Understand it · practical guide
The bathroom that stays wet: fan path, door gap, mould, and the cold wall
In one line: A noisy fan that exhausts into the ceiling is not ventilation, and a clean grille cannot cure a blocked duct or a room with nowhere for replacement air to enter.
Steam on the mirror is normal for a hot shower; mouldy corners, peeling paint and a ceiling that stays damp for hours mean the moisture is not leaving. The fan has to move air out of the room, the duct has to reach outdoors, and replacement air has to enter through a door gap or another planned opening. Remove one of those three and the fan only stirs the problem.
Trace the air
- Start with the grille. Switch off the fan circuit, remove dust from the grille, and check that the flap or shutter is not glued shut by grime. A fan that hums but barely moves air may have a blocked impeller or a tired motor.
- Find the outlet. The duct should discharge outside, not into a roof void, ceiling cavity or closed service cupboard. Warm wet air left above the ceiling condenses on cold surfaces and creates the hidden mould story the bathroom cannot see.
- Give the room replacement air. Close the door and the fan tries to pull against a vacuum. A modest under-door gap or transfer grille lets the fan move the wet air it is designed to move. Keep the bathroom door open after bathing when privacy and the house layout allow it.
- Run it for the moisture, not the switch. Turn the fan on before or at the start of the shower and leave it running afterward. A timer or humidity control can help, but it does not replace a clear duct and a working outlet.
The cold-surface problem
If the air path works and one outside corner still grows mould, the cause may be a cold bridge, a roof leak or water entering around a window rather than shower steam. Clean visible mould with the correct product and protection, let the surface dry, and repair the moisture source before painting. Do not put wet hands near a fan switch or open a mains fan housing as a first test; electrical isolation and fixed wiring are an electrician's job. The useful rule is not that a bigger fan solves everything. It is that air must leave the room, the replacement path must exist, and the surface that stays wet needs its own diagnosis.
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General information, not professional advice. Homes differ — if a job is beyond your confidence or the guide’s boundary, that is what tradespeople are for.