Solar Transmission (G-value)

Some light and heat energy passes directly through transparent and translucent materials, but there will always be a part of this energy that is reflected from the surface, known as reflectance, and a part that is absorbed by the material, known as ‘absorptance’. 

The energy that is absorbed by the material is converted to heat and re-radiated both internally and externally. The greater the mass of the rooflight assembly or material generally, the greater is the capacity for heat build-up and re-radiation. This is referred to as the ‘secondary’ component in solar heat gain.

The combination of the directly transmitted heat energy and the re-radiated secondary component is referred to as the total solar transmittance, or g-value.

The primary reason for including rooflights into a building is to allow the entry of natural daylight and to take advantage of all of the benefits associated with it. It is also worth considering that a well designed building with a good spread of natural daylight will also benefit from passive solar gain that can reduce the demand for space heating for many months of the year.

Reducing solar gain and the risk of overheating would normally only be considered where people might be occupying or working in a certain part of the building for a substantial part of the day. It is not a requirement for areas not expected to be occupied for any duration such as circulation spaces, store rooms, toilets etc. Due to stratification in high or double-height industrial buildings, and accumulated dirt on rooflights combined with internal absorption, the impact of solar gain in occupied space is often reduced, justifying an increase in rooflight area.

Reliable data on internal gains for typical warehouse buildings is difficult to obtain, but a figure of 5W/m² is widely accepted. This value is entirely attributable to artificial lighting.