Choosing a Bathroom Exhaust Fan: CFM, Sones and Smart Controls

The exhaust fan is the cheapest part of a bathroom remodel to get right and one of the most expensive to get wrong. A fan that is too weak, too loud or ducted badly leaves moisture in the room, and that moisture shows up later as peeling paint, black grout and mold. This page is about choosing the product: reading the rating label, comparing fan types and controls, and what to check before you buy. For why Northwest bathrooms need good ventilation, see bathroom exhaust fans and moisture control. For what Washington's codes require, see the bathroom ventilation code page.

Reading the rating label: CFM and sones

Two numbers describe a fan's performance, and the Home Ventilating Institute (HVI) defines both.

• CFM is "the number of cubic feet of air that a fan will move in one minute." It tells you how much air the fan can move, measured under test conditions.
• Sones measure loudness. HVI says one sone "is approximately equivalent to the sound of a quiet refrigerator in a quiet kitchen", and the scale is linear: a 3-sone fan is half as loud as a 6-sone fan. HVI's lowest certifiable rating for residential products is currently below 0.3 sone.

Sones matter more than people expect. A loud fan gets switched off as soon as the person leaves the room, and then it is not removing moisture. A quiet fan can run as long as it needs to.

HVI-certified fans are tested by independent HVI-designated labs, and their ratings are listed in HVI's Certified Products Directory. Washington's residential code requires bathroom fans to be HVI-rated, so the HVI listing is the label to look for.

Sizing the fan

HVI's sizing recommendation is simple. "For bathrooms up to 100 square feet in area, HVI recommends that an exhaust fan provide 1 CFM per square foot," which works out to about eight air changes an hour. For larger bathrooms, HVI sizes by fixture instead:

HVI also recommends placing exhaust points "over or near the shower or tub and in an enclosed water closet." In a primary bath with a separate toilet room and a large shower, that can mean two fans, or one inline fan with two grilles. HVI suggests a separate fan for a steam shower.

These are HVI's recommendations. Washington's code sets its own minimum (50 CFM intermittent or 20 CFM continuous for a bathroom, in Table M1505.4.4.1) and counts the airflow the fan actually delivers as installed, which depends on the duct. The details are on the ventilation code page.

ENERGY STAR criteria

ENERGY STAR sets efficiency and sound limits for ventilating fans. The current criteria for bathroom and utility room fans (Version 4.2) are:

• Multi-speed fans must meet the criteria at every speed.
• To show the fan holds up against real ductwork, its airflow at 0.25 inches of water column static pressure must be at least 70 percent of its airflow at 0.1 inches. In plain terms, a qualifying fan does not lose most of its airflow when the duct pushes back.
• Certified fans carry at least a one-year warranty.

Washington's appliance efficiency rules (WAC 194-24-151) also require residential ventilating fans manufactured on or after January 1, 2024 to meet ENERGY STAR criteria. An ENERGY STAR fan is effectively the baseline for a new fan here, not an upgrade.

Fan types compared

The electrical rules for lights and fans near tubs and showers are on our bathroom electrical code page.

Controls: timers, humidity sensors and continuous fans

A good fan on a bad control does little. The control that works is the one that keeps the fan running after the room is empty.

• Timer switch. Turns the fan off after a set time. The Department of Energy's Building America guidance recommends a timer rather than a standard wall switch. HVI recommends running the fan 20 minutes after using the bathroom; DOE suggests an hour after a shower.
• Humidity sensor. Some fans and wall controls have a built-in humidity sensor that turns the fan on and off automatically as moisture rises and falls. Neither ENERGY STAR nor HVI sets a standard sensor setting, so compare how each model lets you adjust it.
• Occupancy sensor. Starts the fan when someone enters. Useful in toilet rooms and powder rooms.
• Continuous low speed. Some fans run quietly all the time at a low rate and boost when needed. A bathroom fan can also serve as the house's whole-house ventilation fan, which brings extra code requirements; the ventilation code page explains them.

Washington's code accepts an on/off switch as a manual control, but requires the manual control to be readily accessible in the room the fan serves.

The duct decides real performance

A fan's CFM rating is measured in a lab. In the house, the duct pushes back, and a long, kinked or undersized duct can cut delivered airflow sharply. Washington's code counts the airflow as installed and gives a table of duct sizes and lengths for common fans.

• Duct straight to the outside, never into the attic. EPA advises using exhaust fans "to remove moisture to the outside (not into the attic)."
• Keep the run short, with as few elbows as possible. Smooth metal duct carries more air than flexible duct of the same size.
• Match the duct to the fan's outlet size; do not reduce it.
• Use a wall or roof cap with a backdraft damper, and insulate the duct where it passes through a cold attic so moisture does not condense inside it.

What to check before you buy

1. The fan is HVI-certified and ENERGY STAR qualified.
2. CFM suits the room: about 1 CFM per square foot up to 100 square feet, or HVI's per-fixture figures above that.
3. Sones: 2.0 or lower for a typical bathroom fan; lower is better if you want people to leave it running.
4. Duct outlet size matches the duct you can actually run, and the fan's performance at 0.25 inches of static pressure is listed.
5. Housing size fits between your joists, and the housing depth fits the ceiling cavity.
6. For a fan-light: the light is rated for its location and is switched separately from the fan.
7. The control: timer, humidity sensor, occupancy sensor or continuous low speed, and whether it is built in or a separate wall control.
8. For a large bath or a separate toilet room: whether one inline fan with two grilles serves better than two fans.

How JDI handles the fan

On every bathroom we remodel, the fan and its duct are part of the design, not an afterthought. We size the fan to the room, run the duct to the outside, and pick a control that keeps it running after a shower. The fan is a small line on a bathroom cost estimate, and it protects everything else in the room. To have your current fan and duct checked as part of a remodel, request a free in-home consultation.