All major standards and programs for window energy efficiency — including building energy codes, tax credits and utility incentives, ENERGY STAR, and others — base their criteria on certified ratings by the National Fenestration Rating Council (NFRC). To ensure that their products are recognized, manufacturers must therefore participate in the NFRC certification and labeling program.
The NFRC label provides the only reliable way to determine the window energy properties and to compare products. The NFRC label appears on all products certified to the NFRC standards and on all window, door, and skylight products which are part of the ENERGY STAR program. At this time, NFRC labels on window units give ratings for U-factor, Solar Heat Gain Coefficient (SHGC), Visible Light Transmittance (VT), and optionally Air Leakage (AL) and Condensation Resistance (CR) ratings.