If you have come across a G-value on a glazing specification and wondered what the number actually tells you, the basic idea is simple. It tells you how much solar energy gets through the glazing and ends up inside the building.
- Higher G-value = more solar heat enters
- Lower G-value = less solar heat enters
What does G-value actually mean?
In technical terms, G-value means the total solar energy transmittance of glazing. That sounds more complicated than it needs to.
Imagine sunlight reaching a window. Some of that solar energy will eventually end up inside the room. The G-value is a way of describing how much.
A G-value is normally written as a number between 0 and 1. The same value can also be thought of as a percentage.
- A G-value of 0.60 means roughly 60% of the solar energy is admitted
- A G-value of 0.30 means roughly 30% is admitted
There is one extra detail
When we say solar energy “gets through” the glazing, it does not all pass straight through the glass.
When sunlight reaches glazing, three basic things can happen. Some energy is transmitted through the glass, some is reflected back outside, and some is absorbed by the glass itself.
The absorbed energy warms the glass. Part of that heat can then move towards the inside of the building. G-value includes both the solar energy transmitted directly through the glazing and the inward-flowing part of the energy absorbed by it.
You do not need to remember those parts separately to use the concept. The useful question is still: how much of the solar energy reaching the glazing ultimately ends up inside?
Why does G-value matter?
Solar gain affects both comfort and energy use. A room with a large amount of glazing can receive a significant amount of heat from the Sun, particularly when the glazing receives strong direct sunlight.
In colder weather, some of that gain can be useful because the Sun is providing free heat. In warmer weather, exactly the same solar gain may contribute to overheating. In an air-conditioned building, the cooling system may then have to remove the additional heat.
So a high or low G-value is not automatically good or bad. It depends on what the building needs.
Is a lower G-value always better?
No. If overheating is a concern, a lower G-value may help by reducing the amount of solar heat entering through the glazing. But choosing the lowest possible value is not automatically good design.
A lower G-value can also reduce useful winter solar gains. Depending on the glazing product, solar-control choices may also affect visible light. The right value therefore depends on the wider design, including orientation, glazing area, shading, ventilation, cooling, daylight and how the room is used.
G-value is not U-value
These two values are often shown together, but they answer different questions.
G-value is about solar energy entering through the glazing. U-value is about heat transfer through the window because the temperatures on the two sides are different.
On a cold winter night, when there is no sunlight, G-value is not the main measure of heat loss through the window. U-value is.
The main thing to remember
G-value tells you how much solar energy gets through the glazing and ends up inside. A higher number means more solar gain. A lower number means less.
Continue with the next guide
G-value vs U-value: What’s the Difference?
What G-value Should I Use for My Windows?
Sources and further reading
The following sources are useful if you want to go further into the technical and practitioner detail.