Condensation forms when moist air meets a surface cold enough to turn the vapour back into water. On a window, where it appears tells you almost everything.
On the room side of the glass
This is a sign of too much moisture in the air inside the house, not of a faulty window. A family of four puts several litres of water into the air every day through cooking, showers, drying clothes and simply breathing. If it cannot escape, it settles on the coolest surface it can find.
- Keep trickle vents open, especially in bedrooms overnight
- Use extractor fans when cooking and bathing, and leave them running for a while afterwards
- Put lids on pans and close kitchen and bathroom doors
- Dry washing outdoors or in a ventilated room where you can
- Keep the heating at a low, steady level in cold weather so surfaces never get very cold
New A-rated windows help, because the inner pane stays warmer. But they also seal the house more tightly, which is why ventilation matters more after new windows than before.
On the outside of the glass
Dew on the outside on a still, clear morning is good news. It means so little heat is escaping through the glass that the outer pane has cooled below the dew point of the air outside. It appears in spring and autumn, clears as the day warms up, and is evidence that the window is doing its job.
Between the panes
Mist or droplets inside the sealed unit, where you cannot wipe them away, mean the edge seal has failed and moist air has got in. The insulating gas has gone with it. The unit will not recover and should be replaced.
The frame is usually fine. In most cases we can supply and fit a new sealed unit into the existing frame, which costs far less than a new window. Our sealed units are guaranteed against this kind of failure for five years.
