The right and Wrong of Bathroom Exhaust Fan Installation

Something so simple is often done incorrectly. I am talking about bathroom exhaust fans. Bathroom exhaust fans must be ducted to the exterior of the home. They can't be ducted to attics, garages, other enclosed spaces or into other venting systems installed in the attic. These other venting systems include ridge vents, soffit vents and other mechanical vents.   

Windows in bathrooms are nice but truthfully few people are going to open them to allow for proper venting of the bathroom. It is either going to be too hot and the air conditioning will be on or its going to be too cold and the heating will be on. The windows stay closed in most bathrooms.  

A bathroom exhaust fan must be powerful enough to remove moisture from the bathroom and expel it to the exterior of the home. If you have a larger bathroom with a tub and shower the recommendation is to install an 80 CFM fan. For a master bathroom a 100CFM fan is recommended.  CFM refers to cubic feet per minute. The higher the CFM the larger volume of air the fan can move. At a minimum the bathroom fan should be able to change the air in the bathroom at least eight times per hour. 

When ducting a bathroom exhaust fan flexible duct work should be minimized and better yet not used at all. A rigid smooth surface duct is recommended. Duct should be the same size as the vent fan port or opening. Using too large a vent diameter will cause air leakage and moisture loss. Using too small a diameter duct will constrict air flow and cause the fan to work harder and will not move the volume of air necessary. The two most common duct sizes are 4 inches and 6 inches. 

When installing a bathroom exhaust fan, the duct should be as straight as possible. This will allow the fan to operate at its highest efficiency. Elbows and bends should be minimized. Having an elbow too close to the fan is a common installation error. An elbow installed too close to the fan can allow condensation and air to blow back causing damage to the ceiling in the bathroom. 

Bends should be kept to a minimum and they should be sweeping and not tight. Tight bends will lower the efficiency and output of the exhaust fan. Make sure the ducting is not crimped or crushed. 

Bathroom vent fans should vent to the exterior and not into closed areas. Often as NJ home inspectors we will see bathroom vent fans that improperly exhaust into the attic. This is a poor installation because moisture will collect in the attic and over time can cause problems such as moister damage and mold growth. Hot moist air from the improperly installed fan duct will come into contact with the cold attic roof deck and framing causing condensation to form. With condensation typically comes mold growth which no one wants in their home. 

The bathroom exhaust fan should not be discharging into a soffit or a gable vent.  These are intake vents and not designed for exhausting air. The moisture laden air from the exhaust vent can be sucked back into the gable or soffit vent causing moisture damage or mold growth to develop over time.  The bathroom exhaust outlet should be about ten feet away from an attic vent or other air inlet.  The vent fan termination should have a louver style cap that closes when the fan is not in operation. this will keep pests out. 

The bathroom exhaust fan duct work should be insulated when the vent ducting enters an unconditioned space. If the ducting is not insulated condensation can collect both on the inside and the exterior of the duct. Uninsulated duct work can cause condensation to roll back and cause staining or damage to the ceiling in the bathroom.  

As Home inspectors in NJ we owe it to our clients to make sure the bathroom exhaust fans are properly installed and vented to the exterior in the homes that we are inspecting. 

John Martino

LookSmart Home Inspections

NJ Home Inspectors Lic # 24GI00058700

E. Hanover NJ 07936