Which metric measures the overall rate of heat transfer through window components?

• A. U-value
• B. Fenestration
• C. SHGC
• D. Air leakage rate

Answer: (A)

Understanding how much heat moves through a window assembly is about its thermal resistance. The U-value measures the overall rate at which heat transfers through the entire window—through glass, frame, and seals—per unit area for each degree of temperature difference between inside and outside. A lower U-value means better insulation and less heat loss in winter (and less heat gain in summer).

Other terms describe different aspects: Fenestration is simply the term for the window opening itself, not a measuring rate. SHGC (Solar Heat Gain Coefficient) indicates how much solar radiation the window allows to pass through, which affects cooling loads, but not the total heat transfer due to conduction and convection. Air leakage rate measures how much air sneaks through gaps around the window, contributing to infiltration losses but not the conductive heat transfer through the window components.

So the metric that captures the overall rate of heat transfer through the window components is the U-value.