subject: Understanding Energy Efficient Windows [print this page] Understanding Energy Efficient Windows Understanding Energy Efficient Windows
High-performance windows can be a valuable asset for both new and existing homes. Whether you're building a new home or considering a remodel or retrofit project, understanding energy efficient window ratings and metrics starts you down the right path. The National Fenestration Rating Council (NFRC) is a non-profit organization that provides third-party ratings for the energy performance of windows and doors. The NFRC label provides a set of measurements that help prospective buyers understand the performance characteristics of a window. The NFRC label includes the following data: U-Factor, Solar Heat Gain Coefficient (SHGC), Visible Transmittance (VT), Air Leakage (AL) and Condensation Resistance (CR). U-Factor and R-ValueU-factor and R-value both reflect the insulating power of a window, in other words, how easily heat is transferred through it. A lower U-factor corresponds to better-insulating windows. The R-value is the inverse of the U-factor. Both U-factors and R-values can be given for either full frame or center of glass (COG). It is important to focus on the full frame U-factor or R-value because the weakest insulation point of a window is usually the frame. A standard, single-pane window has an R-value of 1 (U-factor of 1), and a clear glass, dual-pane window has an R-value of 2 (U-factor of 0.5). Super-insulating windows with low-e coating, suspended film technology and inert gas fills should have R-values between 5 and 11 (U-factors between 0.2 and 0.09). Solar Heat Gain CoefficientSHGC measures the percent of solar radiation that enters a building through a window in the form of heat gain. SGHC is always given as a number between 0 and 1. The key to understanding the importance of SHGC is the concept of tuning. Tuning means customizing the glazing of a window based on the orientation of a building in order to optimize comfort and efficiency. Installing windows with high SHGC in your home where they receive direct sunlight in the winter, but are shaded in the summer allows the sun to passively heat your home in the winter without overheating in the summer. Prospective window buyers should look for product lines that offer both high and low SHGCs. In high-performance windows, high SHGC usually ranges from 0.33 to 0.50, and low SHGC ranges from 0.17 to 0.27. Visible TransmittanceVT refers to the percentage of light in the visible light spectrum that a window lets in. VT is a measurement between 0 and 1. A high VT score indicates a window that allows a large amount of daylight into a room. High-performance windows feature VT ranging from 0.29 to 0.65. The key point to think about when evaluating a window in terms of VT is that the coatings that achieve low SHGC often lower the VT as well. So, look for a good balance between high VT and low SHGC. Air LeakageAL describes the amount of air that infiltrates through unsealed points in the window assembly. AL is measured in terms the volume (cubic feet) of air that leaks through the window assembly per minute per square foot of window surface (cfm/sq ft). A standard window should have an air leakage rate of 0.3 cfm/sq ft. However, a high-performance window can achieve AL as low as 0.04 cfm/sq ft. According to ENERGY STAR figures, AL accounts for 25-40% of heating and cooling energy in homes so lowering AL can dramatically affect energy consumption. Condensation ResistanceCR is a rating system that evaluates the tendency of a product to resist the formation of condensation on the inner surface. The CR potential of a high-performance window is measured by a score between 0 and 100, higher numbers corresponding to a better product. Condensation on the interior surfaces of windows can obscure visibility, contribute to the growth of mold and mildew, and cause damage to the window and frame. The CR rating of a window depends both on the frame type and the glazing. Windows with aluminum frames and clear glass perform in the lowest range (10-48), with low-e coatings and wood or vinyl frames performance range improves (45-65). With fiberglass frames and low-e coatings even superior CR is possible. When shopping for energy-efficient windows for new or existing construction consider the particularities of your building location and climate and select a product that offers the best combination of the above factors.
