To compare energy performance of different windows, which metric would you examine first?

• A. U-value
• B. Window color
• C. Exterior wall thickness
• D. Roof pitch

Answer: (A)

When comparing energy performance of windows, the most important factor to look at first is the U-value, or thermal transmittance. This metric directly measures how much heat passes through the window per unit area for a given temperature difference, so a lower U-value means better insulation and less unwanted heat loss in winter and heat gain in summer. While other aspects like window color, wall thickness, or roof pitch can influence overall building performance in related ways, they do not quantify a window’s insulating effectiveness as the U-value does. In practice, you start with the U-value to judge energy performance, and then consider other factors like solar heat gain if climate or shading concerns are relevant.