Bathroom Exhausts

Regardless of what kind of ventilation system may be installed for the rest of the house, exhaust fans are recommended in the bathrooms to remove excess moisture, clean chemical fumes, etc. The fan should be ducted to exhaust outside of the home.

Exhaust air from bathrooms, toilet rooms, water closet compartments, and other similar rooms shall not be:

  • exhausted into a ceiling void, eave, ridge vent, subfloor, or other areas inside the building; or
  • recirculated within a residence or to another dwelling unit. 

Operable bathroom windows are a convenient feature, but they should not be relied on for consistently adequate bathroom ventilation.

In meeting modern exhaust airflow standards, bathroom fans can be run intermittently (occupant-controlled) or continuously. Intermittent fans should have a flow rate of 50 cfm or more, and continuous fans should have a flow rate of 20 cfm. If the fan is set to run continuously, the rating should be 1.0 or less. ENERGY STAR-rated exhaust fans can have low ratings, low power draw, and, in some cases, multiple speeds for spot exhaust and continuous ventilation.

Manufacturer’s Instructions

The fan in the bathroom should be installed in accordance with the manufacturer’s instructions. 


Openings in the ceiling or wall should be cut for the fan no bigger than needed to fit the fan. After installation, an air seal between the fan housing and the drywall may be filled with caulk or foam insulation.


The connection of the exhaust duct to the fan port may be made with mechanical fasteners and/or mastic. A smooth-surfaced duct, such as galvanized sheet metal or PVC, that is the size specified by the manufacturer should be installed. If an aluminium flex duct is used, it should be stretched tight to avoid unnecessary sagging and minimize friction. Ideally, the duct should be the same diameter as the exit opening on the fan housing.

Straight as Possible

The fan should exhaust directly from the outside. 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 1 meter 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. The duct should not be crushed or kinked. If possible, the best practice is to have the duct terminal be located on 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. Duct seams should be sealed with mastic or metal tape. To minimize condensation, insulation could be installed on the duct. 


Fans that exhaust into the ceiling, attic or building’s interior can cause problems from excessive moisture. Warm, moist air will condense on cold attic framing, insulation, and other materials. If mould develops, it may cause health problems for sensitive occupants. Condensation could cause damage to building materials. Moisture also reduces the effectiveness of thermal insulation.  

The bathroom exhaust fan can vent out through the wall or up through the roof.

Inspection of Bathroom Fans

  1. Turn the fan on and off.
  2. Listen to its operation.
  3. Check that the fan exhausts directly outside.
  4. Check that any dampers on the outside termination can open freely.
  5. Check that any openings made in the ceiling for the fan or exhaust duct are air-sealed.
  6. Check 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.
  7. Check to see that the component is sealed and insulated.
  8. Check for dust build-up that can impede airflow.
  9. Look for indications of moisture or condensation related to the fan’s installation.
  10. Test the fan’s operation to determine the flow rate using a flow hood, flow grid, anemometer, or another equivalent method. Bathroom fans are typically rated by how many cubic feet per minute the fan will exhaust in a factory setting. Ductwork, termination choices, and installation may decrease the measured cubic feet per minute below the factory-rated value. To ensure that the installed fan exhausts the correct amount of cubic feet per minute, the EPA recommends that a contractor install a fan with a rating higher than the required measured amount.

 Additional Information

  • 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. Examples include bath exhaust fans, range hoods, and clothes dryer exhausts/vents. 
  • A bathroom is commonly considered any room containing a sink (lavatory) and a toilet (water closet), a bathtub, a shower, or a similar source of moisture. 
  • Humidity control may be a separate component of the exhaust fan and typically is not required to be integral.

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