Bathroom Exhaust Fans and Moisture Control in the Pacific Northwest

A shower puts a lot of water into the air of a small room in a few minutes. In a dry climate some of it disappears on its own. In a Pacific Northwest house, through a long damp season, it settles on the coldest surfaces it can find: the ceiling over the shower, the outside wall, the window frame, the back of the toilet. Mildew on a bathroom ceiling is almost always a ventilation problem, not a paint problem.

This page explains what the fan has to do, what Washington's code requires of it, and how to choose and control one so it actually gets used. For the product side, see choosing a bathroom exhaust fan.

Why Northwest bathrooms grow mold

The EPA's summary is short: "The key to mold control is moisture control." Mold needs moisture, and a bathroom supplies it twice a day. The EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent, and drying wet materials within 24 to 48 hours.

Older homes add their own problems: fans that were never ducted outside, fans that vent into the attic, and fans too small or too noisy to be used. If you are remodeling, this is the moment to fix all three, because the ceiling is open.

What Washington's code requires

Washington differs from the national model code in one important way. The model code lets an openable window of a certain size stand in for a fan. Washington did not adopt that section ("R303.3 Bathrooms. This section is not adopted."). Instead, "Bathrooms, toilet rooms, and kitchens shall include a local exhaust system" (WAC 51-51-1505, M1505.4.4.1). The headline rules for a new or replaced fan:

• Airflow: 50 cfm intermittent or 20 cfm continuous for each bathroom or toilet room.
• Controls: a manual switch, timer, occupancy sensor, humidity sensor or pollutant sensor; an on/off switch counts, and a manual control must be readily accessible in the room.
• Ducting: ducted directly to the outside with a backdraft damper; ducts in unconditioned space such as an attic insulated to at least R-4.
• Verified airflow: the installed fan is tested with a flow hood or similar device, unless the fan and duct meet the code's prescriptive duct table.

Those are the main points. Duct length limits by fan size, termination distances and the energy code's fan efficiency figures are on our bathroom ventilation code page.

How big a fan you need

The code sets a minimum, not a target. The Home Ventilating Institute (HVI), which certifies fan ratings, recommends about 1 cfm per square foot for bathrooms up to 100 square feet. A 7 by 9 foot bathroom is 63 square feet, so HVI's guideline points to a fan around 63 cfm, above the code's 50 cfm floor. For larger bathrooms, HVI sizes by fixture: 50 cfm each for a toilet, a shower and a bathtub, and 100 cfm for a jetted tub.

Two adjustments matter in practice:

• Rated airflow is not delivered airflow. A fan's rating assumes a short, smooth duct. Long runs, flexible duct, tight elbows and crushed sections cut real airflow. That is why Washington requires a test unless the duct falls within the prescriptive table, and why ENERGY STAR checks that a certified fan still delivers at least 70 percent of its airflow at a higher duct resistance.
• Location matters as much as size. HVI recommends exhaust points "over or near the shower or tub and in an enclosed water closet." A fan by the door clears the doorway, not the shower. A separate toilet room with a door usually needs its own fan.

Noise: why sones matter

A fan nobody runs does nothing. Sound is rated in sones. HVI describes one sone as about 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. The lowest rating HVI currently certifies is under 0.3 sone. ENERGY STAR's criteria for bathroom fans up to 200 cfm include a maximum of 2.0 sones.

Compare fans using their HVI-certified ratings, which come from independent labs and are listed in HVI's directory. A quiet fan is the single easiest way to make sure the fan actually runs through a whole shower.

Ducting to the outside, not the attic

Washington's code says it directly: "All exhaust ducts shall terminate outside the building." The EPA's advice is the same: exhaust bathroom moisture "to the outside (not into the attic)." Moist air dumped into an attic or crawl space condenses on cold framing and sheathing, where it can cause the very mold and rot the fan was meant to prevent.

A good duct run is short, as straight as the framing allows, sloped so condensation does not pool, insulated where it crosses unconditioned space, and finished at a wall or roof cap with a working damper. The termination has to stay away from operable windows, doors and other air intakes. When the ceiling is open in a remodel, rerouting a bad duct costs little compared with doing it later.

Timers, humidity controls and run time

How long should the fan run? HVI recommends 20 minutes after using the bathroom; the Department of Energy's Building America guidance says one hour, and recommends a timer switch over a standard wall switch. Either way, the fan should keep running after you leave, which a plain switch rarely achieves.

• Countdown timer: simple and reliable; you choose the run time.
• Humidity-sensing control: some fans have built-in humidity sensors that turn the fan on and off automatically, which helps in shared bathrooms where nobody remembers.
• Occupancy sensor: starts the fan when someone enters; useful in toilet rooms.
• Continuous low speed with a boost: one fan running all the time at a low rate and stepping up when needed.

The bath fan's second job: whole-house ventilation

Washington requires every dwelling to have a whole-house ventilation system that runs continuously at a minimum rate (WAC 51-51-1505, M1505.4). In many newer homes, one bathroom fan is that system, running quietly all day. If yours is, its rules are stricter: a fan designed to run at a higher intermittent rate needs an occupancy or humidity sensor to boost it automatically, and whole-house fans are rated at a maximum of 1.0 sone.

Before replacing a fan in a newer home, check whether it is labelled as the whole-house fan, often with a control marked "Leave on unless outdoor air quality is very poor." Swapping it for an ordinary on/off fan can quietly take away the house's fresh air supply. Whether remodeling a bathroom in an older home obliges you to add whole-house ventilation where none exists was not confirmed for this guide; ask the permit office if it matters to your plans.

Makeup air: where the replacement air comes from

Every cubic foot a fan pushes out has to be replaced from somewhere. In a tight, modern house with the bathroom door closed, a fan can struggle to move its rated air because nothing is coming in. A gap under the door or a transfer grille lets air reach the fan. Larger exhaust loads, such as a big primary suite with several fans or a kitchen range hood on at the same time, deserve a look at the whole house's air balance during design.

Planning ventilation into a remodel

A remodel is the cheapest time to get ventilation right. Decide fan location and size with the shower layout, plan the duct route before framing is closed, choose the control, and make sure the airflow is verified. JDI plans the fan, duct and control as part of the bathroom design, and the work is inspected under the permit. If moisture or mold is already a problem in your bathroom, read mold in bathrooms, then talk to us through our Vancouver bathroom remodeling page.