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U-FACTOR

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•ft^{2}•F)
or (W/m^{2}•C).