Glossary of Terms

Thermal Transmittance
Thermal transmittance (U-value) defines the ability of an element of structure, consisting of given thicknesses of material, air spaces etc. to transmit heat under steady state conditions. It is a measure of the quantity of heat that will flow through unit area in unit time, per unit difference of temperature of the individual environments between which the structure intervenes, being calculated as the reciprocal of the sum of the resistances of each component of the structure, including the resistance contributed by inner and outer surfaces and by any air spaces or cavities. Its units are W/m²K. When dealing with thermal insulation, the difference between common terms should be appreciated.

Surface Resistance
Surface resistance (R_s) is the reciprocal of surface coefficient. Its units are m²K/W.

Thermal Resistance
Since the primary purpose of thermal insulation is to frustrate the flow of heat, it is both appropriate and convenient to measure performance directly in terms of a material’s thermal resistance (R-value) which is obtained by dividing thickness in metres by thermal conductivity in W/mK, the result being expressed in m²K/W. Being additive, thermal resistances facilitate the computation of overall transmittance values (U-values).

Thermal Conductance
In contrast, thermal conductance (C) defines a material's ability to transmit heat measured in watts per square metre of surface area for a temperature gradient of one Kelvin in terms of a specific thickness expressed in metres. Its units are W/m²K. It is to be noted that, where a structure incorporates a number of component materials, airspaces etc. individual conductance values cannot be added directly for the purpose of calculating an overall rate of heat transfer without the necessity for first deriving reciprocal values.

Thermal Conductivity
Thermal conductivity (lambda value) defines a material's ability to transmit heat being measured in watts per square metre of surface area for a temperature gradient of one Kelvin (K) per unit thickness of one metre. W/mK.

Surface Coefficient
The surface coefficient (f) is the rate of heat transfer from a surface to the surrounding air (or fluid) due to conduction, convection and radiation. It is generally used only in still air conditions and when the temperature difference between surface and ambient is in the order of 30 K. It is obtained by dividing the thermal transmission per unit area in W/m² by the temperature difference between the surface and the surrounding air. Its units are W/m²K.

Temperature
For the purpose of ready identification, actual temperature levels are expressed in degrees Celsius (°C) whilst temperature difference (interval or gradient) is expressed in Kelvins (K).

Heat
The unit of quantity of heat is the joule (J). Heat flow may be expressed as joules per second (J/s), but as a heat flow of one Joule per second equals one Watt, the unit Watt (W) is adopted for practical purposes in calculating U-values.