How to Plan Your Bathroom Ventilation

How to Plan Your Bathroom Ventilation

Bathroom ventilation is essential for a well-maintained home.

The only proper source of water in the bathroom is the water supply lines. Any other source is a problem that must be eliminated. Chief among these is condensation.

Condensation is a continual problem wherever there are wide variances in temperatures present, such as the hot water of a shower or whirlpool, or the heat from a steam or sauna unit. Vaporized water will condense on any cold surface and can potentially cause damage.

To prevent this, the super-saturated air must be removed from the room. Building codes dictate that this air should be vented to the outside. The ventilation fan serves this purpose.

Bathroom 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). Most 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:

Step 1:Calculate the cubic feet of your bathroom
(length x width x height)
Step 2:Divide by 60
(the number of minutes in an hour)
Step 3:Multiply the result by 8
(the number of recommended air changes per hour)

For bathrooms above 100 square feet in area, HVI recommends a ventilation rate based on the number and type of fixtures present, according to the following table:

Bath Tub
50 CFM
50 CFM
50 CFM
100 CFM

This table is cumulative, so that a room containing all of these fixtures would require a fan rated at 250 CFM, but usually multiple fans are installed to achieve the desired results.


  • Steam showers
    it is best to have a separate fan in the steam room that can be turned on after use.
  • Tub/Shower
    Typically the exhaust points are located over or near the shower or tub.
  • Enclosed toilet rooms
    must have an operable window or a fan for ventilation. With windows closed, exhausted air will be replaced by makeup air from adjacent rooms or forced air system registers.
  • 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.


Most fans are rated between 50 CFM (cubic feet per minute) and 250 CFM. These fans are 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 generally 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 must be installed in the wall, the duct needs to be pointed up to allow the damper to operate. However, the bearings may not last as long due to insufficient oil flow within the sleeve bearings.

Fans installed above kitchen ranges must be listed for that application by UL and must be designed to handle the grease in the exhaust air and the high temperatures of a potential grease fire. Most bathroom ventilation fans are not suitable for this purpose.

HVI recommends that the fan be left on for 20 minutes after use of the bathroom. 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

The Home Ventilating Institute (HVI) publishes a directory of certified products. (Call HVI in Chicago at 1-847-526-2010 to request a copy).

Virtually all fans lose flow as the static pressure (resistance) increases.

This is measured in inches of water gauge, expressed as 0.1″ w.g. or 0.25″ w.g. (equal to a column of water one-tenth or one-quarter of an inch tall).

Most bath fans operate at between 0.1″ and 0.25″ w.g. The Home Ventilating Institute rates almost all bath fans at 0.1″ w.g.

Undersized ducts or tight roof caps can increase this static pressure to 0.5″ or 0.7″ w.g. and there will be virtually no flow.

Most fans are designed for use with a four-inch duct.

Avoid using three-inch duct; this creates very high static pressure, reduces airflow dramatically, and over the long term, reduces motor life in the fan. The ability of a fan to overcome static pressure is shown in the fan curve for a particular fan. In the example below, note how the airflow drops off as the static pressure increases.

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.


The loudness of a ventilation fan is measured in Sones. The sone is an internationally recognized measurement of sound output.

Sones translate decibel readings into numbers that correspond to the way people sense loudness.

Sones follow a linear scale, like inches. Double the sones is double the loudness.

In contrast, decibels follow a logarithmic scale which is a multiplying of numbers instead of adding.

Sones readings offer easy, quick and accurate comparisons for laymen and engineers. In technical terms, the sone is equal in loudness to a pure 1,000 cycles per second at tone at 40 decibels above the listener’s threshold of hearing. One sone is equivalent to the sound of a quiet refrigerator in a quiet kitchen (source HVI). The loudness of most fans ranges between .3 Sones and 2.5 Sones, with the average around 1.0.


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.

Some models can be installed in the ceilings of showers where they are protected by GFCI 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.