There is no single G-value that is right for every window. The useful question is not “what is the best G-value?” but “how much solar gain does this particular space need, and how much can it cope with?”
- A lower G-value lets in less solar heat
- A higher G-value lets in more solar heat
The right choice depends on the building around the glazing.
Start with the problem, not the number
If a room is already at risk of overheating, reducing solar gain may be useful. If a space has limited solar exposure and useful winter gains are important, pushing the G-value as low as possible may not make sense.
That is why glazing selection should start with what the design is trying to achieve. Are you trying to reduce overheating? Limit cooling demand? Make better use of winter solar gain? Maintain useful daylight? In many projects, the answer is a balance of several of these.
The same G-value can behave differently in different buildings
The glazing may have one product G-value, but the room does not experience solar gain in isolation. The building determines how much sunlight actually reaches the glass and what happens to the heat once it is inside.
Orientation
Different façades receive sunlight at different times and intensities. A west-facing window, for example, can receive strong afternoon sun. The same glazing used on a more shaded façade may experience a very different solar load.
Glazing area
A large glazed area can admit a lot of solar energy even when the G-value is relatively low. The G-value tells you the proportion admitted, but the total glazed area still matters.
External shading
Overhangs, fins, awnings and other external shading can stop solar radiation before it reaches the glass. In some situations, shading can therefore reduce solar gain more effectively than changing the glazing alone.
Surroundings
Nearby buildings, trees and other obstructions can alter how much direct sunlight reaches the window. A glazing specification should not be considered separately from its actual site conditions.
Ventilation and cooling
Once heat has entered the space, ventilation or mechanical cooling may be able to remove it. A room that can easily reject excess heat may respond differently from one with limited ventilation and no active cooling.
What points towards a lower or higher G-value?
A lower G-value may be worth considering where windows receive a lot of direct sun, there is a large amount of glazing, external shading is limited, overheating is a concern, or the space has difficulty removing excess heat.
A higher G-value may be more acceptable where solar exposure is limited, glazing areas are modest, useful shading is already present, overheating risk is relatively low, excess heat can be removed effectively, or useful winter solar gain is important.
These are not fixed rules. They are design clues.
What actually changes the G-value of the glazing?
G-value is not a separate material inside the window. It is a performance result of the complete glazing make-up.
| Part of the glazing | How it can affect G-value |
|---|---|
| Glass type or glazing make-up | Different products transmit and absorb different proportions of solar energy |
| Solar-control coatings | These can reduce solar gain by changing how the glazing reflects, transmits or absorbs solar radiation |
| Tint or body colour | Tinted glass can absorb more incoming solar energy and reduce the amount transmitted |
| Number and arrangement of panes | Different combinations of panes and coatings create different overall solar performance |
| Low-E coatings | Their main role is associated with long-wave heat transfer, but the coating type can also influence solar transmission |
For a real project, use product-specific data
Generic values are useful when you are learning or carrying out early-stage option studies. Once the glazing is being specified, the actual manufacturer data for the proposed glazing make-up becomes much more important.
Practitioners should also check what the published figure represents before entering it into a calculation or energy model. Depending on the software, calculation method and product data, you may need to distinguish between a centre-pane glazing value and a value representing the wider window system.
A short note for UK practitioners
In the UK, G-values may appear in Approved Document L calculations, energy models, LETI guidance and manufacturer literature. A reference value used for a particular calculation is not automatically the correct product specification for a real building.
Use the current guidance that applies to the project and, where the design is sensitive to solar gain or overheating, review the glazing as part of the wider building rather than as an isolated number.
The key idea
Do not choose a G-value in isolation. Look at orientation, glazing area, shading, surroundings, ventilation, cooling, daylight and overheating risk, then select and test the glazing that works with the whole design.
Related guides
What Does the G-value of a Window Mean?
G-value vs U-value: What’s the Difference?
Sources and further reading
The following sources are useful if you want to go further into the technical and practitioner detail.