Learn how U-factor, SHGC, R-value, and air infiltration ratings determine window energy efficiency. Interactive guide to reading NFRC labels and choosing the right performance specs for your climate.
U-factor measures how well a window prevents heat from escaping. It ranges from 0.20 to 1.20, with lower numbers indicating better insulation. For cold climates like Chicago, look for windows with a U-factor of 0.30 or lower. A window with a U-factor of 0.25 will lose roughly 40% less heat than one rated at 0.40.
U-factor and R-value are inverses of each other. U-factor measures heat transfer rate (lower is better), while R-value measures resistance to heat flow (higher is better). R-value equals 1 divided by U-factor. A window with a U-factor of 0.25 has an R-value of 4. The window industry primarily uses U-factor, while wall insulation typically uses R-value.
Solar Heat Gain Coefficient (SHGC) measures how much solar radiation passes through a window, rated from 0 to 1. A lower SHGC (0.25 or less) blocks more solar heat, reducing cooling costs in summer. A higher SHGC (0.40+) lets in more solar warmth, which can reduce heating costs in winter. The ideal SHGC depends on your climate and which direction the windows face.
Triple-pane windows offer a U-factor of 0.18-0.22, compared to 0.28-0.35 for double-pane. They reduce energy loss by 30-50% over double-pane and significantly decrease outside noise. In cold climates with harsh winters, the energy savings typically pay back the 15-25% price premium within 7-12 years, while also improving comfort by eliminating cold spots near windows.
The NFRC (National Fenestration Rating Council) label shows five key ratings: U-Factor (insulation), SHGC (solar heat gain), Visible Transmittance (natural light), Air Leakage (draft resistance), and Condensation Resistance. Compare the numbers to NFRC requirements for your climate zone to verify the window meets minimum efficiency standards.