Today was the first day of September, which from a meteorological perspective is the first day of Autumn. That’s come round pretty quick! Those in the window industry will also know that this is around the time where external condensation on energy efficient windows becomes a bit more common. Yes, it’s also that season again.

Managing expectations

The phenomenon of external condensation became apparent after the rise in popularity of A rated windows in the late 00’s. Then it became even more apparent when WERs were introduced and even more energy efficient windows were being installed.

Not sure on how external condensation forms? This is how Saint-Gobain explains it:

Surface condensation will appear on face 1 of the insulating glazing if the temperature on this face of the glazing is significantly lower than the external air temperature and if the dew point (i.e. temperature at which water vapour becomes liquid) of the external air is higher than the temperature of the glass.

The surface temperature on the outside of glazing is dependent on:

-The heat flow from the interior passing through the glass. This depends on the difference in temperature between the internal surface and the external surface of the glazing and the U-value of the glass

-The heat exchange by convection with the external air

-Heat loss by radiation mainly to the sky.

Various studies and measurements carried out by the CSTC have shown that heat exchange by radiation is relatively limited in overcast weather. However, when the sky is clear at night, there are significant heat losses to the sky.

Pretty through explanation as I’m sure you’ll agree. But it is a natural phenomenon, and physical proof for home owners that their windows are working.

But installers should be making people aware of this from the outset. It’s a known issue now to the industry, and it is only right that home owners be made aware that when the climate is like that described above, condensation on certain windows in their home should be expected.


For most, this won’t be that much of an issue. A hour or two of sun, a bit of a breeze and it will all have gone. But there will be those who will want a solution to what they might consider a problem.

Coated glasses seem to do the trick. Pilkington say that their self-cleaning Activ glass is less prone to external condensation:

Those people who have fitted Pilkington Activ™ self-cleaning glass may also notice that they get fewer occurrences.  Pilkington Activ™ is as prone to condensation as any other glass, but the properties of this product means it does not allow the water to form beads on the surface of the glass. So you tend not see the effect to the same degree.

So the trick here being the coating limiting the effects of external condensation, but not eradicating completely. Better than nothing I guess. I hear that Ritec’s ClearShield EcoSystem product is pretty good at stopping external condensation.

Of course these coated glasses come at a cost. Whether home owners choose these upgrades will depend on how big they see external condensation as a problem.

In reality, external condensation really only occurs during Spring and Autumn, and only when the external air temperature allows it. The very least the industry should be doing is to explain the phenomenon to home owners so that disgruntled phone calls are kept to a minimum.

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