Window Glazing Options
"Glazing" refers to the glass and the glass construction used in a window and it impacts both the look and energy efficiency of a window.
Double-Glazing is pretty much the standard for windows sold today. They're made by combining two layers of glass with a space between them to form a single window pane. That greatly increases the window's insulating value, as does the use of Argon Gas or other inert gases in the space between the layers of glass. A Low-E Coating is another option for keeping heat in or out (depending on the season) and for reducing ultraviolet penetration which can fade carpets and fabrics.
Triple-Glazing adds a third layer of glass to the glazing mix to increase the insulating value of the window. Despite that advantage it's less common because of the weight it adds to the window.
Safety Glass or Tempered Glass is required by code in certain applications, like in windows above bath tubs or where there is a danger that someone might accidentally walk through the glass.
Grilles are used to achieve the look of a divided lite window. They usually fit between a sash's rails and stiles, right over the interior surface of the glass, and they're generally removable. Grilles can also be glued to the glass's interior and exterior surfaces at the factory for a more realistic divided-lite appearance.
True Divided Lite Windows have individual sections of glass mounted in a single sash and separated by muntins. It's a more traditional style that was actually the cheaper alternative in the days when large sheets of glass were far more expensive than small ones. Nowadays it's a more expensive option because of the millwork involved.
Combination Storm Windows consist of a fixed glass sash mounted over a fixed screen sash, with a second operable glass sash in its own track to let fresh air in. Storm windows add insulating value to older windows but are somewhat redundant when used with energy-efficient windows.
Window Energy Ratings
The U-Factor is the most useful way to rate a window's energy efficiency. It's a measure of the tendency to conduct heat of an entire window (the glass AND the frame and sash components). The lower the U-Factor, the better insulating ability a unit has.
U-Factors are determined by the National Fenestration Rating Council, and the numbers should be indicated on an NFRC label on every window.
An efficient window may have a U-Factor of .35 or less. Less efficient units would have a higher U-Factor.