What is a g-value?

Typical figures

• Clear double glazing: around 0.7, so seventy per cent of solar energy enters
• Low emissivity double glazing: around 0.6
• Solar control glass: 0.3 to 0.4
• Heavy solar control: below 0.3

Why it matters more overhead

In summer the sun is high and shines almost straight down. A vertical window catches that obliquely. A rooflight catches it nearly square on, so overhead glass receives more solar energy than any other glass in the building.

In a kitchen, which already produces heat from cooking, the effect compounds.

This is why the g-value is the more consequential number on a rooflight, and the U-value is the more consequential one on a wall.

The trade-off

Solar control glass also reduces daylight a little and can carry a faint tint, usually a light grey or blue-green. Most people do not notice it once it is in.

It also blocks some winter solar gain, which is a genuine loss on a south facing rooflight. In this climate, summer overheating is the bigger problem and the trade is worth making.

By orientation

South and west: specify a low g-value. Not optional.

North: ignore it. There is no direct sun to control, and specifying solar control there just makes a good room darker.

East: moderate. Morning gain is real but shorter and cooler.

The one thing to remember

The g-value is fixed at manufacture. It cannot be changed afterwards without replacing the glass, and internal blinds do not substitute for it.

The proportion of solar energy that gets through

The g value, sometimes called the solar factor, expresses how much of the sun’s energy striking a glazing unit ends up inside the room. It runs from 0 to 1, and lower means less heat coming in.

A clear double glazed rooflight is somewhere around 0.6 to 0.7. A solar control unit might be 0.3 to 0.4. That difference is the whole of what solar control glazing does.

Why it matters more on a rooflight

A rooflight faces the sky, so it collects the sun more directly than a wall window does, particularly through the middle of the day when it is strongest. A near horizontal flat rooflight or lantern collects more still, because it has no orientation to shelter behind.

Overheating is the most common regret about rooflights, and the g value is the number that governs it.

The other number to look at alongside it

Light transmission, the proportion of visible light that gets through. Clear glazing is around 75 to 80 percent; solar control around 60 to 70.

The ratio between the two is how a product is judged. You want the g value down and the light transmission as high as possible, because a unit that stops heat by simply being dark defeats the purpose of a rooflight.

Modern selective coatings achieve that separation well, which is why solar control glass does not darken a room the way older bronze tinted glass did.

How it differs from the U-value

Two different things, frequently confused.

The U-value measures how much heat escapes from inside to outside. It matters in winter and it is what Part L regulates.

The g value measures how much solar energy comes in. It matters in summer and it is not regulated for a replacement rooflight, though it appears in the Part O overheating calculation for new dwellings.

Triple glazing improves the U-value and does very little to the g value, which is why it is not the answer to an overheating room.

Where to specify a low g value

South and west facing units of any size. Any near horizontal flat rooflight or lantern. Any glazed extension on a modern airtight house.

Not on north facing units, where there is no direct sun and the reduced light transmission is all cost and no benefit.

Where to find it and what to compare

The g value appears on a manufacturer’s technical data sheet rather than on a sales page, and it is worth asking for in writing alongside the whole unit U-value.

When comparing two units, compare g values and light transmission together. A unit with a very low g value and poor light transmission is stopping heat by being dark, which defeats the purpose of putting glass in a roof. A good selective coating gives you a low g value with light transmission still in the sixties or better.

Be careful about which surface the figure describes. Some data sheets quote a coated glass pane in isolation rather than the assembled unit, and the assembled figure is what the building experiences.

How much difference it makes in practice

On a two square metre rooflight facing south, the difference between a g value of 0.65 and one of 0.35 is a substantial amount of energy arriving in the room on a July afternoon. It is the difference between a room that is warm and one that is unusable.

On a north facing unit of the same size, the difference is close to nothing, because there is no direct sun to admit or exclude.

That is why specifying by orientation matters more than specifying a single glass type across a whole project, and why a quotation with identical glazing on every unit regardless of aspect has not been thought about.