G-value in buildings

Understanding the solar transmittance through translucent and transparent materials such as glass is important for determining the solar heat gain into the space they enclose during sunny conditions. Solar heat gain can be beneficial in the winter, as it reduces the need for heating, but in the summer can cause overheating.

The total solar heat transmittance through transparent and translucent materials is equal to the solar heat that is transmitted through the material directly, plus the solar heat that is absorbed by the material and then re-emitted into the enclosed space.

Traditionally this was expressed in terms of a shading coefficient which described the amount of solar heat transmitted through a material compared to the amount of solar heat transmitted through a standard sheet of clear float glass 3 mm thick.

However, manufacturers are now moving away from shading coefficients. In the USA, they are moving towards the use of solar heat gain coefficients (SHGC) and in Europe, g-values (window solar factors, solar factors or total energy transmittance (TET)). In essence, these both represent 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.

g-value = total solar heat gain / incident solar radiation

The difference between g-values and solar heat gain coefficients is that they use a different value for air mass.

g-values can refer to the centre-of-glass g-value, or can relate to the entire window, including frame (gglass or gwindow). Generally, a higher g-value will be beneficial in cooler climates and a lower g-value in warmer climates. Typically g-values will range between 0.2 and 0.7, with solar control glazing having a g-value of less than 0.5.

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