Mould and damp in rental properties, and the pattern behind the post-winter spike in reports

It's Monday morning and there are three reports in your inbox that are all about the same thing, even though they read differently. The first tenant has "discovered a stain behind the sofa", the second writes about "a musty smell in the bedroom that won't go away", and the third sends a photo of a window frame that has gone dark at the bottom. All three only reported it now, even though, looking back, they could easily point to having noticed the spot for weeks.

Most managers recognise that pattern. As soon as the first warm days arrive, the tenant moves the sofa to give it a spring clean or leaves the window open for longer, and only then does what has been simmering all winter become visible. For your inbox, that means a spike in reports that has nothing to do with a sudden deterioration of the building, and everything to do with visibility and routine. The tricky part is that all these reports come in as separate cases, not as a single pattern.

It's a bigger problem than the spike in reports suggests

In April, the CBS (the Dutch national statistics office) published data showing that 1 in 5 households experience damp or mould in their home, compared with 15 percent in 2021. For tenants, that share is structurally higher: 30 percent in rental properties versus 14 percent in owner-occupied homes. In the four largest cities, it rises to 31 percent. The bathroom is the recurring problem area in 59 percent of cases.

On top of that, the Woonbond (the Dutch tenants' association) shows that mould is now the most reported housing problem at their reporting point, accounting for 23 percent of all reports over the second half of 2025. That's higher than rent price complaints and higher than maintenance backlogs more broadly. What you see in your inbox today isn't a seasonal effect you simply have to wait out, it's the visible tip of something that has been building all winter.

For a large share of these reports, a structural cause turns out to be behind them. The Woonbond reports that tenants are now more often proven right in disputes, since judges and the Huurcommissie (the Dutch rent tribunal) are taking a stricter view of what can reasonably be expected from each party. The tenant needs to ventilate and heat the home reasonably, while the landlord is responsible for removing any structural cause. A report you file away today as "it's down to the occupant" can still land back on your desk in six months via the Huurcommissie.

What you could already have seen in January

What's striking is that the signals are usually there months before the report is actually made. In late December, someone complains about "a draught in the bedroom" or "it takes a lot to heat the living room". In February, there's a question about a loose window seal. In March, an email about a squeaking ventilation grille. Each one a separate report, each one rightly handled as a minor issue, and only in May and June do they add up to become visible as one underlying damp problem.

Two things help you spot that pattern earlier. The first is linking reports to the property rather than to the ticket. If you see the same home appearing three times in your system over four months, even for seemingly unrelated complaints, that raises a flag before the tenant makes the connection themselves. The second is adding a few standard questions to every winter heating complaint. "Is there condensation on the windows?", "Do you feel a draught when the window frames are closed?" and "How do you ventilate at night?" are three questions few tenants answer proactively, but which take ten seconds to give you a fundamentally different picture than a bare report that "it's cold".

What this means for today's inbox

The spike in reports you're dealing with right now is an opportunity, not a backlog. Three identical reports in one day doesn't mean you need to schedule three repairs, it means you should first check whether the three reports share one underlying cause or not. You can make that distinction in five minutes: a quick structural intake over the phone or through a short questionnaire in the app, covering the location in the home, how long it's been going on and whether there's ventilation.

The gain lies in the order you work in. If you categorise first and act second, you'll find that some reports need the same contractor or the same diagnosis, and that others can't be closed off with "clean it and ventilate for a bit". Volkshuisvesting Nederland endorses this approach: woningcorporaties (Dutch social housing associations) must look at every report for a possible structural cause straight away, and bring in an independent expert if needed. The same principle applies just as much to private portfolios, since the Huurcommissie applies the same reasoning in a dispute.

The reading that comes back in September

In September, the same three tenants show up in your system again, only this time it's not about mould. It's about "last winter's heating costs". Deal with it promptly now, including the structural intake, and you prevent that. Respond now with "clean it and ventilate for a bit", and you'll get the same report back in October, via the tenants' association in November, and as a request to the Huurcommissie in January. Today's inbox is therefore not just a to-do list, it's an early inspection that measurably reduces the number of cases in your portfolio six months from now.