The how and why of condensation

Condensation on windows can occur internally, externally and between the glazing. Condensation has several causes, but basically it is down to moist air coming into contact with a cold surface. Because it is subject to the laws of physics, there is no universal solution – but the best medicine is good ventilation and well-insulated windows.

Internal condensation can damage the window

Old windows sometimes develop condensation on the inside of the interior glass pane, mainly during the winter months. This is because the window does not insulate well and the interior glass is therefore colder than the air in the room. Condensation occurs when, among other things, heat and moist indoor air meets cold glass. Other causes can be poor ventilation, high relative humidity or if curtains obstruct the circulation of air, e.g. from the radiator, which is usually under the window.

Windows are designed to deal with rain and moisture on the outside, but not to withstand long-term contact with moisture on the inside. If internal condensation occurs over a long period, the window may become damaged. The best measures? Replace with well-insulated and energy-efficient windows.

Condensation between the glazing requires ventilation

If condensation occurs on a window with a double-glazed sash unit plus a single-glazed sash, this is down to warm indoor air leaking out between the sashes and condensing on the inside of the external glazing. Energy windows are particularly sensitive to this, as the external glazing is very cold. The solution is to adjust the ventilation so that a slight underpressure is created in the room and warm air can be forced out. Because warm air rises, windows on the house’s upper floor may be more susceptible to this problem.

The windows themselves should have been correctly assembled so that no gaps are present between the sash and frame, and all points of closure in the espagnolette work as they should. Remember that an integral blind conduit will allow a small amount of warm air through if there is overpressure.

If condensation occurs between the glazing in insulated panes, this is down to moisture having penetrated the seals. Where glazing with failed seals is to be changed, it should be replaced with insulated glazing that is more energy-efficient. Take care to keep external ventilation holes on the bottom section of the frame free of snow or dirt, so as not to prevent air circulation between the outer and inner sashes.

External condensation – a sign of good insulation

Energy-efficient windows prevent the room heat from radiating to the outside, which means that the outer glazing is significantly colder than it would be on a less energy-effective window. This can produce effects under certain conditions during spring and autumn, on cold, cloudless nights, as the outer glazing then loses heat through radiation to the night sky. When the temperature on the outer glazing is lower than the outdoor air and also below the air’s dewpoint, and relative humidity is simultaneously high, condensation forms on the cold pane. If the temperature of the outer surface of the glazing drops below freezing, ice crystals may form on the pane.

External condensation is not harmful. It is merely a sign that the window has good insulating capacity. The condensation disappears late in the morning once the temperature increases under the warming effect of the sun.

The Technical Research Institute of Sweden (now RISE) has conducted an investigation of external condensation on energy-efficient windows based on climate data for Stockholm. The results show that the phenomenon generally occurs over the period from August to October, but can also occur at certain hours during the spring.