Hugh Cairns: Choosing a bathroom fan


Regardless of your bathroom fan location, the air that it moves should be discharged directly outside. Never vent a fan directly into an attic or within the building envelope. (Photo: Hugh Cairns)

Choosing a bathroom fan

By Hugh Cairns

Bathroom exhaust fans are an important part of a home’s ventilation system. They eliminate odors, improve indoor air quality, and remove moisture and humidity that can lead to structural damage or mildew and mold growth. Unless a bathroom is properly ventilated, the moisture from a shower has no place to go and can penetrate into drywall, attic insulation and structural components.



Since the purpose of an exhaust fan is to expel air from a room, the selection of a bathroom fan begins with the ability of the fan to remove the total volume of air in the room where it is installed.

To determine the fan you need, first you’ll have to measure the square footage of the room to be exhausted. The square footage of the room has a direct correlation to the amount of air to be exhausted every minute. All bathroom exhaust fans have an airflow rating measured in cubic feet per minute (CFM).  To find the CFM rating you need, measure the room to calculate its square footage.

Making up the air that leaves the room when the fan is in operation is very important. Make up air is the air needed to replace air being exhausted by the fan. If your bathroom has a window you can go ahead and open it. If you don’t have a window then the makeup air is drawn from adjacent rooms or forced air system registers. Try and place your fan away from the supply of makeup air so that the fan can pull the air through the room. Bathroom doors need to be undercut about a half inch to allow makeup air to enter the room. Inadequate make up air will diminish performance of the system.

The bottom line is choose a good fan. Fans with a low noise level and good performance will ultimately payoff against a cheap noisy fan that you’ll probably never want to turn on.

My bathroom fan is noisy and old. Can I upgrade to a newer, quieter fan?

You bet. In most cases you will install an entirely new unit. In some cases fan motors are available for older housings but, unfortunately, most bathroom fans models have unique housings so the chances are that you will need to replace the housing, and in some cases, the associated ductwork.

How long should you run your exhaust fan?

You should run the fan continuously when the bathroom is in use. In addition it is recommended that you continue the operation of the fan for an additional 15 to 20 minutes after use of the bathroom. A bathroom fan timer is a good solution.

If you are lucky enough to have a fan with a humidistat control, the fan will do all the work for you. Alternatively you can install an aftermarket humidistat to control your bathroom fan.

Bathroom exhaust fans and home inspections

The reality is most homeowners don’t evaluate their ventilation systems let alone their bathroom fans. During a home inspection your CAHPI BC (Canadian Association of Home & Property Inspectors BC) home inspector will inspect and report on your mechanical ventilation systems. Specifically, your inspector will inspect for ventilation systems in areas where moisture is generated such as kitchen, bathrooms, laundry rooms and report on them. Home inspections don’t determine indoor air quality or your ventilation systems adequacy or distribution balance.


From experience I can tell you that the biggest problem with bathroom exhaust fans that I see on the job is where they terminate in attic spaces. When a bathroom fan pulls warm moist air from the bathroom, and distributes the air to the attic. It can condense on anything below the dew point. This can be the nails, the sheathing, the rafters and more.

What makes matters worse is that a lot of home owners never venture into their attic. There could be a problem up there festering for months or years until the home inspector finds an unfortunate condition.