I had insulated, double-paned replacement windows put in my old home in Virginia. One of the things I do remember is that the dealer told me the way to make sure there really is argon gas between the two panes of glass is to light a lighter or hold a candle flame in front of the window and you should see a mirror images of the flame. I'm no window expert but it worked.
Here is info that might help
http://www.bestinclassnetwork.com/bl...ement-windows/