Rooflights and G Values – Why one size doesn’t fit all
When specifying rooflights, architects are often faced with a balancing act between daylight and solar control. At the centre of this decision lies the Solar Factor, commonly referred to as the g value. While it is tempting to assume that lower g values are always better, the reality is far more nuanced and selecting the wrong g value for a given application can compromise both comfort and performance. Opting for a value which is too low is often just as bad as choosing one that is too high. Rooflight specifiers must carefully consider project-specific requirements when selecting glazing, and how g values directly influence both solar gain and light transmittance.
What is the g Value?
The g value measures the proportion of solar energy that passes through glazing into a building. Expressed as a number between 0 and 1:
- Low g value (e.g. 0.25–0.35): Less solar heat enters the space
- High g value (e.g. 0.5–0.65): More solar heat is transmitted indoors
This includes both direct solar transmission as well as secondary heat re-radiated inward from the glass.
A key point often overlooked is that solar control and light transmission are closely linked. In general lower g values are achieved using coatings or tints but this will also reduce visible light transmittance (LT). Higher g values typically allow more daylight into the space but increase the amount of solar gain/heat entering the room and so this creates an inherent trade-off; do you reduce the light transmittance for a better g value or do you want more natural daylight at the risk of the room overheating.
There is no universally “correct” g value and the ideal specification depends on a range of factors unique to each project. A south facing rooflight will experience significantly more solar gain than a north facing one. Therefore south facing rooflights will benefit from lower g values to control overheating whereas north facing rooflights can tolerate and even benefit from a higher g value to maximise daylight. Now whilst you are probably thinking that this all makes sense, how often do you, or even can you, specify rooflights with differing g values based on their fitting position?
Different environments have different thermal and lighting priorities so the internal use of the space is an important consideration. Whilst residential living spaces may benefit from a higher g value for warmth in the winter, it can also result in rooms becoming too hot in the summer. Offices and classrooms typically require a balanced light with controlled heat gain and retail and gallery spaces regularly prioritise the need to protect against glare and overheating.
Another important thing to consider is the rooflight size and the overall glazed area as large expanses of roof glazing amplify both benefits and risks. Having a large rooflight with a higher g value can quickly lead to overheating but a small rooflight with a low g value may unnecessarily reduce daylight without significant thermal benefit.
You should also factor in the ventilation and cooling strategy as buildings with strong natural ventilation or active cooling systems are likely to tolerate higher solar gain more effectively. Conversely, poorly ventilated spaces may require stricter solar control. This is where opting for opening rooflights can provide significant benefits in providing cooling to a room. At Stella we offer a range of automated opening rooflights that can operate with rain, wind or even temperature sensors.
The ventilation and cooling of a building is also important when considering seasonal performance. Considering year-round performance is essential as higher g values can contribute to passive solar heating in the winter and lower g values help mitigate overheating in the summer. The geographical location of the project can have a big influence on the requirements and the challenge is to find the right balance for both winter and summer conditions.
When miscalculating g values can become a problem –
The majority of enquiries that we receive which actually discuss g values, require a low figure, and the reason given is always to prevent overheating. It appears that there is an assume that reducing solar gain is always beneficial, but this is not necessarily the case.
A low g value in the wrong location can lead to underlit interiors with a reliance on artificial lighting, reduced occupant wellbeing due to lack of natural daylight and also missed passive heating opportunities in cooler months. We would always recommend that you evaluate your g value requirements as, overly aggressive solar control can undermine the very purpose of installing a rooflight.
Equally, specifying a g value that is too high can result in overheating, particularly in summer, increased cooling loads and subsequent energy use, glare discomfort which impacts the usability of the space and reduced occupant comfort and productivity.
It is important that you strike the right balance and we recommend that g value selection should never be isolated from the broader design strategy. Instead, it should be considered alongside the following:
- Orientation and shading
- Glazing size and positioning
- Ventilation strategy
- Desired daylight levels
- Building use and occupancy patterns
In many cases, the optimal solution lies somewhere in the middle not at the extremes.
Rather than defaulting to the lowest or highest possible g value, architects should adopt a performance-led approach and evaluate project-specific solar exposure and wherever possible model daylight and thermal performance whilst giving consideration as to how glazing contributes to both energy efficiency and the user experience.
A well-chosen g value enhances comfort, reduces energy demand, and ensures that rooflights deliver on their primary purpose which is bringing natural light into a space in a controlled and beneficial way.
The Solar Factor (g value) is a powerful tool but only when used thoughtfully. Both high and low values have their place, and neither is inherently “better” without context. For architects, the challenge is not simply to minimise solar gain, but to optimise it whilst balancing heat, light, and comfort to suit the unique demands of each project and client.
When it comes to Stella rooflights, our preferred solar control glass is a Saint Gobain product called COOL-LITE® SKN which is a high performance solar control glass with advanced thermal insulation properties and excellent neutrality. This range is specifically designed to provide neutral solar control for large glazed apertures in residential projects. The SKN range of solar control glass has been carefully engineered to prioritise comfort and energy efficiency, making it an ideal solution for combating overheating whilst enhancing the overall energy performance of a building space.
From aesthetics to interior design, noise levels, and natural light, various factors contribute to the overall feeling of comfort and well-being within a building. This is why a thoughtfully designed and carefully constructed space has the power to elevate our mood, enhance productivity, and even promote better health.
