Condensation on the outside

Leo Pyrah, product manager at Pilkington UK, reveals why external condensation can appear on new windows. Got a question? Head to to find the answer.

Are new windows faulty if condensation appears on the external glass pane?

External condensation occurs naturally and isn’t a fault in the glass or window itself. In order to save energy, maintain a comfortable internal environment and meet building regulations, windows are now much more thermally efficient than they have previously been.

These new, high performance windows are preventing heat loss and when external condensation appears it’s actually an indication that the windows are working as they should.

External condensation happens because of a natural phenomenon called dew point, and is more likely to occur in the spring and autumn.

This is the temperature at which a concentration of water vapour in the air will form dew. If air cools down sufficiently to the dew point, it can no longer hold the moisture, forcing the water vapour to condense.

The temperature of the outer pane of a window can fall to a low level during the night, and the dew point can be comparatively high. When warm, moist air from outside comes into contact with a cooler external glass pane, the excess moisture in the air condenses. As a result, the external glass pane presents the perfect place for water vapour from the atmosphere to condense into water droplets on its surface.

While condensation doesn’t usually stick around for long, there is little that can be done to avoid it appearing on the outside of a window entirely. The heat from the sun or a gentle breeze will help the moisture evaporate.

Choosing a product with a second low-emissivity glass, which is designed to delay the onset of external condensation, will help reduce the likelihood of condensation appearing. Pilkington Anti-condensation Glass is designed to do just that. This particular product can be used in combination with other energy-efficient ranges such as Pilkington K Glass and Pilkington Optitherm to create the optimal glass solution for any requirement.