The short answer
The g-value is the share of the sun's energy that passes through the glass and into the room, written as a number between 0 and 1. A low g-value lets in less solar heat and keeps a room cooler in summer. A higher one lets in more and can help in winter. Read it beside the U-value, which measures heat escaping the other way.
One number for the sun's heat
A rooflight has two jobs with heat. It should hold warmth in during the cold months, and it should keep the sun from baking the room in summer. Two different figures describe those two jobs.
The U-value is about heat leaving. The g-value, sometimes called the solar factor or total solar energy transmittance, is about heat arriving. It counts the sunlight that goes straight through the glass and also the part that the glass absorbs and then passes inward as warmth. A g-value of 0.5 means roughly half of the sun's energy reaches the room. A g-value of 0.3 means about a third. The rest is reflected or absorbed and shed outdoors.
Clear, uncoated glass lets a lot through, so its g-value is high. Coatings and tints bring it down. The figure belongs to the glass unit, not to the frame, and it is measured for the sun striking squarely. A roof sees the sun at a steep angle in high summer, which is the time it matters most.
Why a rooflight needs more thought than a wall window
A rooflight faces the sky. At midday in June the sun is high, and a horizontal or gently sloping pane catches it almost head on.
A wall window sits side on to the sun at that hour and gets some shade from the eaves. A roof window or flat rooflight gets neither. The heat comes straight in and lands on the floor and furniture. Hot air then collects under the ceiling, which is why a loft bedroom under a rooflight can be the warmest room in the house on a summer evening.
That does not mean you need a dark, low-light unit. It means the g-value deserves a place in the decision. A north-facing roof pitch might happily take a high g-value, because daylight is the aim and the sun barely touches it. A south or west pitch is where a solar control coating earns its keep. Our page on choosing energy efficient rooflights by orientation goes through each compass direction.
Reading the g-value next to the U-value
Neither figure tells the story alone. The pair shows which way a unit leans.
| Room need | g-value | U-value |
|---|---|---|
| Cool a sunny roof | Lower | Low |
| Borrow winter sun | Higher | Low |
| Shaded or north roof | Either | Low |
The U-value is kept low in every row, because losing heat is never useful. The g-value is the one you tune to the room. Some coatings do both jobs at once, and our page on low-e coatings on rooflight glass explains how.
The two figures can pull against each other slightly. A very strong solar control coating lowers the g-value, and it also cuts some of the daylight. That is a third number to look at, which the page on light transmittance on a rooflight explains. Picking a unit is a balance of the three, which is why we write the choice down for your room rather than quoting a catalogue default.
Which way the g-value should lean for your room
Start from the room and work back to the glass.
A south-facing loft bedroom, slept in and often warm by evening, leans towards a lower g-value. A kitchen lantern over a north-facing garden has no such pressure, so clear daylight matters most. A home office with a screen needs to avoid both glare and heat, which suggests a moderate g-value and a blind you can close. The guide to stopping a rooflight room overheating puts these cases together with shading, ventilation and opening units.
Winter is the other half. In the cold months a higher g-value lets low sun warm the room a little. The gain is real but smaller than people hope, because winter days are short and the sun sits low. It is useful to know about, and our page on whether a rooflight can help warm a room in winter covers it honestly.
Where sun fading is a worry, the coating matters too. See whether rooflight glass can stop furniture fading for what a coating can and cannot do.
What goes in the written spec
A number on a data sheet is not much use to you unless it is tied to your roof.
At the survey we note which way the roof faces, what sits above and beside it, and what the room is used for. The glass we propose, its coating and its stated g-value go into the quiet spec with the U-value, the pane build and the seals. You can then see why that particular unit was chosen, and ask for a different balance if you would rather have more light or more shade. The full range is on energy efficient rooflights in Crawley.
Short questions about g-values
Is a lower g-value always better?
No. It depends on the room. A lower figure keeps a sunny room cooler, but it also admits less free warmth in winter and can trim daylight slightly. It is a setting to match to the room, and not a score to maximise.
Does a lower g-value make a room darker?
A little, in some cases. Good solar control glass is made to keep the heat out while letting most of the light in. See whether solar control glass makes a room darker.
Is g-value the same as a solar control rating?
It is the measure behind it. Solar control glass is simply glass built to have a lower g-value than clear glass, and our page on solar control glass for rooflights shows how.
Can a blind do the same job as a low g-value?
Partly. An external blind stops the sun before it reaches the glass, which is very effective, while an internal blind works after the heat is already through the pane. Coated glass works whether or not anyone remembers to close a blind.
To have the g-value matched to your roof and your room, get a quote.