Ventilating rooms in winter: dry rooms without wasting energy

01Where does all that moisture come from?

Moisture in a house is not a sign of bad residents. It is the result of ordinary living. A household easily puts 10 to 15 litres of water into the air every day as vapour. A large part comes from the people themselves: through breathing and skin, one resident evaporates more than a litre of water a day. Cooking, washing up, showering and drying laundry come on top of that.

In a house where six to eight people live, the amount is therefore much larger than in an ordinary family home. Breathing and skin alone account for six to eight litres or more a day, before anyone has had a shower. Everyone cooks at their own time, the shower runs morning and evening, and laundry often hangs indoors.

In summer this is no problem: windows are open and walls are warm. In winter it is. Cold outside air can hold little moisture. When that air warms up indoors, it can take up moisture from inside and carry it out. That is exactly what ventilation does. If it does not happen, the moisture finds the coldest spot: a corner behind a wardrobe, an outside wall or a window frame. That is where mould starts.

02What humidity is right for a room?

A relative humidity of 40 to 60 per cent is good. After a shower or cooking it may briefly be higher. What matters is that it comes down again afterwards. A room that rises high every night and does not recover during the day is not getting enough fresh air.

A simple humidity meter costs about ten euros. A CO2 meter shows whether enough fresh air comes in: up to 750 ppm the air quality is good, above 1,200 ppm it is unhealthy. In a small bedroom with two people and a closed door, CO2 rises quickly at night.

Why this matters: the humidity in the middle of the room is not the same as at a cold wall. At the surface of a cold outside wall the humidity is much higher. Mould can grow there even when the air in the room still feels normal. Read how that works in our article on preventing mould in worker housing.

03Ventilating or airing: what is the difference?

Ventilating and airing are often used as if they were the same, but they are two different things.

• Ventilating is the continuous, small flow of fresh air through the house. Through trickle vents above the window, small top-hung windows or a mechanical system. This is needed day and night, all year round, including in winter.
• Airing is opening the windows wide for a short time to refresh all the air at once. Usually 10 to 30 minutes is enough. Turn the heating in that room off while you do it.

Airing does not replace ventilating. A room that is aired for ten minutes in the morning but then stays shut all day with closed vents will still become damp. The reverse is also true: if the vents stay open, the window does not need to be open all day.

Without a good ventilation system, the advice is to air the whole house two or three times a day, with windows and doors opposite each other open. That creates a through-draught that removes moisture quickly.

04Does a window on tilt help?

A window that stays on tilt all day feels like good ventilation. In winter it is often the worst of both worlds. The air is refreshed slowly, while the wall and frame around the window cool down. Moisture then condenses exactly there. And the radiator under the window heats the street for a large part.

Ventilation always costs some heat. In a well-ventilated house, roughly a quarter of the heating costs go into warming up fresh air. That is the price of healthy air, and it is worth paying. A window left ajar for hours with the heating on underneath costs far more than necessary.

A better combination:

1. trickle vents or small windows always open; they do not need to be fully open;
2. with a mechanical system, the fan always on;
3. short, thorough airing when needed, with the heating in that room off;
4. extra extraction where the moisture is produced: bathroom and kitchen.

05How should residents use vents and mechanical ventilation?

Residents often close vents because of draughts. Or they switch off the mechanical ventilation because it hums or uses electricity. In both cases the removal of moisture stops.

Mechanical ventilation should never be switched off, except for maintenance or in an emergency where the authorities advise closing windows and doors. If the unit has no plug, you can label its circuit in the meter cupboard so nobody switches it off by accident. With three settings: setting 1 when nobody is home, setting 2 when people are home (including at night) and setting 3 while cooking and showering. Also turn it up when many people are home or when laundry is drying indoors.

A few practical points for the manager:

• Leave at least 1.5 cm under interior doors, or fit a grille in the door. Otherwise the extraction in the bathroom and kitchen cannot draw air from the bedrooms.
• Clean vents and valves at least once a year.
• Have the ventilation unit serviced every two years and the whole system checked about every four years.
• Have the exhaust ducts cleaned at least once every eight years.
• If a vent really causes an annoying draught, consider a wind-pressure-controlled vent instead of taping the vent shut.

06Bathroom, kitchen and wet laundry: where is most moisture produced?

Most moisture is released at a few fixed moments. You can tackle those directly.

A bathroom fan should be able to extract at least 50 cubic metres of air an hour. A cooker hood with its own motor (around 300 m³ an hour) and an outside exhaust works much better than a hood without a motor on the existing duct.

Wet laundry is often the biggest silent source of moisture in shared houses. A rack with a whole week's laundry in a bedroom can keep that room damp for days. A place to dry laundry, or a dryer, is a small investment compared with repainting a room.

07What does ventilating cost compared with mould?

Ventilating costs heating money. That is true. But the comparison is not ventilating versus doing nothing. It is ventilating versus the consequences of moisture: mould on walls and frames, damaged mattresses, complaints from residents and arguments about the deposit.

Moisture is also a health matter. According to the World Health Organization guidelines, indoor dampness and mould are linked to more respiratory symptoms, allergies and asthma. The main recommendation is therefore to prevent persistent dampness and mould growth on surfaces and in building structures.

A few rules of thumb for practice, as an indication and not a promise:

• short, thorough airing with the heating off costs less heat than a window left ajar for hours;
• open vents with a closed window give a small, constant flow that is enough for most rooms;
• a room that no longer gets mould does not have to be treated and repainted every year.

Want to know what an electric heater costs when residents add one because a room feels cold and clammy? Read our article on the electricity cost of a plug-in heater.

08Which house rules work, and in which language?

Rules only work if residents understand them. The SNF standard for housing labour migrants (version 7.3), for example, requires short house and living rules in the residents' language on a central information card. Municipalities ask for the same in their own rules for flexible housing.

Keep the rules on ventilation short and concrete. Below are four sentences in four languages that you can put on the card or by the bathroom door:

Add the reason briefly: not to check up on people, but to prevent mould and cold rooms. A rule that residents understand is followed better than a ban without an explanation.

09How can you see which rooms need attention?

Most moisture problems only become visible when there are black spots on the wall. By then it is really too late. With measurements per room you see it earlier: a bedroom that rises high every night and no longer drops below 65 per cent during the day is getting too little fresh air. Often the vent is closed, the ventilation is off or laundry is drying there.

eyesonrooms places a wireless sensor in each room that measures temperature and humidity. You see the mould risk per room, and a summary arrives in your inbox every morning at 07:30. That lets you talk to the residents of one room, or have one vent repaired, instead of checking the whole house. The system measures only the indoor climate: there is no camera, microphone or motion sensor.

Tip: compare the rooms in the same house with each other. If one room is always damper than the rest, the problem usually lies in how that room is used or in that one room, not in the whole house.

10Frequently asked questions

11Sources

• Milieu Centraal: Ventilating your home (accessed October 2026, Dutch)
• Milieu Centraal: Natural ventilation (accessed October 2026, Dutch)
• Milieu Centraal: Mechanical ventilation (accessed October 2026, Dutch)
• Stadlander: Ventilation leaflet (2021, Dutch)
• Domus Medica: Moisture in the home (Dutch)
• WHO: Guidelines for indoor air quality: dampness and mould (2009)
• Stichting Normering Flexwonen: Standard for housing labour migrants, version 7.3 (2018, Dutch)

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