Last edited 01 Nov 2015

# Thermal indices for the built environment

Degree days. The number of days and degrees that the outside air temperature is above or below a specified base temperature. See Heating degree days.

Emissivity. The amount of long wave infra-red radiation which a surface of known temperature (such as the façade of a building) will emit to its surroundings.

g-value (sometimes referred to as window solar factor, solar factor or total energy transmittance (TET)). The fraction of incident solar radiation transmitted by a window, expressed as a number between 1 and 0, where 1 indicates the maximum possible solar heat gain, and zero, no solar heat gain. NB The difference between g-values and solar heat gain coefficients is that they use a different value for air mass.

k value (or lambda value λ). A measure of the intrinsic thermal conductivity of a material, that is, how easily heat passes across it. This is a fundamental property independent of thickness.

Psychometric charts. Graphs that can be used to assess the physical and thermodynamic properties of gas-vapour mixtures at a constant pressure.

R-value. A measure of the thermal resistance of a material of a specific thickness.

Shading coefficient. The amount of solar heat that passes through a transparent or translucent material compared to the amount of solar heat that passes through a sheet of clear float glass with a total solar heat gain coefficient of 0.87 (ie a sheet of clear float glass 3mm thick which has a shading coefficient of 1).

Solar heat gain coefficient. The fraction of incident solar radiation transmitted by a window, expressed as a number between 1 and 0, where 1 indicates the maximum possible solar heat gain, and zero, no solar heat gain. NB The difference between g-values and solar heat gain coefficients is that they use a different value for air mass.

Thermal comfort. ‘…that condition of mind which expresses satisfaction with the thermal environment’.

Thermal optical properties. Characteristic radiant behaviour within the thermal spectrum.

U-value (or heat transfer coefficient). A measure of the thermal conductivity of a complete building element, including its surface conductivities.