Preventing mould in worker housing

01Why these houses in particular?

A family of four already puts around 12 litres of water into the air every day, through showering, cooking, drying laundry and breathing. Housing for labour migrants often has more people per house, and a few things make it worse:

Laundry drying indoors. Without a dryer or a drying room, the washing hangs in the bedroom, and all that water ends up in the air of one small room.
Shift work. Someone who sleeps during the day keeps windows and curtains closed at exactly the time when ventilating works best.
Taped-up vents. Trickle vents are closed or taped over against draughts and cold, and mechanical ventilation is switched off because of the noise.
Changing residents. Someone who stays for a few months rarely feels responsible for a black patch behind the wardrobe, and a house rule written in Dutch often does not get through.
Cold bedrooms. When the thermostat is in the living room, a north-facing bedroom can stay much colder than the rest of the house.

02How mould forms: moisture, a cold wall and time

Air at 20 °C with 60 % relative humidity feels normal. But against a colder wall that air cools down, and cold air can hold less moisture. At the surface of the wall the relative humidity therefore rises:

MoistureShowers, cooking, drying laundry: litres a day

Cold wallBehind the wardrobe, in the corner: 13 °C

TimeWeeks above 80 % humidity at the wall

MouldBlack patches, smell, damage

Mould is not bad luck: it is moisture, a cold wall and time.

Wall surface temperature	Room 20 °C, 50 %	Room 20 °C, 60 %
18 °C	57 %	68 %
17 °C	60 %	72 %
16 °C	64 %	77 %
15 °C	69 %	82 %
14 °C	73 %	88 %
13 °C	78 %	94 %

Relative humidity at the surface, calculated with the Magnus formula for saturation vapour pressure. Values above 80 % are highlighted.

If the humidity at a surface stays above roughly 80 % for a longer period, mould can grow. The same limit is used in the standard for moisture calculations in buildings, EN ISO 13788. So there does not have to be any condensation on the wall: a corner that stays just too cold for weeks is enough.

How cold the inside of a wall gets depends on the insulation and on the weather. Building engineers express this as a temperature factor. At a factor of 0.65, the minimum for new-build homes in the Netherlands, the coldest spot is about 13 °C when it is 0 °C outside and 20 °C inside. Older houses often do not reach that factor. That is why mould almost always starts in the same places:

in the corners of external walls, especially near the ceiling;
behind wardrobes and beds placed against an external wall;
around window frames and in the window reveals;
on the bathroom ceiling and in the tile joints.

Cross-section of an outside wall. On the inside of the cold wall, humidity rises above 80 %, and that is where mould starts.

03What does the SNF standard say?

Many Dutch staffing agencies are registered with the Stichting Normering Flexwonen (SNF), which sets the standard for housing labour migrants in the Netherlands, and their houses are inspected. Version 7.3 of the SNF standard contained two requirements on this subject (our translation):

There is a sound and demonstrably maintained mechanical installation or natural ventilation, in combination with sufficient airing.

There is no mould growth in the bathrooms, kitchens and/or other rooms.

In that version, an inspector could record mould as a minor or a major non-conformity. The standard is tightened regularly; from 1 July 2025, for example, stricter rules on space per resident applied to new locations. Check the current text at normeringflexwonen.nl. In practice little changes: mould that an inspector finds is mould you would rather have seen weeks earlier. How to keep that in view per room is described under the SNF package.

04Measures that work

Ventilation open and on. On every visit, check that the vents are open and the mechanical ventilation is running. A taped-up vent is the first sign.
A place to dry laundry. A dryer or a shared drying room takes the biggest source of moisture out of the bedroom.
Extract after showering and cooking. Bathroom door shut, extractor on, and a window open for a while. The moisture has to go outside, not into the hallway.
Furniture away from external walls. A few centimetres behind a wardrobe or bed lets air move along the wall, so it does not stay cold and damp.
Do not let rooms get too cold. A room that stays below 16 to 17 °C for days has a colder wall and therefore a higher humidity at the surface.
Instructions in the residents' own language. A short instruction with pictograms in Polish, Romanian or another language the residents speak works better than a house rule in Dutch.
Measure instead of guessing. A single hygrometer only gives a snapshot. Continuous readings per room show whether a room dries out again after a shower, or stays damp for days.

05What humidity is normal?

Indoors, a relative humidity of 40 to 60 % is usually recommended. After showering or cooking it may be higher for a while; what matters is that it comes down again afterwards. A bedroom that goes above 70 % every night and no longer drops below 65 % during the day is a room to keep an eye on, especially if it has a cold external wall.

06Seeing the risk per room before the wall turns black

eyesonrooms places a wireless sensor in every room that measures temperature and humidity at least once every ten minutes. The system fetches the outdoor temperature for the address and uses it to estimate how damp the coldest wall of each room gets. Every room gets a risk level from green to red, and a room that stays red for days triggers an alert with the address and the room.

That way you can talk to the residents about ventilation, or put in a dryer, weeks before black spots appear. And at an inspection or in a dispute about the deposit, you have the readings to hand.

Want to see which rooms are at risk?

A pilot in a few houses shows within a month in which rooms the moisture lingers.

Request a pilot

Or look at the pricing first: €47.99 per sensor, €89.99 per gateway and €4.99 per room per month (incl. VAT), both with a volume discount from 40 rooms.

07Frequently asked questions

Is a dehumidifier the answer?

For a single room and for a while, yes, but it treats the effect rather than the cause. It uses electricity and has to be emptied, and ventilation is still needed for fresh air.

Is mould in the home bad for health?

According to the World Health Organization (WHO), dampness and mould indoors are associated with respiratory symptoms such as coughing, wheezing and asthma. Prevention is therefore a matter of health too, not just of paintwork.

Who is responsible if mould appears anyway?

That depends on the cause: the building, such as a thermal bridge or broken ventilation, or the use, such as not ventilating or drying laundry in the room. See Mould damage in a rented room: building defect or use?

What humidity is too high in a bedroom?

Between 40 and 60 % is normal. If a bedroom stays above 65 to 70 % for days, there is too little ventilation, and the humidity at a cold wall soon rises above 80 %.