Thermal resistance (R)

The stress-producing phenomenon resulting from sudden temperature changes in a roof membrane when, for example, a cold rain shower follows brilliant sunshine.
Stress introduced by uniform or non-uniform temperature change in a structure or material that is contained against expansion or contraction.
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 x 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.

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