Understanding U-Factor & SHGC: The Definitive Science of Window Ratings

## Decoding the NFRC Energy Performance Label When comparing replacement windows, manufacturer marketing claims can be confusing. The National Fenestration Rating Council (NFRC) label provides certified, standardized test results based on strict laboratory thermodynamics. ### 1. U-Factor (Thermal Transmittance) - **What It Measures:** The rate at which heat flows through the entire window assembly (frame, glass, and spacer). - **Units:** $\text{BTU}/(\text{hr} \cdot \text{ft}^2 \cdot ^\circ\text{F})$. - **Benchmark:** Single-pane windows have a U-Factor around 1.0 to 1.2. High-performance Energy Star windows achieve U-Factors of 0.20 to 0.27. ### 2. Solar Heat Gain Coefficient (SHGC) - **What It Measures:** The percentage of solar infrared radiation passing through the window into the home. - **Benchmark:** Range from 0.00 to 1.00. In southern cooling climates, SHGC should be $\le 0.23$ to block summer heat. In northern heating climates, SHGC of 0.35+ captures passive solar warmth in winter. ### 3. Visible Transmittance (VT) - **What It Measures:** The amount of visible light spectrum entering the room. - **Benchmark:** Higher numbers (0.50 to 0.70) maximize natural daylighting, reducing artificial lighting electrical needs. Contact ServiceFind Windows Pro for certified NFRC-tested window installations and an upfront **Fair Quote**.