Damp and mould: where does it come from, and what does it cost?
Damp patches are rarely the problem itself, they are the symptom. The costs depend entirely on the cause, and at a viewing that cause can often already be traced.
In short
- There are four main causes: rising damp, penetrating damp, condensation and leaks.
- Condensation is usually the cheapest to solve; rising and penetrating damp the most expensive.
- A fresh coat of paint on one specific wall is a classic sign that something has been covered up.
- Mould is a consequence of persistent damp, cleaning without tackling the cause does not work.
Four causes, four price tags
1. Condensation
Moisture from the air settling on cold surfaces: windows, cold bridges, external walls behind a cupboard. Recognisable by mould in corners, behind furniture and around window frames, usually at eye level or higher. The cause is often a combination of too little ventilation and poor insulation. This is generally the most easily solved variant: ventilate better, repair mechanical ventilation or improve insulation.
2. Rising damp
Groundwater drawn up through the wall because a damp-proof course is missing or has deteriorated. Recognisable by a damp line at the bottom of the wall, usually up to a metre high, often with salt efflorescence (a white powdery deposit) and plaster coming away. Solution: injecting a new damp-proof course or applying a vapour barrier. More disruptive and more expensive, and the repair work to plaster and skirting comes on top.
3. Penetrating damp
Rainwater driven in through the facade, usually on the side that takes the most driving rain (in the Netherlands, south-west). Recognisable by damp patches that appear after rain and by facade damage, weathered pointing or a cavity that has filled with water. Solution: repair the pointing, impregnate the facade or repair the cavity.
4. A leak
A defect in the roof, a pipe, a window frame or a dormer. Recognisable by a sharply bounded patch, often on the ceiling or around a penetration. Costs depend entirely on the source: a broken roof tile is nothing, a leaking pipe in a concrete floor is a renovation.
What to do at a viewing
- Smell. A musty cellar smell in a living room is more informative than what you see.
- Look behind cupboards, in the corners of external walls and under window sills.
- Watch for one wall that has just been painted while the rest of the home has not been touched up.
- Open the crawl space if you can. Water on the bottom, condensation against the floor or a musty smell are relevant observations.
- Check whether there is mechanical ventilation and whether it is running. Taped-over vents are a signal.
- Preferably also view the property once after a spell of rain.
In the documents
Damp leaves traces in paperwork. The seller's questionnaire contains an explicit question about damp problems, read the answer literally. With an apartment, leaks and damp complaints almost always come back in the VvE minutes, including what was done about them. And if there is a structural survey, check whether the surveyor took moisture readings or only assessed visually.
Frequently asked questions
Is mould in the bathroom a problem when buying?
Usually not. Mould in the grouting of a shower is almost always condensation from insufficient ventilation and is easy to solve. More worrying is mould on an external wall in a bedroom or living room.
How do I tell rising damp from condensation?
By height: rising damp starts at the floor and does not come much higher than a metre, often with salt efflorescence. Condensation sits higher, in corners and on cold surfaces.
Can I hold the seller liable for concealed damp?
That can be an issue if the seller concealed a known defect that prevents normal use, the seller has a duty to disclose. But you also have a duty to investigate, and the purchase agreement often contains a clause limiting liability. Put a situation like this to a lawyer before you take any steps.
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