Concepts are explained and the definitions of Thermal Resistance and Thermal Capacity, as well as the formulae for the calculation thereof, are provided.
Building elements, particularly those in the shell of a structure give protection from the outside environment, and extreme conditions such as, heat and cold. The measures of how effectively the shell of a building maintains an equable temperature for the benefit of occupants is in part gauged via the thermal resistance of the shell. In mathematical terms the Thermal Resistance of an element is the inverse of its Thermal Transmittance.
The Thermal Resistance of a building element or the materials used provide for the flow of heat (always from a hot to cold temperature) to be impeded. This impedance is measured in terms of the total R-value for a building element, which equates to the sum of the component R-values of the various materials, including the inner and outer air surfaces and any air-spaces that make up the composite building element.