under steady conditions, the mean temperature difference between two defined surfaces of material or construction that induces unit heat flow through unit area. Note: Thermal resistance and thermal conductance are reciprocals. Thermal resistances are R‑values; to obtain the U‑factor, overall thermal transmittance, the R‑value for either materials or constructions must first be evaluated because, by definition, the U‑factor is the reciprocal of the R‑value.