Selecting Bathroom Ventilation

Selecting the proper ventilation for a bathroom requires four considerations: size, sound, operation and location. But first, it’s important to understand codes. For the past few years the International Residential Code has allowed a window to replace a mechanical ventilation unit if the glazing is at least 3 square feet and one-half of the window opening. The problem is how can we be sure the person taking a shower or using a toilet will open the window?

Inclement weather or security issues can prevent this from happening. What if the window is left open for long periods of time? Will children know to open the window or is it out of their reach? The reasonable solution is to always install mechanical ventilation in the bath space. The IRC requires a ventilation unit rated at a minimum 50 cubic feet per minute (cfm) and ducted to the outside of the home.


To meet the IRC requirement, the minimum-size exhaust system is 50 cfm. What about larger bath spaces? The Wauconda, Ill.-based Home Ventilating Institute recommends eight air changes per hour. The following formula will determine the cfm for a properly sized exhaust system for a space: W x L x H x 8ACH = cfh. Then divide the cfh figure by 60 for the required cfm.

HVI has a shortcut if the ceiling is 8 feet: Plan 1 cfm per square foot of bath space. Therefore, the requirement for a 9- by 8-foot bath (72 square feet) would be 72 cfm. However, as bathrooms increase in size, clients may request an enclosed toilet compartment. In this case, the IRC will require two ventilation units: one for the enclosed toilet space and another for the remaining bath space.

It is important to stay within the recommended cfm range for projects. Oversized exhaust units add energy costs. To eliminate odors and moisture from a home, a fixed amount of air must be exhausted. The air removed beyond the required amount has to be heated or cooled. Another factor is the make-up air required to replace the air discharging outdoors. As air is removed from a toilet compartment, additional air must come into the space. If the bottom of the door to the toilet area or entrance to the bathroom is too close to the floor, air cannot enter the space at the same rate it is leaving. Without the incoming air, the fan can’t do its job.


Once proper cfm is determined, a remodeler must consider the sound level of the ventilation unit. Bathroom fans are rated in sones. One sone is similar to the sound of an operating refrigerator. If the fan is too loud, the homeowner might not turn it on. Exhaust systems that operate at less than one sone are available. Remotely located fans are a quiet option and could vent two bath spaces with a single fan located in the attic.


The exhaust fan should be controlled by a timer switch or humidistat. The advantage of a timer switch in an enclosed toilet compartment needs no further explanation. A timed switch near the shower is less obvious. Most people will turn off a bath exhaust fan as soon as the steam leaves the bath space. Unfortunately, moisture could remain in the ductwork for hours. It is recommended a bath fan remain on for 20 minutes after the bather finishes showering to remove this moisture.

A humidity-activated switch is an option. The humidity sensor is located in the wall switches or built into the fan. Some humidity-activated switches can be set to operate after the humidity level drops to remove moisture from the ductwork. Regardless of the type of switch chosen, the IRC requires that it not be located near the bathtub or shower. If combining the bath fan with a light or heater, be sure each item is switched separately. There are few occasions when a fan and light or fan and heater need to be on at the same time.


Bathroom ventilation is needed for two reasons: odors and moisture. Therefore, the best location for the exhaust points will be near the toilet and above the bathtub or shower. Typically, the fan housing and intake is located in the ceiling. This works great for the tub or shower but not as efficiently for the toilet. Consider locating the intake near the toilet in a side wall. In a powder room or enclosed toilet compartment, the exhaust fan could be mounted in a wall adjacent to the toilet and almost hidden from view while being more efficient.

Before installing an exhaust fan in a vertical installation, make sure it is designed to operate efficiently in that position. It is possible for sone levels to increase if a fan is installed vertically. Another consideration would be the thickness of the stud wall. Some of the more efficient bath exhaust fans are using 5- to 6-inch duct. If the bath space is against an outside wall, you can vent directly through the wall with a wall-mount unit. Finally, a remote exhaust system that can serve a toilet compartment and the remaining bath space might be the best option.

Three bath exhaust fans might be required in larger baths where the shower and bathtub are in separate areas and the toilet is in a compartment. HVI recommends cfm ratings for each individual fixture in this situation as long as the cfm total equals the requirement for the entire space. See the chart on this page for cfm recommendations for individual fixtures.

CFM recommendations for individual fixtures

Toilet 50 CFM
Shower 50 CFM
Bathtub 50 CFM
Jetted Tub 100 CFM

Chart courtesy of Home Ventilating Institute,