To determine the overall coefficient of heat transfer (U-value, or the inverse of RSI value) and the overall solar heat gain coefficient (SHGC) of windows, the CSA A440.2, NFRC 100, and NFRC 200 test standards specify testing using standard‑sized windows. The overall U-value and SHGC for the tested product are then applied to all windows of the same product category regardless of size. This is done to facilitate the comparison of thermal performance for different window designs.

The overall U-value and overall SGHC of windows with dimensions that do not match those in the standards are adjusted by HOT2000 to compensate for size effects.

NOTE: The minimum thermal performance specifications required by the National Building Code apply to standard‑sized windows.