Background

has gained widespread participation by window manufacturers throughout the United States as a means to identify and market high performance residential window, skylight, and door products1. ENERGY STAR qualifying criteria are based on the two main thermal indices typically used to quantify the energy performance of window and other fenestration products: the U-factor and the Solar Heat Gain Coefficient (SHGC)2. These properties respectively quantify heat loss (or gain) as a function of a temperature difference and the fraction of solar energy transmitted by the window. Complete product U-factors and SHGCs are required to be calculated in accordance with the procedures developed by the National Fenestration Rating Council (NFRC), a non-profit group representing a diverse set of stakeholders in the window and building industry. U-factors are ideal for quantifying heat flow through a window at a specific set of conditions (i.e. outdoor temperatures, indoor temperature, wind speed). Similarly, 1 Throughout this white paper, the term windows will be used to refer to all fenestration products including doors and skylights. Proposed tradeoffs are presented only for windows; ultimately, alternative tradeoffs will have to be prepared for skylights and doors separately. 2 In this paper, references to ENERGY STAR are references to the ENERGY STAR Windows criteria