What does U-value measure in window assemblies?

What does U-value measure in window assemblies?

• A. Overall heat transfer through window components
• B. Thermal resistance of insulation
• C. Amount of sunlight transmitted
• D. Air leakage rate through openings

Answer: (A)

U-value measures how much heat passes through the entire window assembly for each degree of temperature difference between indoors and outdoors. It captures the combined heat transfer through glass, frame, spacers, and any gas fills, representing overall heat loss or gain. The lower the U-value, the better the window insulates. U-value is the reciprocal of the total thermal resistance (R-value) of the window assembly, with common units in W/m^2·K (metric) or Btu/h·ft^2·°F (imperial). Other properties like how much sunlight gets through or the rate of air leakage are different metrics and not what U-value measures.

U-value measures how much heat passes through the entire window assembly for each degree of temperature difference between indoors and outdoors. It captures the combined heat transfer through glass, frame, spacers, and any gas fills, representing overall heat loss or gain. The lower the U-value, the better the window insulates. U-value is the reciprocal of the total thermal resistance (R-value) of the window assembly, with common units in W/m^2·K (metric) or Btu/h·ft^2·°F (imperial). Other properties like how much sunlight gets through or the rate of air leakage are different metrics and not what U-value measures.