Ivan Landa
Window condensation is common in winter, but it is not always just the result of high humidity in the home. Find out why windows fog up on the inside, on the outside or between the panes, when condensation is normal and when it may point to a thermal bridge or an insulation problem. We'll also show you how to reduce window condensation and what to watch out for when windows are installed.
Window condensation is a fairly common problem, especially in the winter months. Water droplets appear on the glass, the area around the frame may be damp, and in some cases black spots or even mould start to form. But condensation doesn't always mean a bad window. It is often caused by a combination of higher indoor humidity and a low surface temperature.
So why do windows fog up, when is it still normal, and when does the problem need to be addressed?
Window condensation is caused by water vapour condensing. Warm indoor air holds a certain amount of moisture. When it reaches a cold surface, the air cools down and the water vapour begins to turn into water droplets.
The window glass is a typical spot, because in winter its surface can be much colder than the air in the room.
The higher the air humidity and the lower the surface temperature of the window, the greater the risk of condensation.
Window condensation is therefore most often caused by a combination of:
Occasional condensation on a window does not have to mean there is a defect. It typically appears in winter, for example in the morning after a night when the room was closed and humidity rose while you were asleep.
Moisture is produced by everyday life at home. Cooking, showering, drying laundry, houseplants and people themselves all add to it. If the humid air has nowhere to go, it starts to condense on the coldest surfaces.
It becomes a problem when windows fog up regularly, water runs down onto the window sill or frame, and surfaces stay damp for long periods. At that point, materials may start to get damaged and mould may develop.
Where the condensation appears matters, because each situation can have a different cause.
If water forms on the side of the glass facing the room, the most common cause is high indoor humidity combined with a low glass surface temperature.
Regular ventilation, heating the room and reducing humidity can help. It is also important to let warm air circulate freely around the window. The problem can be made worse by, for example, a wide window sill, heavy curtains or furniture placed right in front of the radiator.
Condensation on the outside of modern windows can, on the contrary, be a sign of their good insulating properties.
If the glass lets only a small amount of heat escape from the interior, its outer surface stays relatively cold. Under certain conditions, moisture from the outdoor air can then condense on the glass.
This kind of condensation is therefore not necessarily a defect. In fact, you are most likely to see it on high-quality insulating glass.
This is a different situation. If moisture gets between the individual panes of insulating glass, it may mean that the seal of the insulating glass unit has failed.
Under normal conditions, the space between the panes is sealed. If humid air gets in, water can start to condense on the inner surfaces of the glass.
In this case, ventilating or reducing humidity in the room usually won't help. The condition of the insulating glass unit needs to be checked and, if necessary, the unit replaced.
Water droplets don't only form on the glass. If moisture appears mainly near the frame, in the corners or on the window reveal, the problem may lie in the window detail itself.
One possible cause is a thermal bridge. Where the insulation is not continuous, the surface of the structure can be much colder than the surrounding area. Once its temperature drops low enough, moisture starts to condense on it.
Thermal bridges can occur, for example, at the window reveals, below the window or where the window frame connects to the building structure.
The area around the window is a very sensitive detail in terms of thermal insulation. Properly insulated reveals help keep surface temperatures higher and reduce the risk of condensation and mould.
First, it is a good idea to find out why the condensation is happening. There is no single solution that fits every case.
Short, intensive ventilation helps remove humid air from the interior. In winter, it is better to open the windows wide for a few minutes several times a day than to leave them tilted open for long periods.
The ideal humidity level can vary depending on the temperature and the specific conditions in your home, but in living spaces it is worth keeping an eye on whether it stays too high over time.
A hygrometer is a simple tool for this. It will show you whether the condensation is linked to excessive humidity.
There is a reason the radiator sits under the window. Warm air rises along the glass and helps raise its surface temperature.
If long curtains, a deep window sill or furniture block the airflow, condensation can get worse.
If moisture appears mainly in the corners, around the frame or on the reveals, a thermal bridge may be the cause.
In that case, ventilation alone may not solve the problem. It is important to check how the window detail was built and whether the insulation is continuous.
Properly installed reveal insulation helps limit heat loss while reducing the risk of condensation and subsequent mould.
In new builds and renovations, it is best to deal with the problem before it appears.
The window frame, the installation joint, the reveals and the surrounding thermal insulation should work together as one functional whole. A high-quality window alone is therefore not enough. Installing it correctly and getting the detail around it right matter just as much.
Weak spots can include, for example, poorly insulated reveals or the area below the window. This is exactly where a thermal bridge can form, leading to condensation and, in the long term, mould.
One of the materials used to solve details around windows is Purenit. Thanks to its combination of strength and thermal insulation properties, it is used, for example, as a support base under windows or for structural elements around window openings.
If your windows fog up only occasionally and the condensation disappears quickly after airing the room, it may be nothing unusual.
Pay closer attention mainly when:
In such cases, it makes sense to look not just at the window itself, but at the entire window detail. If a thermal bridge is suspected, a thermal imaging survey can help by showing the spots with significantly lower surface temperatures.
Condensation can have many causes. Sometimes it is enough to adjust ventilation and lower the humidity in the room. At other times, you need to focus on the airflow around the window or its technical condition.
However, if moisture regularly appears around the frame, in the corners or on the reveals, the cause may be a thermal bridge and a poorly designed detail.
With windows, it is therefore not enough to look only at the quality of the product itself. Correct installation, insulation of the installation joint and continuity with the surrounding structure are just as important.
