WINDOW TECHNOLOGIES: Assembly
Glass Panes (Layers)
One of the shortcomings of glass is its relatively poor insulating qualities. Multiple panes of glass with air spaces in between improve the insulating value considerably (see figure to the right). Relative to all other glazing options, clear single glazing allows the highest transfer of solar energy while permitting the highest daylight transmission.
Double glazing reduces heat loss (as reflected by the U-factor) by more than 50% in comparison to single glazing. Although U-factor is reduced significantly, the VT and SHGC for a double-glazed unit with clear glass remain relatively high. Adding a low-E coating to a surface of the double-pane unit will increase the energy performance. Depending on the type of low-E coating, the SHGC and VT will also be affected. Adding a gas fill between the layers of glass will also improve energy performance.
As each additional pane of glass adds to the insulating value of the assembly, it also reduces the visible light transmission and the solar heat gain coefficient. Adding a low-E coating to a surface, or multiple surfaces, of the triple-pane unit will increase the energy performance. Depending on the type of low-E coating, the SHGC and VT will also be affected. Adding gas fills between the layers of glass will also improve energy performance.
Additional panes of glass increase the weight and thickness of the unit, which makes mounting and handling more difficult and transportation more expensive. There are physical and economic limits to the number of glass panes that can be added to a window assembly. However, multiple-pane units are not limited to glass assemblies.