Inspecting the Bathroom Exhaust Fan

by Nick Gromicko and Ben Gromicko 
Bathroom ceiling fan
According to the InterNACHI® Home Inspection Standards of Practice, the inspector shall inspect the mechanical exhaust systems in the bathrooms. 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, cleaning 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 an attic, soffit, ridge vent, crawlspace, or other areas inside the building; or
  • recirculated within a residence or to another dwelling unit.  

The inspection image is of an inspector checking a bathroom exhaust fan installed above the light fixture on the wall.

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 sone rating should be 1.0 or less. ENERGY STAR-rated exhaust fans can have low sone 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. 
Port's Direction
The fan’s exhaust port should point in the direction of the termination point.  
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 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.
Straight as Possible
The fan should exhaust directly to 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 3 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. 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. 
Illustration courtesy of Pacific Northwest National Laboratory.
For better performance, duct runs should be as straight as possible. If a bend is necessary, 2 to 3 feet of straight duct should be allowed to run from the fan exhaust port to the first elbow in the duct run. 

Illustration courtesy of Pacific Northwest National Laboratory.
Elbows in the exhaust duct should be gradual rather than 90-degree turns, if possible. A smooth, rigid duct is preferred over a ridged flex 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 mold 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 inspection image above shows a bathroom fan exhausting into the insulation at the attic floor. Indications of watermarks on the drywall can be seen.

Exhaust Outlet
The exhaust duct outlet vent at the exterior of the home should be at least 10 feet away from any air inlet. The wall cap should include a damper that closes when the fan is not exhausting. The wall cap may come with a screen or grille to keep out birds and other animals. The exhaust air should not be directed onto a walkway.

Illustration courtesy of Pacific Northwest National Laboratory.
The bathroom exhaust fan can vent out through the wall or up through the roof.


The box might be covered with attic insulation.  The exhaust fan housing should be covered with an insulated, airtight box.  The box can be made from rigid foam.  The seams can be taped with housewrap tape.  Caulk or spray foam can be used to seal the box to the ceiling drywall and to seal around the exhaust pipe.

The box and the duct might be covered with attic insulation. A box from rigid foam to cover the exhaust fan housing could be created. The seams can be taped and the edges of the box caulked to the ceiling drywall to make the box airtight. 
(llustration above from Lstiburek, J. 2010. Guide to Attic Air Sealing. Prepared by the Building Science Corporation for the U.S. Department of Energy, Building America Program.)

Inspection Checklist
An inspector may follow the following inspection procedure (parts of which far exceed the InterNACHI® Home Inspection Standards of Practice):
  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 are able to 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 buildup that can impede airflow. 
  9. Look for indications of moisture or condensation related to the fan's installation.
  10. Check the sone rating. ASHRAE 62.2-2007 requires 3 sones or less for intermittent (occupant-controlled) kitchen or bath exhaust fans, and 1.0 sone or less for continuous fans.
  11. 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 and openable windows. If the bathroom fan is used as the primary means for meeting the code-required ventilation, calculate the ventilation rate required based on the size of the home, and ensure that the fan’s tested flow rate meets this requirement.
  12. Test the fan’s operation to determine the flow rate using a flow hood, flow grid, anemometer (in accordance with AABC, NEBB, or ASHRAE procedures), or other 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.
  • A humidity control may be a separate component of the exhaust fan and typically is not required to be integral.
  • Bathrooms, water closet compartments, and other similar rooms shall be provided with an aggregate glazing area in windows of not less than 3 square feet, one-half of which must be openable. Exception: The glazed areas shall not be required where artificial light and a mechanical ventilation system are provided.