Bathroom Ventilation Fans

Bathroom Ventilation Ceiling FanNot all bathrooms need a ventilation fan; most building codes specify that if a window in the bathroom can be opened and has an opening area equal to at least 10 percent of the room’s floor area, a ventilation fan is not required.

While opening a window to bring in fresh air, as opposed to operating an electrical fan, is arguably more energy efficient, (though not necessarily in cold climates), there are other considerations. If the home has small children, safety plays a role, and for the elderly and handicapped, accessibility and ease of use come into play.

Unfortunately, all too many bathrooms are equipped with a noisy fan that never gets used. If the bathroom happens to be drafty, which is also all too common, things are even worse. Without proper ventilation, the excess humidity in a bathroom can cause wallpaper to peel, encourage mold and mildew to grow, wood to rot, and metals to rust. There are several ways to address the ventilation problem, and when you are planning on renovating your bathroom you may wish to consider them.


The first and best option is to hook up a humidistat to the fan. A humidstat is like a thermostat for humidity; it automatically turns on the fan when it senses the humidity in the bathroom reaching a pre-set level, and turns off when it falls below that level. If you are installing a new fan, there are fans available which have built-in humdstats.

Another approach is to install a timer on the fan electrical circuit, so that the fan is turned off. This doesn’t work as well, obviously, since there is still no way to get people to turn the fan on in the first place.


You could also wire the fan and the bathroom lighting to the same switch. This is also a less than ideal solution; it wastes energy when the bathroom is used for anything other than showering and bathing. Heated air is pumped out of the house during the fan’s operation and the fan itself consumes electricity. True, most modern fans use relatively low amounts of electrical current, but waste is waste.

Fan Noise

Fan specifications include noise ratings and cubic feet per minute volume sizes. Noise is rated by a measurement called sones. One sone is equal in loudness to a tone of 1000 cycles per second at 40 decibels, which turns out to be about the volume of a quiet refrigerator in a quiet kitchen, according to the Home Ventilating Institute. Sone measurements of 2 are twice the volume of a sone rating of one, and 4 sones is 4 times as loud as 1 sone. (Sone measurements are linear, unlike decibels measurements, which are logarithmic.)

A fan rated at 3 sones or more will be too loud for the home, these are usually intended for commercial applications. Low sone fans are now available that go as quiet as 0.3 sones, these are ideal for making bathroom ventilation less annoying.

Another option is a remotely mounted ventilator fan, located in an isolated but accessible place, like the basement. These are available with multiple ports so they handle more than one room’s ventilation, and are designed specifically for longer ducting runs.

Fan Sizing

The ideal air change rate for most residential bathrooms is 8 changes per hour. Over the course of one hour, any natural or mechanical ventilation system should have the capability to completely empty and resupply the room with fresh air 8 times. This will make certain that humidity and odors are sufficiently eliminated. You will need to calculate the fan size in cubic feet per minute (CFM).

To size your fan in CFM for the room, take the floor area in square feet and multiply by 1.07 (this is a height factor for an 8 foot ceiling). Then round the result up to the nearest multiple of 5. Example:

    Floor Size: 6 feet by 9/12 feet = 57
    Height Factor: 57 x 1.07 60.99
    Fan Size Required: 65 CFM

Note that a larger capacity fan may be needed if your ventilation system has many ducts with corner bends. Flexible plastic ducting is easy to run around corners and obstructions, but the corrugated surface creates more air resistance that normal ductwork, and it is prone to leaks and deterioration. Stick with old school galvanized aluminum duct material.

Types of Fans

You will find a number of different types of ventilators are available, from combination lighting/heating/ventilation units to stand-alone low noise, high energy efficiency units. If your house is newer and built tightly, it will be important to consider heat loss. Older houses were built looser, with lots of ways for air to come into and exit the house, when heating and cooling bills were cheaper.

To keep as much cold air outside you house and heated air inside (or in the summer, cooled air inside and hot air outside), heat-recovery ventilators are used. These devices are designed to transfer heat that is normally lost from the stale outgoing air to the fresh incoming air. The two air streams pass through an exchanger core operating at up to 75% efficiency. Heat recovery ventilators are well suited for bathroom applications, especially in newer energy-efficient homes, and are available in a good range of sizes and capacities.

Photo by Ken Dyck, Creative Commons Attribution License