Why Do Double Glazed Units Fail ??????

[Gizmonster]

As a window cleaner, I regularly see failed units and was wondering how and why they fail. I know about the misting up of the unit inside and the seal failing, etc ... but does anyone know why they fail ?

I saw a builder the other day and he reckoned that large/heavy vehicles can cause vibrations, that over time could cause units to fail ...... maybe ...... it sounds plausible ?

I have a friend who's a joiner/window fitter and he reckons that if the glazed unit isn't placed on spacers when installed, then pools of water/moisture can gather, which, over time can erode the seal, causing it to fail ..... another possible answer maybe ?

I was cleaning some windows the other day and they'd only been in a few months and for want of a better word, they were goosed. How come some seem to last for years and some (as in this case) only a few months ?

What gives ?

[pdq1]

There could be many possible reasons but my slant on it is due to expansion. Each metal or material has a different coefficient of expansion and glass held so securely does not allow for these small amounts. Obviously the hotter the weather the more the glass will expand relative to other fixtures.

http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

[bookbinder]

Yes, but, for heaven's sake. double glazing isn't exactly a new thing, is it? Why can't window makers get it right first time, after all these years?

[seadogg]

A pal who runs a d.g. firm thinks its the windows in full sun that seem to be worse. That fits your comment pdq

[Gizmonster]

With regards to the windows in the sun, I agree to an extent, but I think it's the sun/heat that causes the condensation to be worse on sunny days, possibly making them appear to be worse than an equivalent window in the shade.
I clean windows on a daily basis and a failed window in the sun appears to look a lot worse on a hot sunny day, than the same window on a cloudy/cold day.

[Wak]

I reckon it's about time that they started putting those magnetic glass cleaners on the inside of the sealed units, like the ones you can use to clean the fish tank but dragging another magnet along the outside of the glass.
But I guess it never will happen as it would reduce sales of new units.

[johnk]

On a new unit the cavity is a vacuum so eventually the seal goes and air that contains a small amount of moisture enters and when cold enough the water vapour condenses on the coldest part and mists up the inside. The vacuum is also a good insulator so when it's gone the glazing is not so efficient.