How do I know it's Argon filled glass?

Given some time in the lab, you could probably do it by measuring the speed of sound through that window compared with a known argon-filled identical window.

However, it's not worth doing, because argon fill makes only the tiniest difference in terms of window performance anyway.

I suspect spectroscopy could also get you there, or focussing a laser beam through the glass, to make an ionised spot, and doing spectra on that.

I don't understand why CO2 isn't used. Slowly reacts with aluminium in the presence of water?

There's no easy way I know... ask the company, they will tell you what you ordered/got - if you did not order the windows "argon filled", they'll probably have ordinary air in 'em. Won't make that a discernable difference IMO.

1. As others have said - no easy method of checking.

1. The units will not be argon "filled" anyway. The injection process does not remove all the air from the unit so it contains air and argon.

2. A change of curtain material would have more effect on heat transfer than the difference between air and argon (not)filled units.

Actually if you look for the injection holes you will see if they are argon filled. Basic double glazed windows are made with the normal spacer bars. Those that are intended to be argon filled will have two rubber bungs inserted into the spacer bar. Double glazed window is made as normal. Then using the rubber bungs probes are inserted and argon process is done. I was under the impression that the process reduced the u value by 0.1.

Good point - but you would have to de-glaze the window to do the check.

The industry standard target for Argon fill is 90% of the available volume. This is rarely reached on the average production line.

This from FENSA:

"Comment:

It should be noted that there is a barely tangible difference between the performance of air filled and argon filled double glazed units.

Because the process of filling double glazed units is not entirely accurately calibrated and that the process generally involves drilling and re-filling of holes or corners to the double glazed unit, leakage is more likely than an air filled double glazed unit.

It is therefore recommended that replacement windows have a gap of 16mm or more and that low E glass is used in a conventional air-filled double glazed unit."

Tests by Pilkington have shown that, although the life of an Argon unit could be up to 20 years, test units have leaked all gas in as little as 3 years.