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Exterior Condensation – what is it any why does it happen?

Cross section diagram of triple glazing

The science

We have recently been informed by the BWF, that some manufacturers have been experiencing the phenomenon of exterior condensation on newly installed windows and doors with highly energy efficient glazing. A householder waking to new windows obscured by condensation may think that the glazing units have failed; in fact, this is  an indication of the efficiency of the glass and not a fault. The condensation will disappear naturally as the outside conditions change and the surface temperature of the glass increases.

Why does this happen?

In a nutshell; the higher insulating properties of the improved glazing units reduce the amount of heat escaping from the property and the external face of glass may not be warmed to a temperature above the “dew-point” of the outside air and thus condensation occurs.

Predicting when windows will be susceptible to external condensation is difficult, and if it does occur, it typically will not occur on all windows in a property. The cause seems to be related to meeting a combination of three conditions; high humidity, no wind and a low overnight temperature.

Until recently, most double and triple glazing used Aluminium spacer bars.  Heat conducted through the spacer bar would warm the edges of the glass and reduce external condensation. Where the thermal properties of the glazing units are enhanced with the use of a “warm-edge” spacer bar, this effect is reduced and condensation becomes more common.

Why not reduce the glass U-values to prevent this?

The changes to the Building Regulations that have increased the requirements for the thermal performance of doors and windows came into effect in 2010. Replacement windows now have to meet a U-value of 1.6 W/m2K, and replacement door-sets to have to reach 1.8 W/m2K in England and Wales, and 1.6 W/m2K in Scotland.

In order for a timber window to meet these standards, glazing typically has to be 1.2W/m2K or lower.

In Summary

If you experience condensation on the external face of your glass, then don’t be alarmed, it is a sign of the high energy efficiency of the glass and the condensation will disappear quickly as the conditions change during the day. However, if you are getting condensation on an internal face of your double or triple glazing (inside the unit and between the panes of glass), then the unit may have broken down and you should contact your manufacturer/installer for replacement.

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