Bathroom Condensation, Mould, And The Exhaust Fan

Black mould in the corner of a bathroom ceiling gets reported as a cleaning issue. It is cleaned, it comes back, and it gets cleaned again.

It keeps coming back because the cause has not moved. Warm wet air is condensing on a cold surface and staying there long enough for mould to establish. Wiping it off does nothing about that.

What the fan is actually meant to do

An exhaust fan is there to move moist air out of the room and outside the building.

Two things stop that happening, and both are common in apartments.

The fan is undersized or worn, so it moves far less air than its rating suggests. Fans age badly, and a fifteen year old fan is often running at a fraction of what it did new.

Or the fan works and the air has nowhere to go. This is the bigger one. In a lot of apartments the duct terminates in the ceiling space rather than outside. The fan pulls the moisture out of the bathroom and deposits it directly above the ceiling, where it condenses on the underside of the slab and eventually comes back through as a stain.

If a bathroom is being ventilated into its own ceiling cavity, no amount of cleaning is going to fix it.

The other half of the problem

A fan can only extract if air can get in to replace it. A bathroom with a well sealed door and no undercut is a sealed box, and the fan cannot pull against it.

That is why a fan that sounds like it is working can move almost nothing. The simple test is to hold a sheet of paper against the grille with the door shut, then with it open. If it only holds with the door open, the room needs a proper undercut or a vent.

Where the mould appears tells you something

In the corners of the ceiling, particularly on an external wall, points at cold surfaces and poor air movement.

On the ceiling directly above the shower means the extraction is not reaching the source.

Around a window, on the reveal, is condensation forming on the coldest surface in the room and usually means the whole apartment is holding too much moisture rather than just the bathroom.

Behind furniture on an external wall is a different problem again, and it is worth checking whether water is entering the wall from outside rather than condensing on it.

What actually fixes it

Confirm where the duct goes. If it does not terminate outside the building, that is the job, and it is usually the whole job.

Replace a tired fan with one sized for the room rather than the smallest one that fits the hole.

Give the room make-up air, through a door undercut or a vent.

Consider a fan on a timer or a humidity sensor, so it keeps running after the shower stops rather than switching off with the light. Most of the moisture is still in the room at that point.

Then treat the affected surfaces properly and repaint. Painting over mould without killing it means it comes through the new paint.

In a strata building

The duct and the point where it discharges are often common property, which means the useful fix may not be the owner's to make on their own.

That is worth establishing early, because an owner can replace a fan three times and never solve a duct that terminates in the ceiling.

We handle the fan, the duct, the wall or roof penetration and the make-good as one job across electrical, climate and air and painting. If a bathroom in a property you manage keeps coming back, tell us how often and we will start with where the air is going.