The U-factor, or thermal transmittance, is a measure of the thermal conductivity for a building enclosure assembly (e.g., wall, floor, ceiling, roof) or a building component (e.g., window, door). Incorporating parallel and series conductive heat transfer principles and the R-value or thermal conductivities of each component in the assembly, the U-factor can be calculated. The U-factor, when multiplied by the surface area and the indoor-outdoor temperature difference, predicts the amount of heat loss through the assembly, which means assemblies with low U-factor are more energy efficient. U-factors are typically expressed as (Btu/hr•ft2•F) or (W/m2•C).