G Value

In the context of passive solar building design, the aim is to maximise solar gain within the building in the winter (to reduce space heating demand), and to control it in summer (to minimise cooling requirements). 

Solar heat gain or passive solar gain refers to the increase in temperature in a space, or structure that results from solar radiation. The amount of solar gain increases with the strength of the sun, and with the ability of any intervening material to transmit or resist the radiation.

Shading coefficients measure the solar energy transmittance through windows. "G-value" is the coefficient used in Europe to measure the solar energy transmittance of glass - called a Solar Factor (%) ie, 53% = 0.53.

G-values range from 0 to 1, a lower value representing less solar gain. Shading coefficient values are calculated using the sum of the primary solar transmittance (T-value) and the secondary transmittance. Primary transmittance is the fraction of solar radiation that directly enters a building through a window compared to the total solar insolation, the amount of radiation that the window receives. The secondary transmittance is the fraction of inwardly flowing solar energy absorbed in the window (or shading device) again compared to the total solar insolation.

Objects in sunlight absorb short-wave radiation and reradiate the heat at longer infrared wavelengths. Where there is a material such as glass between the sun and the objects that is more transparent to the shorter wavelengths than the longer, when the sun is shining the net result is an increase in temperature - solar gain. 

The La Rochere collection, TF30 & TF60 have been tested & produced the following results: