Energy efficiency windows are defined as windows that decrease the amount of warm or cool air leaving or entering the home. Windows are judged energy efficient when they have a low U factor, which is the factor for the amount of air that can penetrate the glass, and high R factors, the amount that the window is resistant to air flow.  Energy efficiency windows also reduce condensation and penetration of solar UV rays.  The level at which it does these things is the windows insulation value.

Single pane windows often have a high U factor and are poor insulation choices in extreme climates (very hot or very cold). Double pane windows reduce air flow and penetration and solar UV rays. Many manufacturers include new technologies that significantly reduce U factors called low E coatings. These coatings are available for single, double, triple and quadruple pane glass. Another technology being used is to fill the spaces between class on multi-paned widows with a more energy efficient gas than air. These are gases such as argon, krypton or carbon dioxide.  Gas fills only cost a small amount more than air fills and are most effective when used in conjunction with low E coatings.

Along with the number of glass panes in a window, E coatings, and fills, the overall insulation value of a window depends on its frame and spacers. The frame is what holds the window together, a window spacer refers to the component in a window that separates glazing panes. Spacers notoriously reduce insulation at the windows edges, and certain frames can cause a window to be significantly less efficient. The best energy efficient windows make use of this knowledge by offering better insulating frames than the standard metal frames. Frames such as wood, fiberglass or vinyl, or composite frames which are made from a combination of these materials including metal reduce air leakage. Spacers can be aluminum, steel, foam, fiberglass or a combination of these materials. Metal, again, is the least efficient spacer choice.

The NFRC, or, national fenestration rating council is a nonprofit rating service developed in the U.S. and Canada to rate windows for energy efficiency. Its entire purpose is to present buyers and contractors with non-biased, standardized methods of comparing various types of windows for cost and energy efficiency. All labels will have a U factor, a measure of the rate of solar heat flow through the window, a measure of visible light penetration and an air leakage rating. The measure of solar heat flow through the window will be important for reducing cooling costs in hot climates, and heating costs in cool climates by using or blocking solar rays. Visible light penetration will effect energy efficiency by reducing UV rays.

Energy efficient skylights and doors are also now available and will be rated according NFRC specifications. Proper installation of energy efficient windows can save you money in cooling and heating costs and reduce your overall carbon footprint.