Performance · 5 min read · Updated 2026
Condensation on windows: causes and cures
Three different problems get called condensation, and only one of them means the window has failed.
Inside the room
Warm, moist internal air meeting a cold surface. This is a ventilation and humidity issue, not a fault. Use extract ventilation, keep trickle vents open and avoid drying laundry on radiators.
On the outside pane
Counter-intuitively, this is a sign of a very good window. The outer pane stays cold because so little heat escapes through it, so dew forms. It clears with the morning sun.
Between the panes
This one is a failure. The sealed unit has lost its seal and moisture has entered the cavity. The unit needs replacing — often the glass alone, not the whole window, which is far cheaper.