If the temperature of the glass surface facing the room falls below the ‘dew point’ temperature for a given relative humidity of the surrounding air, water vapour from the atmosphere condenses into water droplets on its surface. The dew point depends upon the amount of water in the atmosphere and air temperature.

Condensation on the room side surface of the inner glass means that the temperature of the glass surface is too low given the water vapour content of the air in the room. This is most likely to occur on the surface of single glazed windows, but can happen on double or triple glazed windows if the room isn’t heated and adequately ventilated.

The occurrence of internal condensation can be reduced by removing excess moisture by ventilating the room. One way to do this is to open the window slightly or use the trickle vents, if fitted.

Upgrading the windows (e.g. from single glazing to double or triple glazing units with low emissivity glass such as Pilkington K Glass™ S or Pilkington Optitherm™ S1 Plus) can result in an increase in the temperature of the room-facing surface and therefore reducing the risk of condensation forming on that surface.

For more guidance on internal condensation, please refer to the Glass and Glazing Federation (GGF) publication, 'Condensation – some causes, some advice'.