Under 60% within 20 min — Peak matters less than recovery time
Every other room on this site is judged by its steady reading. A bathroom is not — it will hit 90% or more during a shower no matter what you do, and that is not a fault. Water evaporating off skin, tiles and a wet floor has to go somewhere.
The question is whether that moisture leaves the building or spreads into the rest of the house. Twenty minutes is a reasonable benchmark: if you are still above 60% half an hour after the shower, the extraction is not doing its job.
Why bathrooms grow mold so readily
Three things line up here that rarely line up elsewhere: enormous moisture production, surfaces that stay wet for a long time, and — in most houses — an exterior wall with a cold corner in it.
Grout, silicone sealant and painted ceilings are all excellent surfaces for mold, and they hold water at the surface long after the air has cleared. That is why the growth shows up on the ceiling above the shower and along the sealant line rather than in the middle of the room.
The other factor is timing. Most of the moisture leaves surfaces in the twenty minutes after a shower, which is precisely when most people switch the fan off and shut the door.
Where the moisture comes from
- A ten-minute hot shower — roughly a pint of water into the air
- Wet towels left hanging in an unventilated room
- A wet floor and wet tiles evaporating for an hour afterwards
- Drying laundry over the tub or on a radiator in the same room
- A cold exterior wall or single-pane window that condenses everything the air is carrying
The plan for this room
Ranked by what it costs you. Work down the list — most rooms are fixed before the paid options.
Run the fan during and 20 minutes after every shower, with the door closed so it pulls replacement air through the gap under the door.
Squeegee the shower walls and glass after use. It removes most of the water before it can evaporate.
Hang towels somewhere ventilated, not folded over a rail in the closed bathroom.
Open the window after showering in cool weather — a bathroom is the one room where the cold-air rule almost always applies.
Fit a humidistat or run-on timer switch so the fan runs long enough without anyone remembering to do it.
Check the fan actually moves air: hold a sheet of tissue to the grille. If it does not hold, the duct is blocked, crushed, or the fan has failed.
Size the fan properly — roughly 1 CFM per square foot of floor area, with 50 CFM the practical minimum.
Follow the duct. A fan venting into the attic or a wall cavity is creating a hidden mold problem. It has to terminate outdoors.
What will not work here
-
A recirculating fan or one venting into the attic
The moisture never leaves the building. You have moved the problem into the roof space, where you will not see it until the sheathing rots.
-
A dehumidifier instead of extraction
A shower dumps moisture in fifteen minutes. Removing it at the source in real time is far cheaper and more effective than drying it out of the whole house afterwards.
-
Anti-mold paint on the ceiling
It buys a season. The ceiling is still getting wet after every shower.
-
Leaving the door open during the shower
It distributes the steam through the house rather than letting the fan remove it. Door closed, fan running.
What is your reading telling you?
- Dew point
- 53 °F
- Mold risk, cold walls
- Moderate
Still inside the accepted range, but this is where cold surfaces begin to sweat.
Through the year
| Season | What the room does | What to do about it |
|---|---|---|
| Winter | Peaks near 100% during a shower and falls slowly, because the walls and the mirror are cold and hold the water | Run the fan during and for 15 minutes after; keep the door shut; add a little background heat |
| Spring | Recovers faster as surfaces warm; the reveal and ceiling stop staying wet all morning | Good time to clean and repaint any growth, because the surfaces will actually dry |
| Summer | Recovers fastest, but in a humid climate the room may never drop below 60% between showers | Ventilate to outside rather than into the house; check the exhaust duct is not dumping into an attic |
| Fall | The first cold snap brings the winter pattern back before anyone has turned the heating on | Restart the fan routine early — this is when the ceiling spotting usually starts |
Any season
Re-caulk the sealant line as soon as it discolors. Once mold is inside silicone it cannot be cleaned out.
Where to measure in this room
A hygrometer in a bathroom mostly tells you about the shower you just had. To learn anything useful, read it at fixed points rather than continuously.
-
On the wall opposite the shower, chest height
Away from the direct spray and away from the door, so it reads the room rather than a plume. Never on the wall the shower sprays onto — that reads as saturated regardless of the room.
-
Immediately after, then 20 minutes later
The pair of numbers is the measurement. Ninety-something falling under 60% in twenty minutes is a working room. Ninety-something still in the seventies is an extraction problem.
-
The ceiling above the shower, with an infrared thermometer
This is the coldest surface in most bathrooms and the first place mold appears. If it is more than a few degrees below the room, the ceiling needs insulation above it as much as the room needs a fan.
-
Behind the door and in the corner
Still air collects where the door swings and in the corner furthest from the fan. If growth is appearing there rather than above the shower, the problem is air movement rather than raw extraction capacity.
Common questions
What humidity should a bathroom be?
It will spike above 90% during a shower and that is normal. The target is coming back under 60% within about 20 minutes afterwards.
My bathroom still fogs 30 minutes after a shower — why?
The extraction is not keeping up: the fan is undersized for the room, the duct is blocked or too long, or it does not terminate outdoors. See exhaust fans for the three checks.
Should the bathroom door be open or closed when the fan runs?
Closed, with the normal gap under the door. That makes the fan draw replacement air through the gap and out of the building, instead of letting steam spill into the hallway.
What size exhaust fan do I need?
Roughly 1 CFM per square foot of floor area, with 50 CFM as a practical minimum. A large bathroom or one with a separate shower enclosure needs more.
Is it better to shower with the door open or closed?
Closed, with the fan running and an undercut or vent for make-up air. An open door lets the moisture disperse through the house to condense on colder surfaces in the bedrooms and hallway, which converts a contained, extractable spike into a whole-house problem. The exception is a bathroom with no extraction at all, where an open window in that room beats an open door into the house.
Does a cold shower produce less moisture?
Substantially less, because evaporation depends on the water temperature — but nobody changes their showering habits for humidity. The version worth doing is shortening the shower and turning the temperature down slightly at the end, which also cools the room surfaces less and helps them dry faster afterwards.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions