If you have ever come across the term G-value in relation to windows or glazing and been a bit confused about what it means, this blog post is for you.
Below, we’re going to do our best to simply explain the concept step by step, using some examples (and visuals!), so that you can get a better grasp of what G-value means and why it matters.
What does G-value actually mean?
In technical terms, G-value means “the total solar energy transmittance of glazing”.
That may sound more complicated than it actually is.
In the most basic terms, G-value tells you how much of the sun’s heat gets through the glazing and ends up inside the building.
That is the basic idea. At its simplest, we are asking how much of the solar heat reaching the window ultimately gets into the space behind it.
How is G-value expressed?
G-value is usually expressed as a number between 0 and 1. You may also sometimes see the same value written as a percentage.
At 0, none of the solar heat would get in. At 1, all of it would get in.
So if glazing has a G-value of 0.60, you can think of that as roughly 60% of the sun’s heat getting in. If the glazing has a G-value of 0.30, roughly 30% gets in.
- Higher G-value = more solar heat enters.
- Lower G-value = less solar heat enters.
Just one extra detail:
The explanation above is the easiest way to understand G-value, but there is one technical detail worth knowing.
When we say that the sun’s heat gets through the glazing, not all of it passes straight through the glass.
Sunlight carries energy. When sunlight reaches a window, three basic things can happen to that energy:
- Some passes through the glass.
- Some is reflected back outside.
- Some is absorbed by the glass.
If the glass absorbs some of the solar energy, the glass itself warms up. Part of that heat can then move towards the inside of the building.
The G-value takes account of the solar heat that passes directly through the glazing, as well as the inward part of the heat absorbed by the glazing.
You do not need to remember those two parts separately. The useful question is still: how much of the sun’s heat ultimately ends up inside?
What do we actually mean by “the sun’s heat”?
This is where a small amount of building physics helps. We only need enough to make the glazing behaviour make sense.
Heat can move in three main ways: conduction, convection and radiation.
Conduction: is heat moving through a material from a warmer part to a cooler part. A metal spoon warming up in a hot drink is a familiar example.
Convection: is heat carried by the movement of a fluid, such as air. Warm air rising from a radiator and cooler air falling is a simple example.
Radiation: is energy travelling as electromagnetic waves, so it does not need direct contact between two things. When this radiation transfers thermal energy, we often describe it as radiant heat. The Sun warming your skin is a familiar example.
Sunlight is energy, not just visible light
The phrase electromagnetic radiation can sound intimidating, but the basic idea is simple: it is energy that travels in waves.
Sunlight is a form of electromagnetic radiation. The Sun sends energy towards Earth across a range of wavelengths. Some is ultraviolet, some is visible light and some is infrared radiation.
Our eyes only see the visible part. That is why visible daylight and solar heat are related, but they are not the same measurement.
You do not need to memorise the different wavelengths for this article. The point that matters is that sunlight carries energy. When solar energy enters a building and warms the space, we call that solar heat gain, or simply solar gain.
Why does G-value matter?
G-value matters because solar gain can affect how comfortable a building feels and how much energy it needs.
Imagine a room with a large amount of glazing on a sunny day. If the glazing has a higher G-value, more solar heat can enter the room.
During colder periods, some of that heat can be useful because the Sun is providing free heat. During warm weather, the same effect can become a problem. The room may become uncomfortable or start to overheat. In an air conditioned building, the cooling system may then have to remove that additional heat.
So solar gain is not automatically good or bad. It depends on the building, the weather and what the space needs.
Is a lower G-value always better?
No. Lower is not always the answer.
If a building has large areas of glazing, strong summer sun and a high overheating risk, reducing solar gain may be very useful. A lower G-value can form part of that strategy.
But choosing the lowest possible G-value is not automatically good design. A lower value can also reduce useful winter solar gains. Depending on the glazing product, stronger solar control can also affect the amount of useful daylight entering the space.
The right value depends on the whole design. Orientation, glazing area, shading, ventilation, cooling, room use, daylight and overheating risk all matter.
A useful way to think about it is that the G-value is one design lever. It can help control solar gain, but it cannot fix every problem around the window on its own.
G-value versus U-value
G-value asks how much solar heat gets into the building through the glazing.
U-value asks how easily heat passes through the window because the inside and outside are at different temperatures.
Imagine a cold winter night. It is warm indoors, cold outdoors and there is no sunlight. In that situation, G-value is not the main measure of heat loss through the window. U-value becomes much more important.
A simple way to remember it: G-value is about solar heat coming in. U-value is about heat transfer through the window because one side is warmer than the other.
What about Low-E glazing?
Low-E means low emissivity.
Emissivity describes how readily a surface gives off energy as thermal radiation. In everyday terms, warm walls, furniture, people and glass all give off invisible radiant heat. A surface with lower emissivity gives off less of that thermal radiation.
Low-E glass has a microscopically thin engineered coating on one of its surfaces. The easiest mental picture is to think of that coated surface as being much more reflective to the invisible long wave thermal radiation given off by warm objects indoors. The glass can still look transparent to visible light.
This reduces the radiative part of heat transfer across the glazing and helps improve its insulating performance, which is why Low-E is strongly associated with U-value.
Why do we say long wave? Warm room surfaces are far cooler than the Sun, so the thermal radiation they give off is concentrated at longer infrared wavelengths. You do not need the wavelength numbers. The useful point is simply that a coating can behave differently towards different parts of radiation.
Low-E is a glazing technology. G-value is a performance value. A Low-E coating can influence G-value, but the two terms do not mean the same thing.
Does a low G-value mean dark or tinted glass?
Not necessarily.
Light transmittance tells you how much visible light passes through the glazing. G-value tells you about solar heat gain. They are different measurements.
Modern solar control glazing can be designed to limit a significant amount of solar heat while still allowing useful daylight into the room. Tinting is one way of changing solar performance, but it is not the only way.
That is why it is useful to look at G-value and light transmittance together rather than assuming one tells you everything about the other.
What actually affects the G-value?
G-value is not a separate material or layer inside the window. It is the performance result of the glazing make-up. Several parts of that make-up can change how much solar energy is transmitted, reflected or absorbed.
| Part of the glazing | How it can affect G-value |
|---|---|
| Glazing make-up or glass type | Different glass products, such as clear, tinted or laminated glass, can transmit and absorb different proportions of solar energy. |
| Solar control coatings | These coatings are designed to limit solar gain by changing how the glass reflects, transmits or absorbs parts of solar radiation. They can therefore reduce G-value. |
| Tint or body colour | Tinted glass can absorb more of the incoming solar energy and reduce how much is transmitted. The glass itself may become warmer as a result. |
| Number and arrangement of panes | Adding panes, coatings and different glass layers creates a different glazing system. The complete make-up changes how much solar energy ultimately gets through. |
| Low-E coatings | Their main purpose is to reduce long wave radiative heat transfer, but different Low-E coating types can also change solar transmission and therefore affect G-value. |
These factors do not all change G-value in the same way, and two products that look similar can have different performance. For a real project, use the tested manufacturer value for the actual glazing make-up wherever possible.
The G-value is only part of the solar gain story
It is also worth separating the G-value of the glazing from the solar gain that the room actually experiences.
The glazing may have one fixed product G-value, but the building around it changes how much sunlight reaches that glazing and what happens to the heat afterwards.
Orientation: Different façades receive sunlight at different times and intensities. West facing glazing, for example, can receive strong afternoon sun.
Glazing area: A large glazed area can admit a lot of solar heat even when the G-value is relatively low.
External shading: Overhangs, awnings, fins, brise soleil and other external shading can stop some solar radiation reaching the glass in the first place.
Surroundings: Nearby buildings, trees and other obstructions can change how much direct sun reaches a window.
Ventilation and cooling: Once heat is inside, ventilation or mechanical cooling affects how easily the space can get rid of it.
This is why the same glazing can perform very differently in two different buildings or even on two different façades of the same building.
How to choose the right G-value for your windows
So, you might now be wondering: what G-value should I use for my windows?
The difficulty is that there is no single G-value that is right for every building or every project. As we have seen above, higher and lower G-values have different effects, and whether those effects are helpful depends on what you are trying to achieve.
Rather than starting with a number and trying to make the design fit around it, it is usually more useful to work back from the problem you are trying to solve.
For example, are you trying to reduce overheating? Limit cooling demand? Make better use of winter solar gain? Maintain good daylight? Or balance several of these things at the same time?
Some useful questions to ask are:
- Which direction do the windows face?
- How much glazing is there?
- How much direct sunlight is likely to reach the glazing?
- Is there external shading?
- Do nearby buildings or trees provide shade?
- Is overheating a concern?
- Can the space lose excess heat through ventilation or cooling?
- How important is useful winter solar gain?
- How important is daylight?
A short note for UK practitioners
G-value is relevant wherever glazing is used, but the regulatory and modelling context depends on the country and the project.
In the UK, you may come across G-values in Approved Document L calculations, LETI guidance, energy models and manufacturer data. These values can be useful, but they need to be used in the right context. A reference value used for a particular calculation is not automatically the right product specification for a real building.
When reliable project specific glazing information is available, check what the manufacturer value actually represents before entering it into a model. Practitioners may also need to check whether the figure is a centre pane value or a whole window value, depending on the calculation or software being used.
Always use the current guidance and standards that apply to the project you are working on.
If you work outside the UK
The basic physics does not change, but the terminology and test standards may. In North America, for example, you are more likely to see Solar Heat Gain Coefficient, usually shortened to SHGC.
SHGC answers a closely related solar performance question. For a real project, use the terminology, test method and product information required by the relevant local standard rather than assuming every published G-value and SHGC figure is directly interchangeable.
The main things to remember
- G-value tells you how much of the sun’s heat gets through the glazing and ends up inside the building.
- Higher G-value means more solar heat gain. Lower G-value means less solar heat gain.
- Lower is not automatically better. The right value depends on the building and what the space needs.
- G-value is not the same as U-value, Low-E glazing or light transmittance.
- G-value describes the glazing, while orientation, glazing area, shading, ventilation and cooling affect the solar gain the room actually experiences.
- Where the final specification matters, use project specific product data and appropriate modelling rather than relying on a generic default.
Sources and further reading
The following sources are useful if you want to go further into the technical and practitioner detail.