Mould on the wall: where does it come from and what do you do about it?

Almost all mould in Dutch homes falls into two groups, and the difference decides whether this is an hour of work or a bill of several thousand euro. Group one is condensation: warm, humid indoor air settling on a cold wall. You see it in the corner behind a cupboard, above the window, in the bathroom and on the north wall of a bedroom.

Group two is water getting into the wall from outside or from within: a leaking gutter, a failed silicone seal, rising damp from the ground, a leaking pipe in the floor. Those patches are usually darker, feel genuinely wet, and sit low near the floor or right under a window sill.

The quickest test costs nothing. Tape a piece of clear plastic, for example cut from a bin bag, airtight onto the dry suspect wall and leave it for two days. If the moisture is on the room side of the plastic, it is condensation. If it is on the wall side, the moisture is coming out of the wall itself and better ventilation will not fix it.

Mould is not the problem, it is the evidence. These four together explain almost everything you meet in a home. Work out which one it is before you buy a bottle, because the product is the same for all four and the solution for none of them.

• Condensation and too little ventilation. Black spots in corners, on cold bridges and behind furniture against an outside wall. Worse in winter, almost gone in summer. By far the most common cause.
• Rising damp. A stain that creeps up from the floor to about a metre high, often with white salt deposits and wallpaper coming loose. Mostly in homes built before 1970 without a working damp course.
• A leak from outside. A sharply defined patch under a window, near a gutter, or in the corner of a dormer. Clearly worse after a day of heavy rain and drying back a little afterwards.
• A leak from inside. A pipe in the floor or wall, or a bathroom where the sealant and tiling let water through. The patch stays wet even in a dry week. Close every tap and read your water meter again two hours later; if it has moved, it is a supply pipe.

This sequence is for a patch of no more than half a square metre on tiles, paint or plaster. Larger areas and porous materials such as wood, plasterboard or sealant follow different rules, which are in the stop rule below.

1. Open a window and keep the door to the rest of the house closed. You are loosening spores and you want them going outside, not down the hallway.
2. Put on gloves and an FFP2 dust mask. Wear clothes that can go straight into the wash afterwards.
3. Wet the patch first with a plant sprayer of water. Dry sanding or dry scrubbing blows the spores into the room, which is exactly what you want to avoid.
4. Use an approved mould remover. The packaging carries an authorisation number starting with NL or EU, or ending in an N. Without that number it is not a registered biocide, however confident the bottle sounds.
5. Let it work as long as the packaging says, usually fifteen to thirty minutes, then wipe off with a damp cloth. Throw the cloth away instead of rinsing it out in the basin.
6. Never mix anything yourself. Bleach together with an ammonia-based cleaner or with vinegar gives off toxic fumes, and a home-mixed solution is not an approved biocide anyway.
7. Let the spot dry thoroughly for a few days and keep an eye on it. If the mould is back within two to three weeks, you have not touched the cause and cleaning again is pointless.
8. Only paint over once the wall is genuinely dry. Anti-mould paint on a damp surface will come off again within a year.

There is a point where carrying on yourself only costs time while the underlying problem grows. Stop as soon as one of these five applies.

The patch is bigger than about half a square metre, or it appears in more than one place in the house. At that size it is no longer about showering with the door open.

The mould is in the material rather than on it: soft plasterboard, rotten wood, sealant discoloured all the way through, a chipboard floor that gives. Surface treatment achieves nothing there, the material has to come out.

The patch returns within a month of being removed. That means moisture is still arriving, so you look for the source, not the stain.

Someone in the house has asthma, COPD, another lung condition or a weakened immune system, or there is a baby. Have someone else do the cleaning and get the cause investigated sooner than you otherwise would.

You do not know where the moisture comes from and the patch is on an internal wall. Mould on an internal wall is almost never condensation and often points to a leaking pipe.

Damp companies advertise free inspections that end up recommending an injection treatment remarkably often. There is a sizeable category of mould where you need nobody, and it is more honest to write that down.

• Black spots on the sealant in the shower. That is surface dirt plus condensation. New sealant and wiping the walls down after showering solves it.
• A patch behind a cupboard against the outside wall. Move the cupboard five centimetres off the wall so air can pass behind it. That is the whole repair.
• Steamed-up windows and some mould on the window timber in winter. Leaving vents open and not drying laundry indoors does more than any treatment.
• A one-off patch after a leak that has already been repaired. Let it dry, treat it, done. You do not need a facade injected because water once stood there.
• A musty smell in the crawl space without mould in the living area. Annoying, but no reason for a five thousand euro ground seal as long as the floor is insulated and there is ventilation.

Damp and mould are not automatically a landlord defect, because how a home is used plays a part. But as soon as the cause is structural, think of a leak, a cold bridge or a ventilation system that does not work, it is the landlord's responsibility and not yours.

Always report it in writing, by email or letter, with photos and a date. List the defects and ask for them to be fixed within six weeks. That written report is your most important evidence later; a phone call does not count.

If nothing happens, you can go to the Huurcommissie, the Dutch rent tribunal. It has a defects check that shows in advance whether your situation is serious enough. Mould falls into the heavier categories there, and the Huurcommissie can lower the rent temporarily until it is resolved.

You can also file a report with your municipality's building and housing inspectorate. They can formally instruct the landlord, and in practice that often works faster than a procedure.

Doing it yourself is almost free: a bottle of approved mould remover costs 6 to 12 euro, gloves and an FFP2 mask another 10 euro together. That is the entire budget for a normal bathroom patch.

What makes 2026 different from a few years ago is the balance between labour and materials. The products have barely risen, the man-hours have. So have several spots done in one visit rather than separately.

Ventilation is the only measure that tackles every source of condensation at once, and it costs nothing. You ventilate day and night, all year round: vents open, mechanical ventilation never switched off. Opening a window for five minutes is airing, which is something else.

Showering: ventilation on the highest setting or the window open, and keep that up for twenty to thirty minutes afterwards. Wipe the walls down with a squeegee, which saves literally litres that would otherwise evaporate into the room.

Drying laundry indoors is the biggest silent moisture source there is. One machine load of wet washing releases two to three litres of water into your indoor air. Drying outside or using a condenser dryer with an outlet makes all the difference in a small flat.

Do not push furniture tight against a cold outside wall and do not let the bedroom cool right down. An unheated room that someone still sleeps in is the classic place where mould starts.

Mechanical ventilation should be cleaned every two years. A clogged unit moves a fraction of what it should, and the bathroom is exactly where you notice that.