It is important when selecting a window or door to look for information that lists the products U-value & R-value. U-values illustrate the amount of heat that escapes through a wall, window or roof. You want to look for windows that have a low U-value because it means they are more energy efficient. R-values are the direct opposite, they portray an objects resistance to the ability for heat to flow. In essence the higher a materials R-value, the lower its U-value which means it loses less energy. R-values actually depend on the number of layers of glass a window has, what type of gas is between the glass layers and whether they have been treated with a Low-E coating.