U-factor measures the rate of heat transfer through the complete window assembly — frame, spacer, and glass together. A lower U-factor means better thermal insulation. Reference points: single-pane window approximately 1.0; standard double-pane vinyl approximately 0.45–0.55; double-pane with low-e coating and argon fill approximately 0.28–0.35; triple-pane fiberglass with dual low-e and krypton approximately 0.14–0.22. IECC Zone 5 requires a maximum U-factor of 0.30. Solar Heat Gain Coefficient (SHGC) measures the fraction of incident solar radiation that passes through the window as heat gain — a value from 0 to 1, with lower meaning less solar heat admitted. For Utah orientation-specific design: south-facing windows with SHGC 0.35–0.45 deliver measurable passive solar heating benefit in Utah’s sunny winters; west-facing windows at SHGC 0.25–0.30 control afternoon summer overheating. Both values appear on the NFRC certification label — that label is the only reliable data source.