The reciprocal of thermal conductivity is 'thermal resistivity', usually expressed in kelvin-meters per watt (K⋅m⋅W−1). For a given thickness of a material, that particular construction's thermal resistance and the reciprocal property, thermal conductance, can be calculated. Unfortunately, there are differing definitions for these terms.
Thermal conductivity, k, often depends on temperature. Therefore, the definitions listed below make sense when the thermal conductivity is temperature independent. Otherwise a representative mean value has to be considered; for more, see the equations section below.