It’s getting to that time of year again folks. Nope, not the nights drawing in, though that is a big down side to this time of year. I’m referring to the bane that is external condensation of energy efficient windows.

I’m pretty sure that every installations company has had a call in the past couple of years from a customer either wondering what all that condensation is on their south facing windows or they’re just very disgruntled and accusing you of not telling them that this would happen. To combat the latter point, at this time of year I do point out that during certain temperature differences between the house and externally, and only at certain times of the year, you may get condensation on the outside of the outer pane. This seems to work out fine with all my customers so far.

But what do you say to that customer that is irate down the phone? Well, not much really. The whole reason why A rated windows get external condensation is that the insulated glass and everything else works so well at keeping the heat in, the only place condensation can form is on the outside. This is how Pilkington puts in their much more technical fashion with lots of words: The phenomenon is a natural and predictable event caused by the outer pane of the glazing being colder that the glass that it replaced.  With single glazing and older style double glazing a larger proportion of heat was lost to the outside through the glass.  With modern low e glass products more of the heat is kept inside and the outer pane is not heated as much.  Moisture condenses out of the air onto a cold surface that is said to be below the dew point.  The dew point varies with the air temperature and the amount of moisture it contains.  In spring and autumn in particular the glass temperature can fall to a low level during the night and the dew point can be comparatively high in these seasons.  The glass is more often likely to be below the dew point in these conditions and the moisture condenses onto the surface.

Still, try and explain that to a customer and you might get a sharp response back! The key to this problem is preempting it. By explaining this phenomenon to a client before they go ahead gets rid of the possibility that they’re going to be ringing you back in later months complaining of this problem. Like I’ve said, everyone I’ve explained this to has been happy to expect this once in a while, therefore we don’t get any nasty phone calls!

There’s nothing we as an industry can do about this really, unless we start changing the spec of our windows to an even higher grade. I heard somewhere that if you use two layers of Low ‘E’ you could eradicate it. Not sure if this is proven or not. Probably not worth looking into. But the current regulations mean we have to fit energy efficient windows and there’s nothing we can do about it. So if you explain this phenomenon to a potential customer and they’re not willing to accept it, then is this the sort of business you want anyway?