Short Answer
U-factor measures the rate of heat transfer through a window assembly. Lower values generally indicate better insulating performance.
Detailed Explanation
U-factor (also called U-value) is a measure of how well a window insulates — specifically, how much heat transfers through the window assembly (glass, frame, and spacers) per square foot per hour per degree of temperature difference. The lower the U-factor, the better the window insulates. U-factor is the primary metric for comparing window energy performance, particularly in heating-dominated climates. ENERGY STAR qualified windows have specific U-factor requirements that vary by climate zone. In South Jersey (a mixed climate), both U-factor (for heating performance) and SHGC (for cooling performance) are important.
Why It Matters
U-factor is the key metric for comparing window insulation performance. Lower is better, and ENERGY STAR provides benchmarks for each climate zone.