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Bathroom Exhaust Fans

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

    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 programENERGY STAR Single-Family New Homes, and Indoor airPLUS.

    Description
    Description

    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.

    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. 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. 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
    Figure 1. For better fan 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 (Source: Courtesy of PNNL). 
    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 flex duct (Source: Courtesy of PNNL). 

    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). 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.
    Figure 3. Bathroom exhaust fans can vent out through the wall or up through the roof (Source: Courtesy of PNNL). 

    Step 6: If the fan is located in an attic, cover the fan housing with an insulated, airtight box made of rigid foam. 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.

    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 (Source: Courtesy of PNNL). 

    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. 

    Success
    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 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.

    In homes pursuing ENERGY STAR or Zero Energy Ready Home certification, 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. For more information, see Testing of Mechanical Ventilation Systems and Measuring Mechanical Ventilation Airflow

    Climate
    Climate

    No climate specific information applies.

    Training
    Right and Wrong Images
    Image
    Wrong - the CFM rating may not meet the performance specification once installed, and the sone rating is too high.
    Wrong - the CFM rating may not meet the performance specification once installed, and the sone rating is too high.
    Image
    Right - This manufacturer's label shows a CFM higher than the requirement, increasing the likelihood that it will meet the performance level once installed.
    Right - This manufacturer's label shows a CFM higher than the requirement, increasing the likelihood that it will meet the performance level once installed.
    Image
    Wrong - Two exhaust terminations are joined in a roughly cut, restrictive hole that is not air sealed.
    Wrong - Two exhaust terminations are joined in a roughly cut, restrictive hole that is not air sealed.
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    Right - The exhaust vent duct terminates to the outdoors with a vent cover that has been properly caulked and covered with a metal pest screen.
    Right - The exhaust vent duct terminates to the outdoors with a vent cover that has been properly caulked and covered with a metal pest screen.
    Image
    Right - The exhaust duct has been mechanically fastened and sealed with mastic.
    Right - The exhaust duct has been mechanically fastened and sealed with mastic.
    Image
    Wrong – This exhaust duct extends far beyond what is needed to reach the roof; a short, straight, smooth rigid metal duct is best for exhaust ducts and exhausting to the soffit vent would be preferable.
    Wrong – This exhaust duct extends far beyond what is needed to reach the roof; a short, straight, smooth rigid metal duct is best for exhaust ducts and exhausting to the soffit vent would be preferable.
    Image
    Wrong – The position of this exhaust fan does not allow the duct to route directly outside in as straight a path as possible.
    Wrong – The position of this exhaust fan does not allow the duct to route directly outside in as straight a path as possible.
    Image
    Right – This exhaust fan duct is made of smooth rigid metal and the duct is mastic sealed at the joints.
    Right – This exhaust fan duct is made of smooth rigid metal and the duct is mastic sealed at the joints.
    Image
    Right – This bath fan runs continuously at low speed and is activated by a motion sensor to operate a higher speeds.
    Right – This bath fan runs continuously at low speed and is activated by a motion sensor to operate a higher speeds.
    Image
    Wrong - Bathroom fan should have enough suction to hold a tissue.
    Wrong - Bathroom fan should have enough suction to hold a tissue.
    Videos
    Compliance

    Compliance

    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 Single-Family New Homes, Version 3/3.1/3.2 (Rev. 13)

    National Rater Field Checklist

    HVAC System31.
    7. Dwelling Unit Mechanical Ventilation Systems (“Vent System”) 46 & Inlets In Return Duct 47
    7.4 System fan rated ≤ 3 sones if intermittent and ≤ 1 sone if continuous, or exempted. 52

    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: 48, 58

    Please see the ENERGY STAR Single Family New Homes National Rater Field Checklist for relevant footnotes. 

    Please see the ENERGY STAR Single-Family New Homes Implementation Timeline for the program version and revision currently applicable in your state.

     

    Energy Star Multifamily New Homes, Version 1/1.1/1.2 (Rev. 04)

    National Rater Field Checklist

    7.6 If located in the dwelling unit, system fan rated ≤ 3 sones if intermittent, ≤ 2 sones if continuous, or exempted. 68
    7.8 In-unit bathroom fans or in-line fans are ENERGY STAR certified if used as part of the dwelling-unit mechanical ventilation system. 68

    8. Local Mechanical Exhaust – In each dwelling unit 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. 59, 73

    Please see the ENERGY STAR Multifamily New Construction National Rater Field Checklist for relevant footnotes. 

    Please see the ENERGY STAR Multifamily New Construction Implementation Timeline for the program version and revision currently applicable in your state. 

     

    DOE Zero Energy Ready Home

    Version 1, Rev. 07, Rev. 08, Rev. 09 and Version 2

    Exhibit 1 Mandatory Requirements.
    Exhibit 1, Item 1) (Item 2) in Version 1 Rev. 09 and Version 2) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.
    Exhibit 1, Item 5) (Item 6) in Version 1 Rev. 09 and Version 2) All installed bathroom ventilation and ceiling fans are ENERGY STAR qualified.
    Exhibit 1, Item 6) (Item 7) in Version 1 Rev. 09 and Version 2) Certified under EPA Indoor airPLUS.

     

    American Society of Heating, Air-Conditioning and Refrigeration Engineers (ASHRAE) 62.2 - 2010

    ASHRAE Standard 62.2-2010 Ventilation for Acceptable Indoor Air Quality in Low-Rise Residential Buildings. 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 Society of Heating, Air-Conditioning and Refrigeration Engineers (ASHRAE) 90.1 - 2010, 2013, 2016, and 2019

    ASHRAE Standard 90.1 Energy Standard for Buildings Except Low-Rise Residential Buildings. The standard provides minimum energy-efficient requirements for commercial buildings, including multifamily buildings more than three stories tall. 

    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)

    Residential Requirements

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

    Commercial Requirements

    See section 503.2.4.4 for mandatory exhaust damper requirements and exceptions. 

    2012, 20152018, and 2021 IECC

    Residential Requirements

    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.

    Commercial Requirements

    See section C403.2.4.4 (C403.2.4.3 in 2015 and C403.7.7 in 2018 and 2021) for mandatory exhaust damper requirements and exceptions.

    Retrofit:  2009, 2012, 2015, 2018,  and 2021 IECC

    Section 101.4.3 (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.

     

    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, 20152018, and 2021 IRC

    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.

    Retrofit:  2009, 2012, 2015, 2018,  and 2021 IRC

    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.

    Retrofit
    Existing Homes

    SCOPE

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

    If the current bathroom fan exhausts into the attic, crawlspace, or between floors, replace or reroute the duct so that it 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.

    DESCRIPTION

    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.  Draw and fan efficiency may be improved by simple maintenance steps such as vacuuming the fan cover and fan blades and verifying that the termination of the exhaust duct is not clogged with debris. Follow the installation instructions provided in the Description tab.

    COMPLIANCE

    See Compliance tab. 

    More

    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*
    Author(s)
    Ueno Kohta,
    Bergey Daniel,
    Lstiburek Joseph W
    Organization(s)
    Building Science Corporation,
    BSC,
    National Renewable Energy Laboratory,
    NREL
    Publication Date
    Description
    Research study examining the performance of existing multifamily ventilation central systems and explored alternative solutions in retrofit situations.
    *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.

    Sales
    Building Science Measures
    Building Science-to-Sales Translator

    High-Efficiency Bathroom Fans = High-Efficiency Bath Fan

    Image(s)
    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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