Condensation

Condensation occurs when warm, moisture-laden air comes into contact with a colder surface. The water vapour in the air condenses on the surface as droplets of water.

Dew point

The dew point is the temperature at which the air becomes saturated with water vapour. The higher the relative humidity, the higher the dew point.

Condensation on glazing

There are three locations where condensation can occur on glazing:

Internal surface (room side) — caused by high indoor humidity combined with a cold glass surface. Can be reduced by improving the thermal insulation of the glazing (lower U-value) and ventilating the room.
External surface (outside) — can occur on very high performance glass where the outer pane is so well insulated from the warm interior that its temperature falls below the dew point of the outside air. This is a sign of very good thermal performance and is temporary.
Within the cavity of a double-glazed unit — indicates a failure of the hermetic seal. The unit should be replaced.

Reducing condensation

Specify higher-performance insulating glass (lower U-value).
Increase ventilation to reduce indoor humidity.
Avoid placing furniture or heavy curtains directly in front of windows, which restrict airflow over the glass surface.
Where possible, maintain a constant room temperature, since fluctuations encourage condensation.