Bathroom humidity: the room that spikes and recovers
The only room where a huge humidity spike is normal. The number that matters is how long it takes to fall.
- Target humidity
- 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-glazed 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
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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.
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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.
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Anti-mold paint on the ceiling
It buys a season. The ceiling is still getting wet after every shower.
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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.
Through the year
Winter
Peak risk. Cold walls and glass condense everything the shower produces. Fan run-on matters most now.
Summer
Lower risk indoors, but do not open the window on a muggy day — use the fan instead.
Any season
Re-caulk the sealant line as soon as it discolours. Once mold is inside silicone it cannot be cleaned out.
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.
How long should a bathroom fan run after a shower?
About 20 minutes. Most of the moisture evaporates off wet surfaces after you have finished, which is exactly when the fan usually gets switched off.
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 extractor 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.