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

It's Monday morning and there are three reports sitting 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 just won't go away", and the third sends a photo of a window frame that's gone dark along 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's been simmering all winter become visible. For your inbox, that means a surge 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 still come in as separate cases rather than as one pattern.

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

In April, Statistics Netherlands (CBS) published that 1 in 5 households experience damp or mould in their home, up from 15 per cent in 2021. Among tenants, that share is structurally higher: 30 per cent in rental properties versus 14 per cent in owner-occupied homes. In the four largest cities, it reaches as high as 31 per cent. The bathroom is the problem area in 59 per cent of cases.

On top of that, the Woonbond (the Dutch tenants' association) shows that mould is now the most reported housing problem at their helpdesk, accounting for 23 per cent of all reports in the second half of 2025. That's higher than rent complaints and higher than maintenance backlogs in the broader sense. So what you're seeing in your inbox today isn't a seasonal effect you just need to wait out, it's the visible tip of something that has been building all winter.

In a large share of those reports, a structural cause turns out to be behind it. The Woonbond reports that tenants are more often proven right in disputes now that 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 property reasonably, while the landlord is responsible for removing any structural cause. A report you currently file away as "that's on the resident" can still land back on your desk via the Huurcommissie six months later.

What you could already have seen in January

What's striking is that the signals are usually already there for months before the report actually comes in. In late December, someone complains about "a draught in the bedroom" or "it takes a lot to get the living room warm". 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 reasonably handled as a small 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 in your system three times within four months, even for seemingly unrelated complaints, that raises a flag before the tenant themselves makes the connection. The second is adding a few standard questions to every winter heating complaint. "Is there condensation on the windows?", "Do you feel a draught with the windows closed?" and "How do you ventilate at night?" are three questions that few tenants answer proactively, but which, within ten seconds, give you a fundamentally different picture than a bare report that "it's cold".

What this means for today's inbox

The surge in reports you're in the middle of right now is an opportunity, not a backlog. Three of the same report in one day doesn't mean you need to schedule three repairs, it means you should first work out whether those three reports share one underlying cause or not. That distinction can be made in five minutes: a quick structural intake over the phone or via a short questionnaire in the app, covering where in the home it is, how long it's been there, and whether there's ventilation.

The gain lies in the sequence. If you categorise first and dispatch second, you'll find that some of the reports need the same contractor or the same diagnosis, and that several others can't be closed off with "clean it and open a window". Volkshuisvesting Nederland endorses this approach: woningcorporaties (social housing associations) are expected to look at a structural cause for every single report and bring in an independent expert where needed. That principle applies just as much to private portfolios, since the Huurcommissie applies the same reasoning in a dispute.

The measurement that comes back in September

In September, the same three tenants turn 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 open a window", 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 will measurably leave your portfolio with fewer cases in six months' time.