The g-value of a rooflight, also called the solar factor, is the proportion of the sun’s energy hitting the glass that ends up as heat inside the room. It runs from 0 to 1. Clear double glazing sits at roughly 0.60 to 0.72, so around two thirds of the sun’s energy comes through. Solar control glass brings it down to about 0.28 to 0.38. A lower g-value keeps the room cooler in summer, and the trade-off is that it costs a little daylight, because heat and light travel through the glass together and the coating cannot perfectly separate them.
On a rooflight the g-value matters more than on any window, because a rooflight faces the open sky and collects sun all day.
What the number means
A g-value of 0.62 means 62 per cent of the solar energy arriving at the glass ends up in your room, as direct transmitted heat plus the part the glass absorbs and re-radiates inward. Lower is cooler. The figure includes both the sunlight you can see and the near-infrared you cannot, which is where most of the sun’s heat actually sits. That is the part a solar control coating is designed to reject, which is how it can cut heat sharply while barely touching visible brightness.
| Glazing | g-value | Solar energy into the room | Light transmission |
|---|---|---|---|
| Clear double | 0.60 to 0.72 | Around two thirds | 78 to 82% |
| Light solar control | 0.40 to 0.50 | Around half | 68 to 74% |
| Standard solar control | 0.28 to 0.38 | About a third | 60 to 68% |
| High-performance solar control | 0.22 to 0.28 | About a quarter | 50 to 60% |
| Clear triple | 0.50 to 0.58 | About half | 65 to 70% |
How a lower g-value trades daylight for comfort
You cannot reject heat without giving up some light, because the coating that rejects near-infrared also dims the visible band a little. So every step down in g-value costs a few points of light transmission. The skill is stopping at the point where the room no longer overheats but still reads bright. Standard solar control at 0.28 to 0.38 keeps light transmission above 60 per cent, so the room stays bright and neutral while solar heat is cut to roughly a third. Go below about 60 per cent light transmission and the room starts to feel a little calmer, which suits a west-facing kitchen that bakes but is wrong for a north-facing sitting room where you wanted the light.
g-value is not the same as light. Two figures describe a rooflight’s daylight and heat: the g-value, which is heat in, and light transmission, which is brightness. A good solar control unit separates them as far as physics allows, cutting the g-value hard while keeping light transmission high. Ask for both, because a low g-value with a decent light figure is exactly what you want on a sunny aspect.
How to choose the g-value
Set it from orientation and room use, not from the catalogue. A rooflight facing the open sky, and any flat rooflight regardless of aspect, wants a solar control g-value around 0.30. A north-facing pitched rooflight wants clear glass, a high g-value and all the winter light it can get, because there is little summer heat to reject. The full logic is on solar control glazing for rooflights and controlling solar gain through rooflights.
The g-value belongs on the specification next to the whole-unit U-value, because optimising one without the other gives you a warm room that bakes in July or a cool room that is dark. A complete rooflight quote carries the U-value, the g-value and the light transmission together, as covered on specifying U-values and g-values for rooflights.
What is a good g-value for a rooflight?
It depends on the aspect. For a south or west-facing rooflight, and any flat rooflight, a solar control g-value around 0.28 to 0.38 keeps the room cool without making it dark, since it still passes over 60 per cent of the light. For a north-facing pitched rooflight, clear glass with a g-value of 0.55 to 0.68 is usually right, because you want the winter light and there is little summer heat to reject. Set it from orientation, not from a single ideal number.
Does a low g-value make a rooflight room dark?
Only slightly, if the glass is specified well. g-value measures heat, and light transmission measures brightness, and a modern solar control coating separates the two as far as physics allows. Standard solar control cuts the g-value to about a third while keeping light transmission above 60 per cent, so the room stays bright and neutral. Only high-performance coatings below about 60 per cent light transmission start to feel calmer, and those are reserved for aspects that genuinely bake.
Get the g-value set from the compass
We take orientation at survey and specify the g-value and light transmission to suit the aspect and the room, both written on the quote next to the U-value. A 10-year workmanship guarantee, and installers with 25 years on rooflights.
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