When you think about it, it is two pieces of glass separated by a 5/8" - 7/8" aluminum or stainless bar around the perimeter, then sealed air tight. When the sun hits the glass, the air inside has to heat and expand causing positive air pressure thus putting some stress on the individual panes, and going the other way on a really cold night to create a negative air pressure between the glass causing them to draw towards one another. I'm sure there are certain specifications on glasses ability to flex under these conditions. Just kind of interesting when you think about it. [img]/forums/images/icons/tongue.gif[/img] [img]/forums/images/icons/tongue.gif[/img]