Condensation on the inside of windows: causes and what to do

Steamed-up windows in the morning are very common in autumn and winter. Still, they are a signal: more moisture stays in the home than leaves it. A few fixed habits usually solve it, and stop it from turning into mould in the corners or rot in the frames.

Where does condensation on windows come from?

Air holds moisture, and the warmer the air, the more it can hold. When warm, humid air touches a cold surface such as a window, it cools down. Cold air holds less moisture, so the excess settles as droplets.

We produce that moisture ourselves. Showering, cooking, drying laundry, plants, aquariums and simply breathing quickly add up to litres a day. If it does not leave through ventilation, it finds the coldest spot: usually the window, then the corners of outside walls and the frames.

You see it most in the morning. At night it is coldest outside, so the windows are coldest too. And in a bedroom with the door closed, two people breathe out moisture for hours.

Inside, outside or in between?

Where the condensation is tells you what is going on.

• Inside: too much moisture in the home, a cold window, or both. That is what this article is about.
• Outside: actually a good sign. The glass insulates so well that the outer pane stays cold, because little heat from inside gets through. It disappears once the sun reaches it.
• Between the panes: the seal of the double glazing has failed. Ventilating or cleaning will not help; the unit needs replacing.

When does condensation do harm?

A few drops on a cold morning that dry up during the day are not a problem. It becomes one when:

• windows stay wet all day;
• water runs from the glass onto the frame and stays there;
• you see black spots in the corners of the window, on the sealant or on the wall around it;
• the room smells musty.

Lasting damp leads to mould, and wooden frames eventually rot. Mould is also bad for your airways.

How to prevent condensation inside

Ventilate continuously. Milieu Centraal is clear about this: ventilation should go on day and night, also in winter. So keep trickle vents open, also when it is cold and also at night. Opening windows wide a few times a day is fine, but it does not replace the vents.

Do not switch off mechanical ventilation. If you have a ventilation unit, leave it on and turn it up when showering and cooking.

Extract after showering and cooking. Switch on the extractor and let it run on for a while afterwards; many housing associations advise at least fifteen minutes. Keep the bathroom door closed while showering, so the moisture does not travel through the house. Cook with lids on pans and the cooker hood on.

Dry laundry outside or in a dryer. A rack of wet washing in the living room releases all the water that evaporates from it into the air.

Keep the home evenly warm. A bedroom that gets completely cold has the coldest windows and therefore the most condensation. A few degrees of heating with a vent open does more than a window kept shut all day.

Keep furniture away from outside walls. Leave a few centimetres so air can circulate. A wardrobe against a cold wall is a classic spot for mould.

Wipe the glass dry. If there are drops anyway, remove them in the morning with a squeegee or cloth, so they do not soak into the frame.

Measure instead of guess

To know how humid your home is, buy a simple hygrometer. Milieu Centraal gives a relative humidity of 40 to 60 percent as good. If you are often above that, you are not ventilating enough. Some CO2 meters also measure humidity; a high CO2 level is a second sign that too little fresh air is coming in.

Does a dehumidifier help?

According to Milieu Centraal, a dehumidifier does not solve a moisture problem in the home and does not replace ventilation. It takes some water out of the air, but the cause remains: more moisture is added than leaves. Ventilating well is cheaper and works better.

When is it more than condensation?

If windows stay wet all day although you ventilate well, or mould keeps returning in the same place, something else may be going on: a cold bridge, a leak or rising damp. Read how to remove and prevent mould in the bathroom as well. If you rent and the damp or mould persists, report it to your landlord.

Considering new glazing because the old windows keep steaming up? Keep the invoice and warranty terms safe; our guide to the legal guarantee explains why that matters.

How Domeaz helps

Ventilating is mostly a habit, but vents and extract valves also need to stay clean: a dusty vent lets far less air through. In Domeaz you plan “Clean vents and extract valves” as recurring maintenance, for example every spring and autumn, and you get a reminder automatically. If you have mechanical ventilation, add the unit to your appliances with its manual and a maintenance rhythm for the filters.

A year-round schedule for the rest of the home is in our home maintenance schedule.

Domeaz is free to start and has no ads. Discover Domeaz.