What Everyone Needs To Know About Solar Gain And Glazing

The moment you commit to an ambitious glazing project, solar gain becomes something that you need to actively design around. Get it right, and your glazing works with the building throughout the year. Get it wrong, and a space that looks beautiful on paper becomes uncomfortable to live or work in.

This guide will explain what solar gain is, how glazing choices directly influence it, and what a well-specified glass system can do to give you warmth in winter and comfort in summer, without compromise.

Solar gain is the process by which shortwave radiation from the sun passes through glass, heats the surface and objects inside a space, and raises the internal temperature. The heat re-emitted by those warmed surfaces is long-wave infrared radiation, which cannot pass back out through the glass. This means the thermal energy accumulates, a phenomenon most people have experienced first-hand in a conservatory or south-facing room on a sunny afternoon. 

This is often referred to as the “greenhouse effect”, and it’s neither inherently good nor bad. In winter, it’s free heating, but in the summer, without the right glazing specification, it’s a recipe for overheating.

The G Number

Every piece of glazing is assigned a G-value, which is also called the total solar energy transmittance. It runs on a scale of 0 to 1. A G-value of 1 means all solar heat passes through the glass, and a G-value of 0 means none of it does. Although in practice, no piece of glass sits at either extreme.

What matters is understanding where your glass lands on the scale, and whether it’s the right specification for the orientation, room use and climate that you’re designing for.

As a general guide for UK buildings:

• A G-value of 0.5-0.6 suits south-facing rooms that benefit from winter solar heat but may need some protection in summer

• A G-value of 0.3-0.4 is more appropriate for west-facing buildings or rooms prone to overheating

• North-facing windows are less likely to be impacted by solar control and more likely to be focused on maximising light, which requires a different specification entirely.

One of the most common mistakes we see is treating C-value as a single decision for an entire building. Instead, a well-considered project specifies glazing on an elevation and room-by-room basis.

Glass Coatings

Solar control coatings are thin, invisible layers applied to the surface of the glass. They selectively filter out solar radiation, reducing the amount of heat that enters a space while allowing visible light to pass through largely unaffected. Coated glasses are one of the variants used in glazing units, enhancing energy efficiency, providing UV protection, and sometimes improving aesthetics.

Low-emissive (low-e) coatings work differently, and rather than blocking heat from entering, they reflect long-wave heat back into the room. In the UK climate, where retaining warmth matters as much as excluding it, the combination of a solar control coating and a low-e layer is often the right answer, managing both summer overheating and winter heat loss within the same unit.

The specifics of which coating works depend on orientation, glazing area and the thermal mass of a building. Choosing quality coatings and glazing materials is essential for achieving optimal energy performance and customer satisfaction. If you are unsure about the solution that would be best for your project, don’t hesitate to get in touch with us.

Advanced Glazing Materials

The evolution of glazing materials has transformed the way we approach energy efficiency and comfort in modern buildings. Today, advanced materials like laminated glass and float glass are at the forefront of solar panel installation and high-performance glazing projects. Laminated glass, known for its strength and safety, consists of multiple layers bonded together, making it ideal for both security and solar control. Float glass, the foundation for most architectural glazing, offers clarity and uniformity, and serves as the base for further enhancements like low-e coatings.

In the solar industry, these materials are often selected for their ability to minimise unwanted heat transfer, helping to keep interiors cool during summer and warm in winter. When combined with low-e coatings, advanced glazing materials can significantly reduce cooling costs and buffer temperature fluctuations, making buildings more energy efficient. Whether you’re installing new windows, doors, or planning a solar panel installation, choosing the right glazing materials is essential for maximising energy savings and comfort. The focus on innovative materials ensures that modern glazing not only meets aesthetic goals but also delivers long-term performance and efficiency.

Low Emissivity Glazing

Low emissivity glazing, commonly referred to as low-e glazing, is a game-changer for anyone seeking to create an energy efficient home or business. This technology involves applying a microscopically thin metallic coating to the surface of the glass, which reflects radiant heat while still allowing natural light to pass through. The result is a dramatic reduction in heat transfer between the inside and outside of a building.

By installing low-e glazing in your windows and doors, you can keep valuable warmth inside during winter and block excess heat from entering during summer. This not only reduces the need for air conditioning and heating, but also leads to substantial energy savings over time. In most cases, low-e glazing is paired with double or triple glazing, where two or three panes of glass are separated by air or inert gas, further enhancing insulation. The combination of low emissivity coatings and multiple panes creates a highly efficient barrier against unwanted heat loss or gain, making your building more comfortable and cost-effective to run throughout the year.

Double vs Triple Glazing

Double glazing consists of two panes of glass, while triple glazing uses three panes. Double-glazed windows have two panes separated by a vacuum or a layer of inert gas such as argon or krypton, which reduces thermal conductivity and improves energy efficiency compared to single pane windows. Triple glazing features three panes of glass, each separated by an inert gas or vacuum, providing superior insulation and energy efficiency. In contrast, single pane windows are less effective at preventing heat loss and can make interiors feel cold, especially during winter.

Adding more glass panes reduces the G-value, so triple glazing transmits less solar energy than double glazing. This is commonly presented as an issue, but it’s actually a bit more nuanced than this.

Triple glazing excels at retaining heat inside a building through a better U-value, which is the measure of heat loss. On a north-facing elevation, or in any application where heat loss is the primary concern, triple glazing typically performs better overall despite its lower solar gain. On a south-facing elevation where passive solar heating is a design intent, double glazing with the right coating might deliver a better balance.

Frame Materials and Gas-Filled Cavities

The choice of frame materials and the use of gas-filled cavities are critical factors in the overall performance of any glazing system. Different frame materials—such as wood, aluminium, and PVC—offer varying levels of thermal insulation, durability, and cost-effectiveness. For example, wooden frames provide excellent natural insulation, while PVC frames are known for their low maintenance and affordability. Aluminium frames, when thermally broken, can also deliver strong performance in terms of energy efficiency.

Beyond the frame, the space between glass panes can be filled with inert gases like argon or krypton. These gases are less conductive than air, which means they further reduce heat transfer through the glazing. When combined with low-e coatings, gas-filled cavities help create a system that minimises both heat loss in winter and heat gain in summer, leading to lower cooling costs and improved comfort. By carefully selecting frame materials and incorporating gas-filled cavities, you can create an efficient glazing solution tailored to your building’s needs, maximising energy savings and long-term value.

Large Format and Structural Glazing

Floor-to-ceiling glass windows, glass roofs and structural glazing systems amplify both the benefits and the risks of solar gain. A large south-facing glass wall is significantly more exposed than a standard window, and the solar load starts to increase.

At Fluid Glass, we work with architects from the early stages of a project that involves large glazed areas precisely because these decisions compound quickly. The glass specification, the orientation, the shading strategy and the ventilation approach all interact with each other, and getting one element wrong will have a monumental effect on the others. 

In a well-considered project, architects use orientation, thermal mass, external shading, specialised glazing, and landscaping techniques to optimise solar gain. Shading strategies work alongside glazing specification to resolve solar gain and are essential for preventing overheating. The most effective type of shading is external, as it intercepts solar radiation before it reaches the glass at all.

Effective shading is crucial in passive solar design, as it blocks the high summer sun while allowing the lower winter sun to enter, maintaining comfortable indoor temperatures throughout the year. Overhangs and brise soleil are particularly effective on south-facing elevations because the sun sits much higher in the sky in summer. A correctly sized overhang can significantly reduce overheating in summer by blocking high-angle sunlight, whilst still allowing the low winter sun to enter in December and January.

Incorporating thermal mass materials, such as stone or concrete, in passive solar design can help absorb heat during the day and release it slowly at night, stabilizing indoor temperatures and reducing heating and cooling costs.

Integrated blind systems offer a clean aesthetic for projects where external shading isn’t feasible. They’re less effective than external shading in pure solar control terms, but are significantly more effective than internal blind solutions.

In residential projects, deciduous planting is worth considering as trees lose their leaves in winter, allowing solar gain when it’s most valuable, and providing dense shade in summer when you need to exclude it. Well-managed solar gain also correlates with high levels of natural light, reducing the need for artificial lighting.

The UK Government introduced Approved Document O in 2021, which sets requirements for overheating mitigation in new residential buildings in England. It requires designers to limit unwanted solar gains in summer and provide adequate means of removing heat from the interior. Part L of the Building Regulations also places restrictions on glazing ratios in relation to the building's overall thermal performance.

For projects with large glazed areas where extensions, new builds or whole house refurbishments are required, compliance with these requirements needs to be built into the specification from the outset, not retrofitted at the end. We’re familiar with these requirements and factor them into every project that we work on.

The projects where solar gain is handled best tend to share a few things in common:

• Glazing specification is discussed early, before elevations are fixed and orientations are locked in. The architect, the glazing supplier and sometimes a building physicist are all in the conversation at the same time rather than sequentially. Ideally, buildings should be aligned within 30° of true south to maximise winter sun exposure and facilitate summer shading, which is a key aspect of passive solar design.

• The G-value isn’t treated as a single number for the whole building; different elevations, different rooms and different uses get different specifications. Properly designed solar gain allows for passive heating while creating a comfortable indoor environment, especially when large, south-facing windows are used to provide significant "free" heating, reducing the need for boilers or heaters.

• Shading is designed in, not added afterwards. An overhang that’s right for the geometry of the building is far more effective than roller blinds ordered six months after completion. Passive solar design involves strategically orienting a building and its windows to maximise solar gain, particularly during winter months when the sun is lower in the sky.

• The balance between solar gain and heat loss is held together; a very low G-value that eliminates overheating risk might also eliminate the passive heating benefits that reduce winter energy bills. When a solar system is installed, careful attention should be paid to the edge of the roof to ensure the panels are positioned for optimal performance and compliance. Solar panels generate electricity and produce energy savings, helping homeowners with saving money over time.

While these aren’t complicated principles, they do require the right conversation at the right time.

Passive solar design is a smart, sustainable approach that harnesses the natural energy of the sun to heat and cool buildings with minimal reliance on mechanical systems. By strategically orienting windows and selecting the right glazing materials, architects and homeowners can create spaces that stay warm in winter and cool in summer, all while reducing energy consumption and utility bills.

In the UK, passive solar design is especially valuable for older buildings, where planning permission may be required for solar panel installation or major renovations. By integrating passive solar principles—such as maximising south-facing glazing, using thermal mass to store heat, and incorporating shading devices—property owners can achieve significant energy savings and a more comfortable indoor environment. The careful selection of materials and thoughtful installation of glazing systems are key to creating spaces that naturally regulate temperature, reduce cooling needs, and make the most of available sunlight. Whether you’re upgrading an existing property or planning a new build, passive solar design offers a proven path to energy efficiency and long-term savings.

Recent years have seen remarkable innovations in the glazing industry, driven by the need for more energy efficient buildings and sustainable solutions. One of the most exciting advancements is vacuum glazing, which uses a thin vacuum layer between glass panes to provide exceptional thermal insulation. This technology is ideal for windows, doors, roofs, and even walls, offering superior performance in both new builds and retrofits.

Self-cleaning coatings are another breakthrough, reducing maintenance requirements and keeping glass surfaces clearer for longer. Advances in frame materials and the use of gas-filled cavities have also contributed to the development of highly efficient glazing systems that deliver impressive energy savings. As the demand for sustainable buildings grows, these innovations are helping homeowners and businesses benefit from lower energy bills, reduced environmental impact, and improved comfort.

By choosing to install solar panels and invest in advanced glazing materials, customers can create properties that are not only stylish and functional, but also future-proofed against rising energy costs and climate challenges. The ongoing evolution of glazing technology ensures that every new project can achieve greater efficiency, value, and sustainability.