Damp and Mould on Walls: Causes and Repairs

Mould is a symptom; the cause may be condensation, a leak or rising damp. How to find the source and fix the problem during renovation so it does not return.

A darkened patch in the corner of a wall, a musty smell behind wallpaper or black spots on a window reveal usually trigger the same reaction: wipe the stain and paint over it. A few weeks later the mould is back, because paint covers the symptom rather than the problem. Mould only lives where moisture persists, and until the source of that moisture is removed, any fix is temporary.

The three main sources of moisture

Condensation. Water vapour in room air turns into water when it touches a cold surface. It happens most often in the corners of external walls, on window reveals, along the line where the ceiling meets an external wall and behind large wardrobes. Signs: the patch grows in winter, sits in “cold” corners, and the window glass often has droplets too. The usual causes are poor ventilation, high indoor humidity and thermal bridges.

Leaks. Water comes from outside or from the plumbing: damaged roof or terrace waterproofing, façade joints, poorly sealed window surrounds, concealed pipe joints or water from a neighbouring flat. Signs: the patch spreads after rain or with a particular water use, often has a yellowish edge and depends less on the season.

Rising damp. On ground floors and in basements, moisture from the soil climbs into the wall by capillary action. Signs: a damp band up to a certain height above the floor, salt deposits and crumbling plaster.

How to identify the source

A correct diagnosis is half the repair decision. Taking moisture-meter readings at different heights and points on the wall shows how the moisture is distributed. A thermal camera can reveal cold bridges and hidden leak paths. Leaving a humidity logger in the room for a while helps assess how likely condensation is.

Simple observations help too: does the patch grow after rain, is the neighbour’s bathroom on the other side of this wall, does the window glass mist up in the morning, is furniture pushed tight against the external wall?

The sequence during renovation

Once the source is known, the work follows this order:

Remove the cause: stop the leak, restore waterproofing, fix ventilation or apply an appropriate damp-proof barrier against rising damp.
Strip damaged materials: mouldy wallpaper, swollen filler and crumbling plaster are removed. If mould has got into plasterboard, the board is replaced.
Clean and treat the surface: the remaining surface is treated with an antifungal solution; a respirator, gloves and eye protection should be worn.
Dry out: no further layers go on until the wall’s moisture content has dropped to a normal level. Rushing this stage undoes all the work.
Rebuild: the wall is finished again with vapour-permeable plasters and paints.

Renovation measures against condensation

In flats with condensation, cleaning the wall is not enough. Working extract ventilation in the bathroom and kitchen, trickle vents or supply vents at the windows, and insulation at cold bridges are key measures. Insulating an external wall from the inside needs careful design: a badly built system can move the dew point into the wall and push the problem somewhere you cannot see it.

Furniture matters as well. Behind a wardrobe or headboard pushed against an external wall there is no air circulation, and these corners are often where mould shows first. Keeping a small gap from such walls helps.

Keeping humidity under control after renovation

Even after the wall has been redone, a few simple habits help stop the problem coming back. Keeping a small humidity meter in the room is useful, especially in winter: if the reading stays high for long periods, ventilation needs to be increased.

After a shower, keeping the bathroom door closed and running the extract fan for a while stops steam spreading through the flat. The cooker hood goes on while cooking, and drying laundry in closed rooms is kept to a minimum. Short, intensive airing can be more effective than leaving a window slightly open for hours.

If a leak has been fixed, watch that area through the first rainy season. If a new patch appears, call in a specialist to pin down the source before opening up the finishes again.

What not to do

Wipe mould with bleach alone and paint over it.
Put vapour-tight vinyl wallpaper or dense paint on a damp wall.
Carry on with filling and painting before the wall is dry.
Start interior finishing before a leak has been resolved with the neighbour or building management.

Before starting a renovation, Zaferoğlu İnşaat assesses the condition of walls and ceilings and, where there are signs of damp or mould, schedules the finishing work only after the cause has been dealt with. Note whether the stain grows after rain or in winter and send us photos; that alone often points to the source of the damp.

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