Roughly one in five households in the Netherlands suffers from damp or mould at home, and in rented homes it is even more common. The damage is rarely just cosmetic: mould irritates the airways, damp attacks plaster and wood, and a wrongly chosen solution costs money without solving anything. If you paint over a patch of mould while the wall is getting wet from below, you will be facing the same wall again within a season. This article helps you in three steps: first recognise the cause from the symptom, then decide what you can do yourself and finally call the right tradesperson. Four causes are the most common in practice: rising damp, penetrating damp, condensation on cold spots and leaks.
Four causes, four different approaches
Rising damp is ground moisture that is drawn upwards through the foundations and the brickwork, usually in older homes without a damp-proof course or with an outdated one. It generally stays limited to the bottom metre of the wall and shows up as a more or less horizontal, irregular tide mark. Salts from the ground and the brickwork crystallise on the surface: a white, powdery haze that keeps coming back after you have brushed it off. Plaster and paint come loose and skirting boards rot away.
Penetrating damp is rainwater that gets in through the facade. It sits higher on the wall than rising damp, becomes clearly visible after days of rain and is often on the south-west side, where most driving rain hits the facade. Causes include porous or broken bricks, worn-out pointing, cracks, a filled cavity or a defective gutter or lead flashing that lets water run down the wall. You can read how worn-out pointing is repaired in repairing pointing and cleaning the facade.
Condensation is moisture you bring into the home yourself through showering, cooking, washing and breathing, and which settles on cold surfaces: in external corners, behind cupboards against an outside wall, above windows and where window frames meet the wall. On these cold bridges the characteristic black spotted mould develops. Condensation often increases after insulating or fitting new double glazing without adjusting the ventilation, and after turning the heating down.
You can recognise a leak by a patch with a sharp outline that grows larger and is not tied to a facade or floor zone: under a bathroom, along a chimney, at a roof penetration or in line with a pipe. If the patch only appears after rain, the cause is outside; if it is also there in dry weather, it is usually a pipe. You can read which tradesperson to call in that case in Leaks: which tradesperson should you call?
Symptom by symptom: what you see and what it usually means
| What you see | Most likely cause | First check |
|---|---|---|
| White, powdery efflorescence at the bottom of the wall, returning after brushing off | Rising damp (salt efflorescence) | Measure the height of the tide mark; is it on both sides of the same wall? |
| Bubbling or loose plaster above the skirting board, rotting skirting boards | Rising damp, sometimes a leaking pipe in the floor | Moisture meter at several heights; check water consumption with the taps closed |
| Wet patch high up on an outside wall, worse after rain, often the south-west facade | Penetrating damp | Look outside at the pointing, cracks, gutter and lead flashing above the patch |
| Black spots in external corners, behind cupboards, above windows; misted-up windows | Condensation on cold bridges, too little ventilation | Measure the humidity; check vents and extraction |
| Growing, sharply outlined patch on the ceiling or wall, also in dry weather | Leak from a pipe or drain | Read the water meter with the taps closed; check the bathroom or central heating above |
| Wet patch on the ceiling only after rain | Leak in the roof, gutter or roof penetration | Inspect the loft or roof immediately after a shower |
| Musty smell, damp crawl space, cold floor | Moisture from the crawl space | Open the crawl space hatch: is there standing water, are the air vents clear? |
Two tools make this table much more reliable. A simple hygrometer or a CO2 meter with humidity measurement shows whether the indoor air is structurally too humid; Milieu Centraal (the Dutch independent information centre on energy and the environment) recommends such a meter precisely so you can see when extra ventilation is needed. A moisture meter for walls shows whether a wall is only wet on the surface (condensation) or deep into the material (rising or penetrating damp). Both can be bought or borrowed for little money and stop you calling the wrong tradesperson.
Health: why the GGD takes mould seriously
The GGDs (the Dutch municipal public health services) pool their advice on GGD Leefomgeving. Mould spores can cause a sore throat, sneezing and coughing, a blocked nose, wheezing and shortness of breath. People with an allergy, asthma or a lung condition are affected most; other health complaints are also more common in damp houses. The GGD sums up the approach in four Vs: verwijderen (remove), vocht beperken (limit moisture), ventileren (ventilate) and verwarmen (heat). In concrete terms: take short showers, put lids on pans, dry the washing outside, keep vents open day and night, and set the heating to at least 18 degrees during the day and 15 degrees at night, so that moisture evaporates faster and does not condense on cold walls.
If mould keeps coming back while you are applying the four Vs, there is a structural cause or too little scope for ventilation, and changing your habits will not help against that. In that case the GGD refers you to the owner or landlord; the GGD will help with health questions, but does not solve the structural problem.
Rented home: damp and mould are a defect
If you rent, the landlord is in principle responsible for remedying defects (Articles 7:204 and 7:206 of the Burgerlijk Wetboek, BW, the Dutch Civil Code). A defect is any condition or characteristic of the home that prevents you from having the enjoyment of living you may expect from a well-maintained home. Mould caused by a leaking roof, penetrating or rising damp or a missing ventilation facility falls under this; mould that demonstrably arises only from your own use does not. Report the defect in writing, with photos and the date, and ask for it to be repaired within a reasonable period; the Rijksoverheid (Dutch central government) names six weeks as the usual period.
If the landlord does not carry out the repair, as a tenant of a social rented home you can ask the Huurcommissie (the Dutch Rent Tribunal) to reduce the rent temporarily until the defect has been repaired. The Beleidsboek Gebreken (the Rent Tribunal's policy handbook on defects) lists condensation, penetrating or rising damp with almost constant mould growth or wood rot in living rooms and bedrooms as a serious defect in category B, and serious damp or mould patches and insufficient ventilation of the kitchen, toilet or bathroom in category C. For a defect in category B, the rent can be temporarily reduced to 30 per cent of the current rent, for category C to 40 per cent. The Gebrekencheck (defects check) on huurcommissie.nl shows in advance how the tribunal is likely to assess your complaint. If you rent in the private sector, the same statutory repair obligation applies, but the route runs via the kantonrechter (subdistrict court); Article 7:207 BW gives you the right to a proportionate rent reduction from the date of your report. If in doubt, consult the Juridisch Loket (the Dutch free legal advice service) or a tenants' support team (huurteam).
Which tradesperson to call, per cause
- Rising damp: a bricklayer or a specialist damp-proofing company. Solutions include injecting a damp-proof course into the brickwork, installing a new horizontal barrier or, in the case of a wet crawl space, improving the ventilation or insulating the ground. Only after that does it make sense to have a plasterer replace the affected plaster, preferably with a moisture-regulating renovation plaster.
- Penetrating damp: a bricklayer for pointing, replacing broken bricks and sealing cracks; a roofer if the cause lies in the gutter, lead flashing or roof edge. Impregnating the facade only makes sense once the pointing and bricks are intact, and never for rising damp.
- Condensation and cold bridges: a ventilation installer or central heating engineer for mechanical extraction in the bathroom and kitchen, cleaning or replacing an existing extraction unit and checking the air supply through vents; an insulation company or contractor for removing the cold bridge itself.
- Leaks: a plumber for pipes and drains, a roofer for the roof, a central heating engineer if the water is coming from the boiler or a radiator.
Have the tradesperson make and record the diagnosis first, before you accept a quote for the solution. A provider who only sells their own method, for example exclusively injection or exclusively impregnation, rarely assesses your wall without bias. An independent building surveyor or a second tradesperson costs little compared with the wrong intervention.
What you can do yourself, and what you had better leave alone
Remove small patches of mould with an approved mould remover, recognisable by an authorisation number that starts with NL or EU or ends in N. Dampen the surface first, so that you release fewer spores into the air, wear gloves, ventilate while you work and never mix cleaning products. Open the vents, let mechanical ventilation run on the highest setting for another hour after showering and keep furniture a few centimetres away from cold outside walls.
What you had better not do: paint over mould. The GGD is clear about this: it does not solve the problem, the mould grows through the paint. According to Milieu Centraal, dehumidifiers and moisture absorbers are at most a temporary stopgap and not a solution to the cause. Also keep the ventilation open when you insulate: a better energy label and healthy indoor air only go together if insulation and ventilation are part of one plan, as you can read in Improving your energy label: which measures really count? If you tackle the damp problem at the source and only repair afterwards, you only have to do the work once.