Bathroom mould that keeps coming back: trace the real cause, not just the stain

It starts as a few pale dots above the shower, wiped away with a spray. Then they come back, settle into the silicone joints, creep across the ceiling corners, despite the fresh "anti-mould" paint and the stronger product you bought last time.

From a distance it looks like a cleaning problem. In reality the mark is only the final trace of a simpler chain: a lot of water vapour, a surface that gets too cold, and air that stays humid for too long. As long as one of those three links remains unchanged, the mould colony rebuilds in exactly the same spot.

The chain behind bathroom mould: steam, cold surface, lingering humidity

A hot shower loads the air with water vapour. In a small bathroom the air reaches its capacity quickly. As long as it stays warm that vapour is invisible.

When that humid air touches a colder patch — a ceiling corner, a thermal bridge around a duct, a slab against an unheated volume — its local temperature drops. The excess vapour settles there as liquid water. Day after day, that damp film is exactly what mould spores need to grow.

The black patch is therefore telling you three things at once: your bathroom produces a lot of steam, part of the envelope cools too much, and the air change does not dry the room fast enough between uses.

How to tell if your bathroom really stays humid too long

A bathroom that "never quite dries" is easy to recognise without instruments. Water beads remain on tiles, the mirror clears slowly, towels stay clammy, silicone joints stay soft and dark instead of lightening as they dry.

A simple test is to time, on a typical Brussels day, how long it takes for mist to disappear after a shower. Close door and window, then watch how long the shiny wet look remains on walls and ceiling.

If an hour later ceiling corners are still damp or joints are still glossy, humidity evacuation is probably insufficient. The next question is whether moisture is leaving through an actual extraction path, or whether it is simply trapped in the room and the adjacent corridor.

What really counts as bathroom ventilation — and what only looks like it

A bathroom can be ventilated in several ways, sometimes combined: an extract grille linked to a ventilation system, a separate fan through the wall, a clear gap under the door towards a ventilated corridor, or only a window you open by hand.

Some items look like ventilation but no longer work as such: a decorative grille blocked by paint, a fan whose impeller is clogged, an extract valve screwed almost shut "to avoid draughts". In those cases the steam either remains in the bathroom or migrates to the corridor, which then also feels damp.

If your bathroom has an extract valve, the Belgian standard NBN D50-001 specifies, for a bathroom, a minimum design airflow of 50 m³/h and a maximum of 75 m³/h. That is on paper. In practice the duct and the grille have to be clean and working properly for that airflow to be reached.

Simple checks to find which link fails in your own bathroom

Before another pot of "bathroom paint", it is worth locating the weak link. Three quick checks already narrow it down:

• Steam production: shorten showers and space them out over the day for a few days. If mould growth slows a lot, the room was simply pushed beyond what its ventilation can clear between uses.
• Air extraction: hold a tissue against the extract grille during and just after a shower. If it does not stick clearly, or only when the door is wide open, the pressure difference is weak or the air path is blocked.
• Cold surfaces: once the room has had time to dry, run your hand along the ceiling and corners. Areas that stay noticeably cooler often indicate a thermal bridge. When mould follows those lines precisely despite reasonable ventilation, local insulation is usually the issue.

These tests do not replace an airflow measurement, but they already show whether you need to act first on bathroom habits, on ventilation, or on the building surfaces themselves.

Cleaning, blackened sealant and stained ceiling: what can be reversed

A recent surface stain on washable paint usually comes off. By contrast, silicone that has turned black through its full depth holds mould within its mass: even if you bleach the surface, it darkens again because the material stays damp for a long time after each shower.

A ceiling repainted several times with different products can blister or flake when you scrub it. In that case the substrate often needs to be taken back properly: scraping, partial stripping, and then a paint suitable for humid rooms, but only once steam and drying have been brought under control.

None of this prevents new stains if the bathroom continues to stay humid for too long. Once ventilation and, where needed, cold bridges have been corrected, cleaning becomes an occasional task instead of an endless routine.

When mechanical bathroom ventilation runs but no longer really extracts

An extract grille connected to a mechanical ventilation system, or to a dedicated fan, can give the impression everything is fine: you feel a slight draught, the unit makes noise, the timer runs. Over the years, though, real airflow can drop.

Clogged filters in a heat-recovery unit, an impeller full of lint in an individual fan, a flexible duct crushed above a suspended ceiling, a backdraft damper stuck half shut: each of these reduces how much air actually leaves the bathroom each hour.

Manufacturers generally advise checking filters every 3 to 6 months and a full service once a year. Without at least that, a bathroom with a fan slowly behaves like a room with almost no extraction, even though the motor still turns. When mould keeps coming back despite reasonable use, that is often the sign that the mechanical ventilation itself needs checking and, if required, restoring to proper airflow.