You should beware of any company offering cheap triple glazed vinyl windows. Often they do this by stuffing three panes of glass into a vinyl extrusion intended for a double glazed window. They do this by having 1/4 inch of space between the panes rather than the normal and recommended 1/2 to 5/8 inches of space between panes.
The space between the panes of glass on a window IS IMPORTANT. It's the air space between the panes that determines the insulating ability of the window, and three panes of glass separated by 1/4 inch air spaces won't provide anywhere near the insulating ability that three panes with 1/2 to 5/8 of air space between the panes will.
Consequently, if you come across anyone offering triple glazed windows surprisingly cheap, check the air space between the panes. They may be using less expensive PVC extrusions meant for double glazed windows, and just stuffing three panes in the space typically needed for two panes.
Homeowners often fall for this scam because they presume triple glazed windows are all the same. Triple glazed sealed units stuffed into PVC extrusions designed for double glazed sealed units won't provide appreciably better energy efficiency than double glazed units because it's the air spaces that provide the insulating ability of the window, not the amount of glass the heat has to flow through. Glass, like all masonary materials, has a very low R value. It's the trapped air between the panes that provides all the R value for windows.
Last edited by Nestor_Kelebay; 05-01-2009 at 01:54 AM.