Why does mould come back after I clean it?

What is mould?

It is a fungus that grows where there is moisture and little ventilation. It shows up as dark, green or greyish dots and patches on walls, ceilings, corners, sealant and behind furniture. Cleaning it off the surface does not remove the moisture that feeds it.

Likely causes

1. Condensation. Corners, windows and cold walls collect the vapour from showers, cooking and drying washing, and the water stays on the surface. In Portuguese winters, with damp air and little insulation, this is the most common cause.
2. Water getting in. Water entering from outside or from a pipe keeps the wall damp inside, and the mould appears on top.
3. Lack of ventilation. Closed rooms, furniture pushed against walls and extractor fans left off trap damp air. Many homes also have shutters (estores) kept closed and windows without trickle vents.
4. Moisture from a slow leak. A pipe with a small leak can soak one area for weeks without being noticed.

How to tell them apart

• Dark dots in corners, around windows or behind furniture, worse in the cold: points to condensation.
• A patch with a clear edge, a wall that is damp to the touch, blistered paint, worse with rain: points to water getting in.
• An area next to a pipe, a bath or a sink: worth checking for a leak.
• It comes back in the same place after cleaning and ventilating: the cause is not resolved.

Look at the context too: if the mould only appears in winter and in rooms you rarely use, condensation is the strongest hypothesis. If it appears in any season and beside an outside wall or a pipe, water is more likely to be getting in. A photo taken from the same angle once a week helps you see whether the mould is growing, stable, or returning after each clean.

What to do now

1. Photograph the mould before cleaning, so you can compare afterwards.
2. Clean the surface with gloves and a mask, with the room ventilated. Avoid mixing cleaning products.
3. Ventilate every day, even in winter, and use the extractor fan when showering and cooking.
4. Move furniture away from cold walls and avoid drying washing indoors without ventilation.
5. Watch for two weeks: if it returns in the same spot, look for the source of the water.
6. Avoid painting over it: paint hides the mould but does not fix the cause.

If you rent, send your landlord (senhorio) the before and after photos with the dates.

When to call a professional

Get help if the mould covers a large area, if it keeps returning, if the wall is wet to the touch, or if there is a persistent musty smell. People with asthma, allergies or breathing problems should avoid cleaning large areas themselves. Depending on the source, look for a plumber (canalizador) for leaks, a builder (pedreiro) for water getting in, and a damp-proofing and ventilation specialist (técnico de isolamentos e ventilação) for persistent condensation.