By Vasile Teterea, Owner and CEO, Firm Remodeling LLC | WA Contractor License #FIRMRRL783PU | Published August 6, 2026 | Based at 26424 233rd Ave SE, Maple Valley, WA 98038
The fan on your ceiling is probably not moving the air the box said it would. That is the whole article in one sentence, but it is worth explaining, because the gap between the number printed on the carton and the number actually leaving your roof is where most of the mildew in this town comes from.
Here is the short version. Washington rates bathroom fans at a harder condition than the industry advertises. Most of the CFM figures you see on a shelf are measured at 0.1 inches of water gauge, which is close to a fan blowing into open air. Our state code requires a minimum rating of 50 CFM at 0.25 inches water gauge, tested to the HVI standard. Real attic ductwork in a Maple Valley house often sits at or above that 0.25 number before you have even accounted for the roof cap. So a fan sold as 110 CFM can end up moving something in the range of a 50 or 60 CFM fan once it is installed on four inch flex with two elbows and a twenty foot run.
Then people run it for the eight minutes they are in the shower and shut it off.
Why this matters more here than almost anywhere else
I have had this argument with homeowners who moved here from Arizona or Texas, and I understand the confusion. It never gets that hot, it never gets that cold, so how bad can the humidity problem be.
Two reasons it is worse.
Houses built here since the late 2000s are tight. The energy code kept ratcheting down on air leakage, and it worked. The side effect is that the accidental ventilation that used to dry out a leaky 1978 rambler does not exist in a 2011 house in Cedar Downs. Whatever moisture you put in the air stays in the air until a fan removes it.
The second reason is the one nobody thinks about. In January, ventilation works beautifully. Cold outside air holds almost no moisture, so once it warms up inside your house it is bone dry and it soaks up everything. The problem months are October and April. Fifty five degrees and ninety percent humidity outside. You run the fan, you pull in air that is already nearly saturated and only slightly cooler than the room, and it barely dries anything. Those shoulder seasons are when I see mildew take hold, and they are also when nobody thinks about their bathroom fan because it is not cold and it is not hot.
If your mirror clears in three minutes in December and still looks foggy fifteen minutes later in October, that is not your imagination. That is physics, and it means your fan is undersized for the only weather that actually tests it.
How to size it correctly
The Home Ventilating Institute has two methods, and the honest answer is you run both and take the bigger number.
For a bathroom of 100 square feet or less, use 1 CFM per square foot, with 50 CFM as the floor. A 7 by 9 bathroom is 63 square feet, so 63 CFM, which in practice means a 70 or 80 CFM fan.
For anything over 100 square feet, forget the floor area and count fixtures. Fifty CFM each for a toilet, a shower, and a standard tub. One hundred for a jetted or whirlpool tub. A primary bath with a separate shower, a soaking tub, and an enclosed water closet lands at 150 CFM. Swap that tub for a jetted one and you are at 200.
Then add margin. I add roughly twenty percent on top of whatever the math says, because the math describes a fan in a laboratory and your fan lives in an attic. If the calculation says 100, I am buying 120.
One more thing on large bathrooms. When the room is over about 150 square feet or the water closet is behind its own door, a single big fan in the middle of the ceiling is the wrong answer. Air follows the shortest path from the door undercut to the fan grille, and the enclosed toilet room is not on that path. Two smaller fans, or one over the shower and one in the water closet, will outperform a single monster every time. An enclosed toilet room needs its own fan or an operable window regardless.
Read the 0.25 number, not the 0.1 number
If you take one practical thing away from this, make it this one.
When you look at a fan’s spec sheet, find the airflow rating at 0.25 inches water gauge. That is the number Washington cares about and it is the number closer to reality. Manufacturers publish both, but marketing leans on the friendlier figure.
The fan also has to be HVI certified with the label present, because that is what an inspector will look for. Self reported airflow numbers from a manufacturer that has not gone through HVI 916 testing do not count here.
While you are reading the sheet, look at the sone rating. Sones are a loudness measure, and this is not a comfort nicety. A fan at 3 sones is a fan your family switches off, and a fan that gets switched off is a fan that does nothing. Under 1.0 sone is quiet enough that people forget it is running, which is exactly what you want. For a fan doing double duty as your whole house ventilation, Washington caps it at 1.0 sone anyway.
The duct is the problem, not the fan
I would guess three quarters of the underperforming fans I open up have a fine fan and terrible ductwork. Common failures, roughly in the order I find them:
Four inch duct on a fan that needs six. If the housing has a six inch collar, use six inch duct. Someone reducing to four because that is what was already in the attic has just thrown away a third of your airflow.
Flex duct, run long, sagging between joists, with the interior liner not pulled taut. Corrugated flex has several times the friction of smooth rigid pipe. Every sag is a small elbow. I use rigid metal wherever I can get it, and short, tight, supported flex only where I have to.
Elbows stacked near the fan housing. Every 90 adds the equivalent of several feet of straight run. Give the fan a straight shot for a foot or two before you turn.
Uninsulated duct crossing a cold attic. Warm wet air hits a cold metal pipe, condenses inside it, and runs back down into the housing. If you have ever seen brown staining around a bath fan grille, that is usually this, not a roof leak. Insulated duct, sloped slightly toward the exterior termination, solves it.
Termination into the attic, into a soffit vent, or into the crawlspace. This is not a gray area. Code prohibits discharging bathroom exhaust into an attic or crawlspace, and soffit termination just feeds the moist air straight back through the roof vents above it. It goes out through a roof cap or a wall cap with a functioning backdraft damper, which the code also requires.
And the small one everybody misses: undercut the bathroom door about three quarters of an inch. A fan cannot exhaust air it has no way to replace. A tightly weatherstripped bathroom door turns a 110 CFM fan into a vacuum pump fighting itself.
Controls, or how to stop relying on a person to remember
HVI recommends running the fan at least twenty minutes after a shower. Nobody does this voluntarily.
So take the human out of it. A timer switch is about twenty five dollars and swaps into the existing box in ten minutes, and it is the highest return upgrade in the whole bathroom. Set it for thirty minutes and stop thinking about it. Humidity sensing controls are better still, and Washington’s code has moved in this direction as well: bath fans designed to run at an intermittent rate higher than their continuous rate have to be equipped with occupancy or humidity sensors to kick them to high speed automatically.
The reason I push these on every project is that fan capacity and fan runtime multiply. A 110 CFM fan run for eight minutes removes less moisture than an 80 CFM fan run for thirty. If you only fix one thing in an existing bathroom and you are not opening the ceiling, fix the control.
Also, the electrical side of a fan swap is not city work. In Maple Valley the electrical permit comes from Washington L&I, not from the city, which surprises people. I covered how that split works in the Maple Valley remodel permits post.
How to tell if yours is undersized without any tools
Run through this in your own house tonight.
Does the mirror stay fogged more than about ten minutes after the shower ends? Is there mildew in the grout along the top third of the shower wall, or at the ceiling corner nearest the shower, rather than down low where splashing explains it? Is the paint on the ceiling peeling, blistering, or showing a faint fuzzy discoloration? Do the bathroom window and its sill get wet in the morning? Does the room smell faintly musty an hour after anyone has used it? Is there rust or corrosion on the door hinges, the light fixture screws, or the towel bar brackets?
There is also the tissue test, which is crude but tells you something. Hold a single square of toilet paper against the fan grille with the fan running. It should hold there on its own. If it drifts down, you are moving far less air than the label promised.
Two or more yeses and I would not bother investigating further, I would just plan on replacing the fan and the duct together during your next project. Replacing the fan alone, on the same bad ductwork, is money spent to solve about a third of the problem.
What this actually does to a bathroom over time
The reason I care enough to write two thousand words about a ceiling fan is that I am the person who eventually opens the wall.
Chronic under ventilation does not announce itself. It shows up as mildew you keep scrubbing, then as grout that keeps failing along the top of the shower, then as paint that will not hold, and eventually as soft framing or subfloor around a shower that never actually leaked in the conventional sense. Moisture that lives in the air condenses on the coldest surface it can find, and in an exterior wall bathroom that surface is often inside the wall cavity. A shower can be waterproofed correctly and the room can still rot around it if the air never dries out.
The fix during a remodel costs very little. When the ceiling is already open, upgrading the fan, running proper rigid duct, and adding a humidity control is a small line item on a bathroom project. Doing it later, as its own job, costs several times that because of access alone.
A note on the code, because it is changing right now
Washington has been on the 2021 code editions since March 15, 2024, and those are what Maple Valley enforces today. The State Building Code Council is currently working through adoption of the next cycle, with final adoption of the residential and mechanical codes targeted for late August 2026 and the energy codes toward the end of September. Ventilation requirements have gotten stricter with every cycle for fifteen years and I have no reason to expect that to reverse.
Practical takeaway: if you are speccing a bathroom this year, do not build to the current minimum. Build a little past it. The minimum is a floor for the worst legal installation, not a target.
Frequently asked questions
What size bathroom fan do I need for a standard Maple Valley bathroom? For a bathroom of 100 square feet or less, take the floor area in square feet as your CFM number, with 50 CFM as the absolute minimum. A typical 5 by 8 bathroom calculates to 40, so you would install a 50. I would go to 70 or 80 for the margin. Above 100 square feet, size by fixtures instead: 50 CFM each for the toilet, shower, and standard tub, and 100 for a jetted tub.
Is a 50 CFM fan enough in Washington? It is the code minimum, and it has to hit 50 CFM at 0.25 inches water gauge, not at the easier 0.1 rating manufacturers usually advertise. For a small half bath it is fine. For any bathroom with a shower, it is the bare floor and I would not install one.
Can a bathroom fan vent into the attic or a soffit? No. Code prohibits discharging bathroom exhaust into an attic or crawlspace, and venting into a soffit just recirculates wet air back through the roof vents. It has to terminate outside through a roof or wall cap with a backdraft damper.
How long should I run the bathroom fan after a shower? At least twenty minutes, per the Home Ventilating Institute. Since nobody does that reliably, install a timer switch or a humidity sensing control and let it handle the runtime for you.
Why is my bathroom still humid with a new fan? Usually the ductwork. Undersized four inch duct, sagging flex, too many elbows, or a soffit termination will strip a large fraction of the rated airflow. It can also be a door with no undercut, which starves the fan of makeup air.
Do I need a permit to replace a bathroom fan in Maple Valley? Swapping a fan in the same location is generally minor work, but the electrical portion goes through Washington L&I rather than the city, and any duct rerouting or framing changes can pull the job into permit territory. Call the city’s Planner on Duty if you are unsure, or read our Maple Valley permit guide.
Planning a bathroom project in Maple Valley?
We spec ventilation on every bathroom we build, and we will tell you honestly whether your existing fan is worth keeping. We are based at 26424 233rd Ave SE in Maple Valley and we work throughout Covington, Kent, Renton, Black Diamond, Hobart, Issaquah, Auburn, and Federal Way.
Call (206) 773-8264 or request a free estimate.
Related reading: home remodeling in Maple Valley, WA, tub to shower conversions, choosing tile that stays clean, maintaining new bathroom tile, heated tile floors and whether they are worth it, and how to prepare for a bathroom remodel.
Sizing reference: the Home Ventilating Institute’s bathroom ventilation guidelines, which is where the square footage and fixture methods come from. Code requirements come from the Washington State Residential Code and can change. Confirm current requirements with the City of Maple Valley before relying on anything here.
Vasile Teterea is the owner of Firm Remodeling LLC, a licensed and bonded Washington remodeling contractor based in Maple Valley. Bathroom remodeling is the largest part of the company’s work across south King County. WA License #FIRMRRL783PU.
