What is a g-value?
The g-value is the proportion of solar energy striking a glazing unit that ends up inside the room. It is written as a decimal between 0 and 1. A g-value of 0.60 means sixty per cent of the sun’s energy gets through, by transmission through the panes plus the share absorbed by the glass and then re-radiated inwards.
It is the number that decides whether a room under glass is comfortable in July, and it is the number most often left off a quotation. Alongside the U-value it describes everything a rooflight does thermally, and the two pull in opposite directions. The wider specification sits on energy efficient rooflights.
What the number is actually measuring
Solar radiation hitting a sealed unit splits three ways. Some passes straight through. Some reflects off the surfaces and the coatings. Some is absorbed within the glass itself, warming it, and that warmth then radiates and convects to both sides.
The g-value counts the first part plus the inward share of the third. That second component is why a heavily tinted glass can perform worse than its appearance suggests: it absorbs a great deal of energy, gets hot, and gives a good part of that heat back into the room. A coating that reflects energy before it is absorbed does a better job than a tint that soaks it up.
The two scales, and the older names
You will see the same property expressed several ways, and mixing them up is how comparisons go wrong.
| Term | Scale | Note |
|---|---|---|
| g-value | 0 to 1 | European convention, EN 410 |
| Solar factor | 0 to 1 | Same thing, older wording |
| Total solar energy transmittance | 0 to 1 | The full name, abbreviated to TSET or g |
| SHGC | 0 to 1 | North American, measured slightly differently, close enough to compare |
| Shading coefficient | Roughly 0 to 1.15 | g divided by 0.87. Legacy term, do not treat as g |
Some suppliers quote the figure as a percentage. A “g-value of 35” means 0.35. If a data sheet gives a number above 1, it is a shading coefficient and needs converting before it is compared with anything.
Typical figures for real rooflight glass
The range in domestic units is wider than people expect, and the choice sits with the specifier rather than the manufacturer.
| Make-up | Approximate g-value | Light transmittance |
|---|---|---|
| Clear double glazed, soft coat low-e | 0.60 to 0.70 | 78 to 80% |
| Clear triple glazed | 0.50 to 0.55 | 68 to 72% |
| Neutral solar control double | 0.32 to 0.42 | 60 to 70% |
| High performance solar control | 0.22 to 0.30 | 45 to 60% |
| Body tinted grey or bronze | 0.45 to 0.55 | 35 to 50% |
The last row explains why tinted glass fell out of favour. It removes a lot of daylight for a modest reduction in heat, which is the wrong trade in a British climate.
Turning the number into watts
Peak solar radiation on a horizontal surface in Essex on a clear June day is roughly 800 to 900 watts per square metre at midday. That figure is what makes the g-value concrete.
Three square metres of clear double glazing at g 0.65, under 850 watts per square metre, admits about 1,660 watts into the room. That is more than a large radiator, running for several hours. The same three square metres at g 0.30 admits about 765 watts. The difference, roughly 900 watts, is the entire argument for solar control glass in a flat roof, and it costs a modest uplift at order and nothing afterwards.
Light transmittance is a separate number
The visible light transmittance, written LT or Tv, is the proportion of visible light that passes through. It is measured over the part of the spectrum the eye responds to, roughly 380 to 780 nanometres.
Solar energy is a much wider band. Slightly under half of it is visible light, most of the rest is near infrared, and a little is ultraviolet. That gap is what makes selective glass possible: a coating can be built to reflect the infrared while letting the visible band through, so the room stays bright and the heat stays out. Without that distinction, cutting solar gain would always mean dimming the room.
Selectivity, the ratio worth asking for
Divide the light transmittance by the g-value and you get the selectivity ratio. It is the single most useful figure for comparing solar control options, because it says how much daylight you keep per unit of heat you exclude.
A clear double glazed unit sits around 1.2. A decent modern solar control coating reaches 1.8 to 2.0. Anything above 2.0 is at the top of what is currently available. A body tinted glass may come in below 1.0, which means it is taking out more light than heat. Ask two suppliers for LT and g, do the division yourself, and the better product usually declares itself in one line of arithmetic.
The plane the figure was measured in
As with U-values, the angle matters. Published g-values are normally quoted for radiation striking the glass perpendicular, at normal incidence, which is the most favourable case for transmission.
Real solar radiation arrives at an angle that changes through the day and through the year, and transmission falls as the angle from perpendicular increases. For a rooflight lying flat in June the sun is within thirty degrees of perpendicular for hours at a time, so the published figure is close to what the room will actually receive. For a steeply pitched roof window facing north, the effective figure is much lower than the sheet suggests. The flatter the glass, the more literally the number applies.
Choosing a target by orientation
There is no universally correct g-value. There is a correct one for a given plane and a given glazed area.
A north facing pitched roof window can take a clear unit at 0.65 without complaint, and in winter the higher figure is doing useful work. An east facing loft bedroom is usually comfortable between 0.40 and 0.50. A west or south facing slope with a meaningful glazed area wants 0.30 to 0.40. A flat rooflight over a kitchen, facing the whole sky, wants 0.25 to 0.35 in nearly every case.
Whatever figure is agreed, get it written on the order next to the U-value and the light transmittance. Glass is made to the specification it is sold against, and a g-value that was discussed but never recorded is not a g-value that arrives on site. If a room is already glazed and running hot, the practical measures are set out on how do you stop a rooflight overheating.
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