About 10% of the energy used in U.S. buildings—approximately 4 quads per year—compensates for energy lost through windows. To address this inefficiency, architects, engineers, and home-builders are advocating the use of high-performance windows, which are composed of insulated glass units (IGUs). IGUs lessen unwanted heat gain by combining insulating frames and multiple panes, thin film coatings, and special gas fills between the panes, while selecting for other properties, such as transmittance of visible light and solar heat gain.