Why Bathrooms Here Grow Mould So Fast

Every household here has had the argument about whether the bathroom needs cleaning more often. Usually it does not. A bathroom that regrows mould within days of being scrubbed has a ventilation problem, not a hygiene problem, and no amount of effort at the surface will fix a room that never gets a chance to dry.

The room never fully dries

Mould needs moisture, a surface and time. Warmth speeds it up. A bathroom in Cairns supplies all four generously for most of the year.

The difference from a cooler climate is not that more water goes into the room. It is that far less comes out. In a dry climate, moisture left after a shower evaporates into air that has room to take it, and the surfaces are dry within an hour or two. Here the outside air is frequently already close to saturated, so there is very little drying capacity available, and the film of water on the tiles, the grout, the silicone and the ceiling simply stays.

Advice written for a southern bathroom consistently under-specifies ventilation for this reason. Opening a window in February does not dry a bathroom. It sometimes makes it damper.

The fan is almost always the problem

When we look at a bathroom that keeps going mouldy, the fault is nearly always one of three things about the extraction.

It is undersized. Fans are rated by the volume of air they move, and a room needs enough air changes to clear the moisture generated by a hot shower. A great many bathrooms were fitted with the smallest fan that satisfied the requirement on the day, which is not the same as a fan that works.

It runs for too short a time. The humidity is still in the room when the person leaves it, and a fan wired to the light switch stops at exactly the wrong moment. A run-on timer or a humidity sensor is a small change that does most of the work.

Or, most commonly and most damagingly, it does not go anywhere.

Fans that vent into the roof

This is worth its own explanation because it is so widespread in houses built here from the eighties through the 2000s, and because homeowners almost never know it is the case.

A ceiling exhaust fan needs a duct running to an external vent, through the eave, the gable or the roof. In a great many houses no duct was ever fitted, or one was fitted and has fallen off, and the fan simply discharges into the roof cavity.

The bathroom does get clearer, so the fan appears to work. What has actually happened is that the moisture has been lifted out of the bathroom and delivered into the roof space, where it condenses on the underside of the sheeting and settles into insulation and ceiling battens. Over twenty years the volume involved is substantial. It shows up as staining on a ceiling, as insulation that has slumped and lost its value, and occasionally as far worse.

If you can get into your roof space safely, look at where your bathroom fan sits. It takes thirty seconds to see whether a duct leaves it and goes anywhere.

Where mould means something worse

Surface mould on grout, silicone and the ceiling is a ventilation story. Mould that keeps returning in one specific spot, particularly low on a wall, at the base of a shower, or on the other side of a wet wall, is a different signal. That is water arriving from inside the wall rather than condensing on it, and it points at the membrane rather than the fan.

The tell is location and persistence: ventilation mould is general and appears on cold surfaces across the room, while a leak produces a patch that comes back in the same place regardless of how the room is aired.

If that is what you are seeing, read about bathroom waterproofing, or call 0468 047 067 and describe where it keeps returning.