According to the NFRC, what is another performance factor for windows besides U-factor?

The correct choice, solar heat gain coefficient (SHGC), is a crucial performance factor for windows that helps determine how much solar radiation is transmitted through the window. SHGC quantifies the proportion of incoming solar energy that comes through the window, which directly affects indoor comfort and energy efficiency. A lower SHGC indicates reduced solar heat gain, which can be beneficial in warmer climates to reduce cooling loads.

Understanding SHGC is important for maximizing energy performance in a building, particularly in achieving LEED certification, as it relates to regulating indoor temperatures and minimizing reliance on mechanical cooling systems. A window's U-factor, which measures how well it insulates, is only part of the overall performance profile. Employing both U-factor and SHGC together allows for a more comprehensive evaluation of a window's efficiency in different environmental conditions.

Other performance factors such as visible transmittance, condensation resistance, and air leakage are also important considerations but do not hold the same significance as SHGC in the context of the NFRC's overall assessment framework for window performance.