U-Value (Thermal Transmittance)
Definition
The overall heat transfer coefficient, calculated as U = 1/R_total. It represents the rate of heat transfer per unit area per degree temperature difference through a complete building element or assembly.
Practical Note
Lower U-value means better insulation. Windows typically have U = 1.0-5.0 W/(m²·K). Well-insulated walls have U < 0.3 W/(m²·K).