Condensation on aluminium windows

Aluminium has a reputation for condensation that it earned honestly and no longer deserves. The distinction is the thermal break, and the diagnosis starts with which side of the glass the water is on.

Read the side

Inside face, room side. Warm humid indoor air meeting a cold surface. This is a humidity and ventilation issue, not a broken window — though it does tell you that surface is cold.

Between the panes. The sealed unit has failed. Nothing will dry it out and the unit needs replacing. The frame is almost certainly fine.

Outside face, in the morning. Entirely normal, and a good sign — it means so little heat is escaping that the outer pane has dropped to dew point. Modern efficient windows do this.

Why old aluminium streamed with water

Pre-1990s aluminium windows were a single continuous piece of metal from outside to inside. Aluminium conducts heat extremely well, so the inner frame face sat at nearly outdoor temperature and ran with condensation all winter. That is the reputation, and it was deserved.

What a thermal break does

A modern frame is two aluminium sections separated by a polyamide strip — an insulator. The cold outer half no longer conducts to the warm inner half. The inner face stays close to room temperature and does not condense. Whole-window U-values land around 1.2 to 1.4 W/m²K, meeting Part L.

If you have aluminium windows streaming with water, check whether there is a thermal break. If there is not, the frames are pre-1990s and the condensation is not going to be fixed by anything short of replacing them.

Fixing room-side condensation

The water is coming from inside the house — showering, cooking, drying laundry indoors, and simply breathing. Ventilate at source with extract fans that actually run long enough, keep trickle vents open rather than taped shut, do not dry washing on radiators, and keep a background temperature instead of short intense bursts of heat. Reducing the humidity is the fix. Wiping the glass is not.