What is the g-value (solar factor)?
The g-value, also called the solar factor or total solar energy transmittance, indicates what proportion of the solar energy striking glazing enters the room as heat. It is a number between 0 and 1.
How it works
A g-value of 0.6 means that 60% of the incoming solar heat passes through the glass and enters the room. The higher the g-value, the more free solar heat comes in. This is favourable in winter, because it lowers the heating demand, but unfavourable in summer, because it can lead to overheating.
The g-value is determined mainly by the composition of the glass and any coatings. Solar control glass has a deliberately low g-value, so that less solar heat enters. The g-value is independent of the U-value, which describes the heat loss through the glass. For a good glazing choice, both values are assessed together.
Application
When choosing glazing, the orientation and function of the room are considered. For south-facing rooms or large glazed areas, a lower g-value or solar shading may be needed to prevent overheating. For rooms where passive solar heat is welcome, a higher g-value can be beneficial. The g-value therefore plays an important role in energy-efficient design and comfort.
Related terms
- U-value
- Double glazing
- Insulation
- R-value
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