Bathroom Ventilation


A bathroom with hot water of a shower or whirlpool, or the heat from a steam or sauna unit will definitely have condensation, which is a continual problem wherever there are wide variances in temperatures present. Vapourized water will condense on any cold surface and can potentially cause damage, from rust to your fixtures to flooring or tiling work. 

This condensation should be removed from the room in order to prevent this. A ventilation fan serves this purpose to removed condensation to the outside, which is stipulated in building codes. Ventilation fans are available in a wide variety of sizes and with a number of options. 


The volume of the room that is to be ventilated determines the size of the fan needed. The Home Ventilating Institute (HVI), a trade association representing the manufacturers of 95% of the residential fans in North America recommends that a bathroom exhaust fan be able to deliver eight air changes per hour (ACH). Building codes only require a minimum airflow of 50 cfm from a bathroom, which has a capacity of providing eight ACH for a room 8 ft. x 6 ft. with an 8 ft. ceiling. 

To calculate the cfm rating of the fan you should select, follow the following steps: 

(length x width x height (bathroom cubic feet) / 60*) x 8** 

where * is number of minutes in an hour

** number of recommended air changes per hour

For example, HVI recommends that bathrooms above 100 square feet in areas should have a ventilation rate based on the number and type of fixtures below:

Toilet 50 CFM

Shower 50 CFM

Bath Tub 50 CFM

Whirlpool 100 CFM


A room containing all of these fixtures would require a fan rated at 250 CFM (total of all ventilation rate of all fixtures), but usually multiple fans are installed to achieve the desired results.



  • Steam showers should have a separate fan in the steam room that can be turned on after use. 
  • Tub/Shower – Locate the exhaust points over or near the shower or tub. 
  • Enclosed toilet rooms must have an operable window or a fan for ventilation. With windows closed
  • HVI recommends that the exhaust points be located away from the supply, thereby pulling the supply air through the room. Bathroom doors need to be undercut to allow makeup air to enter the room. 



Fans that are rated between 50 CFM (cubic feet per minute) and 250 CFM are normally mounted in the ceiling. Larger fans can be installed in the wall or ceiling. These fans use ball bearings motors and are not as prone to lubrication concerns when placed in the wall. Smaller ceiling fans up to 110 cfm should not be used in the wall due to concerns about orientation of the motor for bearing lubrication and the built-in damper on the discharge side of the fan. 

If these fans are to be installed in the wall, the duct needs to be pointed up to allow the damper to operate. 

The fan should be left on for 20 minutes after use of the bathroom. Using a timer is a good solution, allowing the fan to turn off automatically at the proper time. Alternatively, ventilation may also be provided on a continuous basis at other rates. This may complement the use of fans to provide the HVI recommended rates 

Most fans are designed for use with a four-inch duct. Avoid using three-inch duct as this creates very high static pressure, reduces airflow dramatically. Over the long term, this will reduce motor life in the fan. The selection of the ducting for a particular installation can drastically affect the performance of a fan. As the duct run gets longer, the static pressure increases and the flow decreases. This limits the size of room that a particular fan can reasonably ventilate.


Optional features on some models are lights, often wired separately for independent operation, motion sensors to turn on when a person enters the room, night-lights, timers, and adjustable humidity sensing circuitry. 

Many are shielded or operate at lower temperatures so they can be installed in insulated ceilings. Most have built in dampers to prevent back draft. They should be equipped with thermal cutoff fuses.