What Is the G-Value of a Rooflight?

Every pane of glass lets some solar energy through and reflects or absorbs the rest. The g-value, sometimes written as the solar factor or the total solar energy transmittance, is the share that ends up as heat in the room, expressed as a decimal. Clear, uncoated double glazing typically sits around 0.6 to 0.7: most of what hits the glass carries on through it. Add a solar control coating and that figure can drop to 0.3 or lower, meaning the same sun striking the same size of glass delivers far less heat to the space below.

A lower g-value is not automatically the right answer. It is a trade against light and against winter warmth, which is why the figure is read alongside the glazing spec as a whole rather than chased on its own.

A vertical window only ever catches the sun at an angle, and a roof overhang, a neighbouring wall or simply the time of day shades it for much of the year. A rooflight sits flat or close to flat against the sky, so on a clear day it faces the sun far more directly, for far more hours, than a wall window ever does. That is the whole appeal of a rooflight over a Fen sky with nothing on the horizon to break the light, and it is also why a poorly chosen g-value shows up as a genuinely warm room rather than a mild difference. On new-town extensions across Bretton, Orton and Werrington, where a flat-roofed kitchen or garden room often sits under a large area of rooflight glass, the g-value of that glass is doing more work than almost any other single figure on the spec sheet.

It is easy to conflate the two because both are single numbers attached to the same pane of glass, but they measure opposite seasons. Our guide to reading a rooflight U-value covers the winter side: how much heat escapes through the unit when it is cold outside and warm in. The g-value covers the summer side: how much of the sun's heat gets in when it is warm outside and the room does not need it.

A glass unit can carry a low U-value and a high g-value at the same time, which is exactly the combination that keeps heat in during winter but also lets in too much sun in summer. Specifying a rooflight well means looking at both figures together, not reading one and assuming the other follows.

The g-value is set mainly by the coatings and the number of panes, not by the frame. A solar control coating, covered in full on our solar control glass for skylights guide, is a very thin metallic layer applied to one face of the glass that reflects a share of the sun's energy back outward before it can pass through. The same family of coating, low-emissivity or low-e, is discussed on the invisible layers: low-e coatings and gas-filled glass, though a low-e coating on its own is tuned mainly for the U-value side and moves the g-value far less than a dedicated solar control coating does.

Adding a third pane, discussed on two panes or three: glazing a rooflight for warmth and clarity, also pulls the g-value down a little, simply because there is more glass and gas for the sun's energy to cross. It is a smaller effect than a coating designed specifically for solar control, and it comes with its own weight and cost factors to weigh separately.

Not directly, though the two are related. A coating that cuts the g-value tends to trim the light transmittance too, since both are measured on the same passing beam of sun. What differs is by how much. Our guide on will solar control glass dim the room below? sets out coatings chosen to keep light transmittance high while still cutting a meaningful share of the heat, against older, cheaper tints that darkened the glass more than they needed to for the heat they saved. The figure to check alongside the g-value, explained on what is light transmittance on skylight glass?, is what tells you whether the sky through that particular glass will still look clear from the room below.

A rooflight's g-value is one part of a bigger sizing question, set out on balancing daylight and heat loss when sizing: the same glass area that brings in a good, even wash of daylight is also the area the summer sun heats through. Orientation changes the sum too. A rooflight facing due south takes far more direct summer sun through the day than one facing north, a point our guide to do north-facing skylights lose more heat? looks at from the winter side of the same question. On larger areas of glass, particularly over a kitchen extension or a garden room used through summer afternoons, the g-value also feeds into the Part O overheating check covered on rooflights and overheating: Part O, which can require a lower g-value, shading, or extra ventilation once the glazed area passes a certain share of the floor.

At survey stage we work through the room's use, its orientation and the sightline together, so the sightline sketch that comes with every quote shows the view you will actually get, and the glazing spec behind it is chosen with that summer sun in mind rather than fixed on daylight alone. The Energy Efficient Rooflights in Peterborough page sets out how U-value, g-value and light transmittance are read together across a whole specification, and the sightline sketch itself is explained on its own page.

A manufacturer's data sheet will usually list the g-value as a plain decimal, sometimes alongside the light transmittance as a percentage, so the two sit close together for comparison. There is no single "right" g-value that suits every rooflight; a landing lit for most of the day benefits from a higher figure that lets more of the available light and warmth through, while a south-facing rooflight over a room used through summer afternoons is generally better served by a lower one. Reading the g-value in isolation, without checking the orientation, the room below and the glazed area against it, is how a rooflight ends up either colder than expected in January or warmer than expected in July.

Is a higher or lower g-value better?

Neither is better on its own. A higher g-value suits a room that benefits from winter solar warmth and even daylight, such as a north-facing landing, while a lower g-value suits a south-facing rooflight over a room used through summer afternoons, where unwanted heat matters more than a little extra warmth in winter.

Does the frame affect the g-value?

No. The g-value is a property of the glass itself, the coatings applied to it and the number of panes. The frame affects the U-value at the edge of the unit and the sightline, but it plays no part in how much solar energy passes through the glazed area.

Can the g-value be checked after a rooflight is fitted?

Not by eye. The figure comes from the manufacturer's test data for that specific glass build, so it needs to be confirmed against the spec sheet at order stage rather than judged once the unit is installed.

Does a low g-value affect the U-value?

Not directly. The two are measured separately, and a coating chosen for solar control is not automatically the same coating that gives the best U-value, which is why both figures need checking against the same spec sheet rather than assumed from one another.