According to the
InterNACHI 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.
Operable bathroom windows are a convenient feature, but they should not be relied on for consistently adequate bathroom ventilation.
In meeting modern exhaust air flow 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.
Sealed
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.
Duct
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.
Bends
If bends are necessary, gradual bends are preferred to 90-degree elbows for optimum flow and less air flow 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 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. 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 exhaust duct should be gradual rather than 90 degree turns if possible. Smooth rigid duct is preferred over ridged flex duct.
Condensation
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 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.
The photo above shows a bathroom fan exhausting into the insulation at the attic floor. Indications of water marks on the drywall can be seen.
Illustration courtesy of Pacific Northwest National Laboratory
The bathroom exhaust fan can vent out through the wall or up through the roof.
Insulated
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)
- 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, shower, or similar source of moisture.
- A humidity control may be a separate component to the exhaust fan and typically is not required to be integral.
- Bathrooms, water closet compartments and other similar rooms shall be provided with 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.
