Condensation on double glazed windows can occur due to a temperature difference between the glass and the air inside the room. Warm air inside the room can enter the gap between the glass panes if the sealant around the double glazing has failed, and when this warm air comes in contact with the cold air between the panes, it condenses. This is the same as condensation on windows without double glazing, but in double glazing, the windows are sealed, so there is nowhere for the moisture to escape to. The appearance of condensation between the panes of a double glazed unit indicates that the windows are not doing their job properly, and the air-tight seal holding the insulating gas inside has failed, letting all the insulating gas out. This means that water vapor can get between the two panes, and the double glazing can collect moisture, which can lead to further damage to the window while wasting energy as you attempt to heat your home and replace the heat being lost.