When specifying windows and glazing, energy performance is often reduced to headline figures, such as U-values. While U-values are important, they only tell part of the story. To truly understand how a building behaves throughout the year, particularly in terms of overheating, daylight, and comfort, it’s essential to understand G-values.
In this guide, we’ll explain what G-values are, why they matter, how they interact with orientation and seasons, and how different glazing configurations affect performance. We’ll also explore how Mumford & Wood balances G-value performance with architectural intent, heritage sensitivity, and refined aesthetics.
What Are G-Values?
In simple terms, a G-value measures the amount of solar energy that passes through glass and enters a building.
Also referred to as the solar factor, the G-value is expressed as a number between 0 and 1:
- A higher G-value allows more solar heat to pass through the glass
- A lower G-value limits how much solar energy enters the building
For example:
- A G-value of 0.60 means 60% of solar radiation enters the space
- A G-value of 0.35 means only 35% is transmitted
This makes G-values a key consideration in managing solar heat gain, especially in homes with large areas of glazing.
Why G-Values Matter for Building Comfort
While U-values focus on heat loss, G-values focus on heat gain, and both are equally important for overall comfort.
Managing Overheating
Highly insulated homes retain heat far more effectively than older buildings. Without careful glazing specification, this can result in uncomfortable internal temperatures, particularly during warmer months.
Appropriate G-values help to:
- Reduce the risk of overheating
- Minimise reliance on mechanical cooling
- Maintain more stable indoor conditions
Balancing Light and Heat
A lower G-value does not necessarily mean less daylight. Advances in glazing technology enable the control of solar heat gain independently of visible light transmission, allowing interiors to remain bright without excessive heat buildup.
Supporting Consistent Internal Temperatures
Rather than blocking solar gain entirely, G-values allow designers to control it intelligently, capturing warmth when it’s beneficial and limiting it when it’s not.
How Orientation Influences G-Value Performance
Orientation plays a significant role in determining how glazing performs throughout the day and across the year.
South-Facing Elevations
South-facing windows receive the most direct sunlight. Higher G-values can be advantageous in winter, as they contribute to passive solar heating. However, without mitigation, they can also lead to overheating in summer.
A balanced G-value is often the most effective solution, particularly when combined with shading or ventilation strategies.
East- and West-Facing Elevations
These elevations are exposed to low-angle sunlight in the morning and evening, which is more challenging to control with external shading. Lower G-values are often more suitable in this context, especially when glazing areas are generous.
North-Facing Elevations
North-facing windows receive little direct sunlight. In these locations, G-values are typically less critical, with greater emphasis placed on thermal insulation, daylight quality, and visual clarity.
Specifying glazing by elevation rather than applying a single solution across the entire building allows performance to be fine-tuned without compromising design.
Seasonal Considerations: Winter Gains and Summer Control
G-values behave very differently depending on the season.
Winter Benefits
In colder months, solar heat gain can:
- Reduce heating demand
- Improve comfort near windows
- Support passive energy strategies
In these conditions, higher G-values, particularly on south-facing elevations, can be advantageous.
Summer Challenges
During warmer months, the same solar gain can lead to:
- Overheating
- Increased glare
- Greater reliance on cooling systems
This is why modern glazing design focuses on selective performance, allowing daylight and views to be maintained while limiting unwanted heat gain.
How Glazing Configuration Affects G-Values
A window’s G-value is influenced by the entire glazing build-up, not just the glass itself.
Low-Emissivity (Low-E) Coatings
Low-E coatings are microscopically thin layers applied to the glass surface. Depending on their formulation, they can:
- Reflect solar radiation
- Reduce heat transfer
- Maintain high levels of visible light
These coatings enable G-values to be carefully controlled without altering the window's appearance.
Tinted and Solar-Control Glass
Tinted or solar-control glass reduces solar transmission by absorbing or reflecting heat. This can be particularly effective for large glazed areas or exposed elevations, though subtle visual changes must be considered, especially on heritage or conservation projects.
Double and Triple Glazing
Triple glazing generally delivers lower G-values than double glazing, alongside improved insulation. While this can help control solar gain, it may also slightly reduce daylight levels, making careful specification essential.
Gas Fills
Argon or krypton gas primarily improves thermal insulation but also contributes to overall glazing performance when combined with coatings and pane configuration.
G-Values in the Context of Regulations and Standards
Although G-values are not always highlighted in headline regulations, they play an increasingly important role in overall building performance.
Building Regulations and Energy Modelling
Modern compliance focuses on whole-building performance rather than individual components. G-values are factored into SAP calculations and dynamic thermal modelling, helping to assess overheating risk and energy efficiency.
Overheating Risk Assessments
With greater emphasis on managing overheating in residential buildings, G-values are now a critical part of:
- Passive cooling strategies
- Future-proof design
- Reducing the need for retrofit solutions
Window Certification G-Values
When reviewing window certification G-value data, it’s important to consider whole-window performance rather than centre-pane figures alone. Frame design, installation quality, and glazing configuration all influence real-world results.
What Is a Good G-Value for Windows?
There is no single answer to the question “What is a good G-value for windows?” It depends on the building's orientation, use, and location.
As a general guide:
- 0.50–0.65 supports useful winter solar gain
- 0.35–0.45 helps manage overheating on exposed elevations
- Below 0.35 is typically reserved for highly glazed or solar-intensive designs
The most effective approach is not selecting one universal value, but tailoring G-values to suit each elevation and space.
G-Values and Timber Windows
Timber windows bring additional considerations beyond thermal performance. Sightlines, proportions, and detailing all play a role in how glazing choices are perceived.
Through careful specification of glazing units, coatings, and configurations, it is possible to achieve excellent G-value windows performance without compromising:
- Visual clarity
- Slim profiles
- Architectural character
This balance is particularly important on heritage, conservation, and design-led projects, where performance upgrades must respect the original intent of the building.
A Considered Approach to Solar Performance
G-values are not simply technical data points; they are a design tool that helps shape how a building feels to live in throughout the year.
When considered alongside orientation, insulation, ventilation, and shading, G-values allow buildings to:
- Stay comfortable in summer
- Capture beneficial warmth in winter
- Reduce energy demand
- Maintain light-filled, well-proportioned interiors
At Mumford & Wood, glazing is specified with this wider context in mind. The aim is never to chase a single performance number, but to create windows that work in harmony with the building as a whole, supporting comfort, efficiency, and longevity while preserving the quiet elegance that defines well-crafted timber windows.
