012). 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.)
Chapter 5 (in 2015, 2018, 2021). The provisions of this chapter shall control the alteration, repair, addition, and change of occupancy of existing buildings and structures.
Section M1507.3 Ventilation rate. Bathrooms - toilet rooms should have mechanical exhaust capacity of at least 50 cfm intermittent or 20 cfm continuous.
Section M1507.2 (M1505.2 in 2018 and 2021 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 and 2021 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 and 2021 IRC), which are based on the home's floor area.
Section M1507.4 (M1505.4.4 in 2018 and 2021 IRC) Local exhaust rates. Bathrooms and toilet rooms should have mechanical exhaust capacity of at least 50 cfm intermittent or 20 cfm continuous.
Section R102.7.1 Additions, alterations, or repairs. 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 the requirements of this code, unless otherwise stated. (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.
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.
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.
The following authors and organizations contributed to the content in this Guide.
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.