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Condensation (water and steam) on glass panes is disturbing. Why is condensation generated just in the edge region of the insulating glass unit when the weather is cold? And this even on modern window frames and insulating glass units with excellent thermal properties. Actually the reason is simple. The warmer the air, the more humidity it binds. Upon contact with the cool surface of the window, part of humidity contained in the air condensates into water. This is foremost a problem in winter, when the external temperatures are low. But condensation may also be generated in summer, when the external air is too humid and the inside temperature is relatively low. In winter condensation is generated on the internal side and in summer on the external side of the glass pane. If condensation generated in summer is disturbing as an aesthetical issue, then in winter condensation (and even glaze frost) may also destroy the opening.

Window glass is the worst insulated part of a wall regardless of what type of window is used and despite of the fact that energy saving glass has been used in the insulating glass unit. Different window types are usually provided with one and the same type of a glass pane and the problem of condensation may occur in all glass panes of all windows if circumstances are unfavourable. If steam condensates between the glass panes of a double frame window, air circulation between the glass panes is too small. In case steam is generated between the glass panes in an insulating glass unit, the reason is poor sealing of the insulating glass unit and the glass unit has to be replaced. In case condensation and ice appear on an internal glass plane facing the room, air humidity in the room is too high. The following text describes the latter case.

An insulating glass unit is built around an aluminium profile spacer. Due to this the insulating glass unit is always the coldest in the edges, where the external temperature is lower than the internal temperature. Due to falling air flow the coldest part of the internal part of the glass pane is its lower edge. If air humidity is too high in the room, water vapours condensate in the air towards the coldest part of the insulating glass unit. If it is cold enough, condensation will freeze. Later ice will melt and water flows down between the frame and the jamb.

The reason why condensation is generated on one window and on one window not, is air humidity in this apartment or room. The fact that the difference comes from different edge temperatures of different types of insulating glass units is not important here but in some conditions one may notice differences. It may come as a surprise but one may notice condensation on a new, freshly installed window, although as you remember, steam condensed much less on old windows. Amongst other things it may mean that new windows are much tighter than the older ones, so that after replacing old, air permeable windows the ventilation of the apartment has become much worse. It is important to avoid generation of condensation and ice. Condensation flows in the window spacer and frame and from between the frame and the sealing further down. Later ice will break these apart from one another. This will become a vicious circle boosting itself. Even if water does not freeze, it will anyhow damage the window.


Pursuant to the building standards all rooms in all apartments and houses shall be provided with air valves or openable windows. In a kitchen and bathroom these shall be connected to a channel, which ends on a roof or is connected to a ventilator. Air valves must be open when cooking, taking a shower or doing laundry. Such activities will most probably create condensation. By no means should the air valves be closed before condensation has disappeared from the windows. The best is to keep air valves open all the time. In an apartment building, where in upper flats natural ventilation is often poor, all floors should be provided with a common mechanical extraction system. Excessive usage of air humidifiers should be avoided.

How to avoid generation of condensation

In many cases you yourself can maintain the temperature of the internal side of the glass pane as high as possible. At first you should try to reduce cold access. This can be done with air valves, which guide air into the room. Then you should direct warm airflow from the heat source along the glass pane towards the upper edge. In case there is a heater under the window, there should not be a windowsill above it, except if there is a sufficient air cavity between the windowsill and the wall. In addition:

• Do not cover the window with heavy curtains blocking air circulation;
• Provide sufficient ventilation in rooms (especially in humid places such as a kitchen, bathroom, etc.);
• The location and output of the heating elements and heaters should provide even temperature throughout the room;
• Avoid very wide windowsills, since these block warm air from reaching to the window edge.

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