The short answer
Every bathroom needs an exhaust fan that moves enough air and vents it outdoors. Washington's ventilation code requires source-specific exhaust in each bathroom, with a fan rated at least 50 cubic feet per minute (cfm) for intermittent operation, or 20 cfm for continuous operation, controlled by a manual switch, timer or dehumidistat, and ducted to the outside with a backdraft damper. In a damp, cool region with closed windows for much of the year, a good fan is one of the cheapest ways to protect a remodeled bathroom.
Why it matters in the Pacific Northwest
Kirkland averages about 36 inches of precipitation a year, with November the wettest month at about 5.8 inches. For much of the fall and winter, windows stay shut and heat is on, and opening a window to let out shower steam isn't practical or comfortable. Without effective exhaust, moisture condenses on walls and ceilings, which can lead to peeling paint, mildew and damage to trim and drywall. A fan that works is what keeps the room dry between showers.
What the code asks for
- A fan in every bathroom. Source-specific exhaust is required in bathrooms, not only in kitchens.
- Airflow: at least 50 cfm for fans operated on demand, or 20 cfm for continuous operation.
- Controls: manual switches, timers, dehumidistats or other approved means, and the control has to be readily accessible.
- Ducting: exhaust must terminate outside the building with a backdraft damper and a screened outlet. Ducts passing through unconditioned spaces such as attics must be insulated.
Building officials may require airflow testing at their discretion, and each city may have local requirements, so ask your contractor to confirm what applies.
Choosing and sizing a fan
Rated cfm versus real airflow
A fan's rating is measured in ideal conditions. Long duct runs, tight bends and a restrictive outside cap all reduce the air that actually moves. Guidance from the U.S. Department of Energy's building-science programs suggests choosing a higher-rated intermittent fan, around 70 cfm or more for many bathrooms, to make up for those losses, especially for larger rooms.
Noise
Loud fans don't get used. Look for a low sone rating, which measures loudness, so people actually turn the fan on.
Controls that help
A timer switch lets the fan run after you leave the room, which is when much of the moisture is still in the air. A humidity-sensing control turns it on automatically. Many guides suggest running the fan for a while after a shower rather than switching it off when you step out.
Installation details that decide whether it works
- Vent to the outdoors, never into the attic, crawl space, garage or a wall cavity.
- Use smooth, rigid duct where possible, short and straight, with joints sealed.
- Insulate ducts in unconditioned spaces to prevent condensation inside the duct.
- Keep the outside cap clear and away from air intakes, and don't aim it onto a walkway.
- Clean the fan periodically. Dust on the blades and grille cuts airflow.
When to upgrade the fan
A remodel is the right time. Signs the existing fan isn't doing its job include a mirror that stays fogged long after a shower, peeling paint on the ceiling, musty odors and a fan that vents into the attic. Because new ducting often means opening the ceiling or roof, it's far easier while the room is torn up. Adding or changing fan wiring is electrical work, which may need a permit in Kirkland; see bathroom remodel permits in Kirkland.
Ventilation is one of several moisture protections that work together with waterproofing, covered in how to waterproof a shower, and a floor that tolerates moisture, covered on our bathroom tile and flooring page. For a view of the whole process, see bathroom remodeling in Kirkland.