Damp and Mould in a Dutch Home: Find the Cause Before You Treat Anything
Damp is a symptom, not a diagnosis. Before anything is sealed, plastered or painted, work out which mechanism is producing the water: condensation, rising damp, a leak, or cold-bridging. Each leaves a different pattern and needs a different repair, and the mould you see is the last link in the chain, not the problem. Fix the source, dry the structure, then finish — skip that order and the damage returns through your new surface inside a winter.
Four mechanisms, four signatures
Condensation is moisture you generate indoors — showering, cooking, laundry, breathing — settling wherever a surface drops below dew point. Look for black speckling in the corners of external walls, behind a wardrobe pushed against an outside wall, on window reveals and in unheated bedrooms. It is worse in winter and sits on the paint or sealant, not in the wall.
Rising damp (optrekkend vocht) is groundwater climbing through porous brick and mortar where no damp-proof course exists or the old one has failed — common in pre-war stock on soft, wet ground. The signature is a level tide mark low on a ground-floor wall, with white salt bloom and crumbling plaster along its top edge. It ignores the season and usually comes with a damp crawl space (kruipruimte).
Penetrating damp is water arriving from outside or from a pipe: a localised patch that darkens after heavy rain or after one particular shower or appliance is used, often nowhere near floor level. Suspects are cracked roof covering, blocked gutters, failed flashing, tired pointing, perished window seals, mortar bridging a cavity, leaking pipework.
Cold-bridging (koudebrug) is a detail that loses heat faster than the wall around it, so that strip condenses first. It shows as crisp, repeating geometric lines: a floor-slab edge in a post-war portiekflat, balcony connections, lintels, frames — often identically placed in the flat above. Combinations are normal; blaming one cause is the classic error.
Why painting over mould fails
Mould is an organism, not a stain. It roots into plaster, wallpaper paste, filler, timber and sealant, so wiping and covering leaves plenty behind to regrow. Paint also removes no water: while the wall keeps reaching dew point, moisture pushes the new film off — blistering, peeling, dark bloom back through the coating within a heating season.
Anti-mould coatings are maintenance, not a cure; they hold only on a surface already dry and already fixed. Vapour-tight paints and impermeable renders are worse, trapping moisture and pushing the wet zone into the next room. Cover-ups also destroy what a specialist reads: the pattern, the tide mark, the salts.
Ventilation in a tightly sealed house
A household puts a serious volume of water vapour into the air daily. In a draughty old house it leaked away unnoticed; retrofit work closes those accidental paths: new frames and seals, insulated glazing, draught-proofing, capped chimneys, taped-over grilles. Single glazing used to be the coldest surface, so condensation formed on the glass where you saw it. Replace it and the moisture moves to the next-coldest surface, an uninsulated wall or corner behind furniture, where it feeds mould instead.
Ventilation only counts if it is used and maintained: extraction running during and after cooking and showering, trickle vents open, filters and ducts cleaned, heat-recovery units serviced. A switched-off system is an unventilated house. The rest is habit — laundry dried indoors, unvented dryers, unheated bedrooms taking humid air from heated rooms.
What a specialist actually inspects
History and pattern first: when it appeared, whether it tracks rain or season, which rooms, and what renovation happened just before. Then numbers: indoor temperature and humidity, wall surface temperatures, and the resulting dew point. That separates condensation from ingress with data instead of opinion.
Then moisture in the fabric: meters to screen, plus drilled masonry samples at several heights, because a rising-damp profile does not look like a leak profile. Thermography maps cold bridges and wet areas behind finishes, read alongside the humidity figures. Then the envelope: roof and flashings, gutters and downpipe discharge, pointing, cavity bridging by borescope, window seals, ground level. Then the crawl space, the pipework, and soft timber checked for dry-rot fungus, which spreads through masonry far beyond the wet spot.
Fix the cause first: the order of works
Diagnose, eliminate the source, dry the structure, strip out contaminated material, repair damaged elements — then plaster, paint, floor and fit. Drying is a real phase with real duration, not a weekend; masonry and screed release water slowly, and finishing over a wet substrate is the commonest reason damp work gets done twice. Soaked porous material goes out rather than getting cleaned: wet insulation, blown gypsum board, delaminated plaster, mouldy sealant.
Match the remedy to the mechanism: condensation on a cold wall wants insulation and ventilation, not injection; rising damp wants a restored barrier, salt-tolerant plaster and crawl-space work; a leak wants the specific building repair; a cold bridge wants a detail-level fix. Internal insulation without proper vapour control just moves the condensation plane inside the construction, where it rots timber invisibly. Re-inspect after a full heating season: work that has not survived a winter is not proven.
What to ask a provider
Ask what they will measure, not only what they will do: dew point and surface temperatures, or just a pin meter on the plaster? Ask whether the survey is separate from the repair, whether crawl space and roof are included, how drying is verified, and what happens if the damp returns. In an apartment, check with the VvE first: facade, roof, gutters and shared pipework are usually collective, and shared buildings often restrict working hours.