When it comes to fighting excess humidity in the bathroom, an extractor fan is anything but an “extra,” or luxury item. By drawing moist air outside, a bathroom fan prevents problems related to condensation and mould, which can be common in today’s increasingly airtight homes.  

For optimum efficiency, however, the fan must be powerful enough, properly installed, and controlled such that a sufficient quantity of air is extracted.  

How to choose a fan  

To find the right fan, therefore, you need a minimum of information. To avoid unpleasant surprises, start by looking for a fan certified by the Home Ventilating Institute (HVI). This label means that the fan’s performance (noise, airflow, energy consumption, etc.) has been verified by an independent laboratory.  

Next, size the fan for your bathroom; i.e., determine the airflow required for proper efficiency:  

  • The Quebec Construction Code requires airflow of at least 25 litres per second (L/s), or about 50 cubic feet per minute (cfm). Note that this standard applies if your home is not also equipped with a heat recovery ventilator (HRV, or air exchanger).  
  • The HVI, meanwhile, recommends calculating 1 cfm for every square foot of space in a bathroom of up to 100 ft2 with an 8-ft ceiling. Following this rule, a fan rated at 80 cfm is properly sized for an 8-by-10-ft bathroom.  
  • For a bathroom larger than 100 ft2, the HVI suggests sizing the fan based on the type and number of fixtures: you should allow 50 cfm for each toilet, bath or shower, and 100 cfm for a whirlpool bath. So if your large bathroom has all four, you will need a fan rated at 250 cfm.  

Also consider the following aspects:  

  • Does your bathroom regularly “suffer through” four long showers one after the other? If so, you may want to buy a higher-capacity fan.  
  • Centrifugal fans (those with a “squirrel-cage” blower), while more expensive, are more efficient and generally quieter than blade-type fans.    

Quiet, please!  

Sound level is a factor not to be ignored: an excessively noisy fan can discourage you from turning it on—or you may find yourself shutting it off too early for it to be effective.  

Bathroom fan noise is measured in “sones.” The best models generate as little as 0.3 sones. If the specifications for a model you looking at say that it produces 2 sones or more, don’t buy that fan. Your ears will thank you!  

The keys to proper installation  

Proper fan installation includes the following steps:  

  • Mount the fan to the ceiling framing to prevent noise due to vibration.  
  • Carefully insulate the fan housing if it is in a cavity adjacent to an unheated space.  
  • Seal ductwork with metal-foil tape instead of regular duct tape, which, despite its name, tends to lose adhesion as it dries out.  
  • Ensure there is a large enough gap under the bathroom door to allow air to enter when the door is closed and the fan is running.  

The right duct  

Ideally, the fan should be connected to a duct made of smooth, rigid sheet metal, and that is as short and straight as possible. Ductwork length has a direct impact on fan efficiency. Note that a 90-degree bend and an outside damper are respectively equivalent to a 15-ft and 30-ft length of 4-inch rigid sheet-metal ductwork. For the best possible efficiency, consult the fan manufacturer’s recommendations on ducting.  

Using flexible ductwork will reduce noise, but will also reduce exhaust efficiency by half, and increase the risk of water building up due to condensation. Using a short length (about 2 ft) of flexible duct to connect the fan housing to the rigid ductwork, however, makes installation simpler—and makes it much easier to remove the fan if there is a problem.  

Note also that any ductwork that passes through an unheated space must be insulated to prevent condensation of the moist air being exhausted. Ductwork should also slope slightly downward to the exhaust outlet so that if water does form, it can’t drain back toward the fan.  

One important detail: the exhaust duct must connect to an outlet with a damper mounted on an exterior wall or, if this is impractical, on the roof. The damper is there to prevent backdrafting, and to keep birds and rodents out. The outlet should not be near any windows, roof soffits or the HRV intake, so as to prevent vented moist air from being drawn back inside the house.  

Working conditions  

A fan is doing its job when all excess humidity is removed from the bathroom. Here is some additional advice to ensure that it is:  

  • For maximum efficiency, the HVI recommends that a fan be kept on for about 20 minutes after use of a shower or bath.  
  • Ideally, the fan switch should be distinct from the bathroom light switch; otherwise, the fan will be turned off when the light is turned off—something we tend to do when exiting the room.  

Various kinds of controls can also be used to ensure more efficient exhaust—for example, a timer (to set the duration of operation) or variable-speed control switch. The fan can also be controlled using a humidistat, which will start and stop the unit automatically depending on a pre-set humidity level.  

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