Of every decision in a bathroom remodel, exhaust fan sizing gets the least attention and does some of the most important work. In Southwest Washington's wet marine climate, outdoor humidity is elevated much of the year, which means the air a bathroom exhaust fan is fighting against is already working against it before the shower even starts. A fan sized for a dry inland climate — or simply matched to whatever was there before — is often not enough here.
We size every exhaust fan to the room, not to habit. That single decision affects how quickly mirrors clear, how fast paint and grout dry after a shower, and over years, whether moisture ever gets a foothold in the framing behind your walls.
CFM (cubic feet per minute) measures how much air a fan moves. The residential ventilation standards body sizes bathroom fans one of two ways, depending on the room:
Standard bathroom (under 100 sq ft)
Size the fan at roughly 1 CFM per square foot of floor area, with a 50 CFM minimum regardless of room size.
Larger or primary-suite bathroom (over 100 sq ft)
Calculate by fixture instead: 50 CFM for the toilet, 50 CFM for the shower, 50 CFM for a separate tub, and 100 CFM for a jetted or soaking tub — then sum the fixtures present.
Enclosed water closet or separate shower room
Give the enclosed space its own dedicated exhaust point rather than relying on the main bathroom fan to pull air through a closed door.
We calculate the specific target CFM for your bathroom's square footage and fixture count during design, then select a fan rated at or above that figure — an undersized fan installed to save a few dollars is one of the most common causes of moisture callbacks we see in older remodels.
CFM only matters if the fan actually gets used, and the single biggest reason homeowners stop using a bathroom fan is noise. Sone is the standard unit for perceived loudness — roughly speaking, each additional sone sounds about twice as loud as the one before it. Many builder-grade fans run loud enough that flipping the switch feels like an event, so people skip it.
We generally spec fans in the quieter range that run comfortably in the background, which matters most for the humidity-sensing or timer-delayed fans we recommend below — a quiet fan running for 20 extra minutes after a shower is far more effective than a loud fan switched off the moment someone steps out.
A correctly sized, quiet fan still fails at its job if the duct run doesn't terminate outside the home. We route every bath fan through insulated, properly sloped ducting to a dedicated exterior vent cap — never into an open attic space, a soffit that just recirculates air back toward the roofline, or an unconditioned wall cavity.
This is one of the most common issues we find when we open up an older Vancouver-area bathroom for a remodel: a fan that was doing exactly what it was told to do, dumping warm, humid air into the attic, where it condensed on cold roof sheathing during our long wet season.
- Mirrors and windows stay fogged well after the shower ends.
- The fan runs but you can't feel airflow at a tissue held near the grille.
- Paint on the ceiling near the shower is peeling, bubbling, or has a dull, weathered look.
- Musty smell that lingers regardless of how often you clean.
- The fan is noticeably loud, which is often why homeowners stopped running it in the first place.