Last edited 01 Nov 2015

Thermal indices for the built environment

This page presents a summary of thermal indices used in the construction industry. For more information go to the articles for each item. To add more items, click 'Edit this article' above.


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.

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