Thermal transmittance (U or U-factor)

The heat transmission in unit time through unit area of a material or construction and the boundary air films induced by unit temperature difference between the environments on each side. In English inch xx pound units, it is expressed as Btu/h x ft2 x F.
Note 1: A thermal transmittance (U) value applies to the overall thermal performance of a system e.g. roof assembly.

Note 2: Thermal transmittance (U) is sometimes called the overall coefficient of heat transfer.

Note 3: Thermal transmittance (U) is the reciprocal of the overall thermal resistance (Rt) of a system i.e. U = 1/Rt.

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