Low-emissivity glass is a non-visible coating that improves the energy efficiency of a thermal glass unit or window. It reflects light passing through the glass, lowering the U-value while increasing the R-value. The amount of heat that escapes through the window is used to calculate the U-value; lowering the amount of heat that escapes increases energy efficiency through addition by subtraction. The R-value is determined by several factors, including the number of glass panes present, the type of gas used between window panes, and whether or not the insulated windows have been coated with Low-E.