Low-E, the abbreviation for low emissivity glass, has an invisible, metallic coating that admits the full spectrum of sunlight but blocks radiant heat from escaping. During the winter months, between 70 and 75 percent of the heat that would otherwise escape from the house is reflected back into the home for energy savings. Because the coating also increases the inside window's surface temperature, areas near them are more comfortable on cold winter nights.
During the cooling season, as much as 25 percent of the unwanted heat that would otherwise enter the house is reflected to the outside. The low-E coating blocks ultraviolet light which would normally fade fabrics and other materials.
It is applied either to one of the inner surfaces of a sealed-double pane window or suspended between the panes on a thin piece of plastic glazing. Suspending it between the panes has the added advantage of raising the window's insulating value to that of a triple glazed window. Some low-E window manufacturers fill the air space between the glazing layers with Argon or other inert gases to further increase the insulating value.
For existing windows, low-E coatings are also available on films which can be applied to the inside surfaces. They are less common than the solar films designed solely to block sunlight.
Low-E windows can achieve R-values as high as R-5, a marked improvement over the R-1 single pane, or even R-2 double pane windows. Low-E windows cost more than standard windows and allow slightly less light to enter, but are often cost effective in extremely hot or cold climates.