We often receive calls at this time of year from customers who are suprised and concerned to see external condensation on their double glazed windows.

This external condensation is actually a reassuring sign that your windows are doing what they are designed to do by keeping the heat indoors and the cold outside where it belongs.

With an increased focus on energy saving and current Building Regulations, external condensation is an inevitable side effect of low U-values in modern glazing because these double or triple glazed windows retain more heat inside the property than single glazed or older style double glazing. Moisture then condenses out of the air onto the outside pane when it is below the dew point. The dew point varies with the air temperature and the amount of moisture it contains. Not all panes are affected, such as where there is a tree, canopy or bush blocking or partially blocking the window from the clear night sky, which changes the surface temperature of the glass. Autumn and Spring are normal times to experience external condensation due to the colder night time temperatures but milder day time temperatures when the dew point can be high and the temperature of the glass falls to a low level during the night. Usually the condensation will subside quickly as the day warms up.

Condensation or misting to the inside surface of the window is more likely to be due to the humidity or moisture content of the inside air being very high, such as in a bathroom or kitchen and increasing the ventilation will help dispel this type of condensation.

Condensation between the panes of a double or triple glazed window may indicate a failed seal within the unit and will require the glass panel to be replaced.