Five real projects reveal what happens when solar gain, glazing and façade design fall out of balance.
For years, one of the central objectives of high-performance glazing has been straightforward: reduce unwanted solar heat gain. Lower solar factors. More advanced solar-control coatings. More selective glass. Less solar radiation entering the building.
The logic appears simple. If excessive solar radiation increases cooling demand and contributes to overheating, reducing the amount of solar energy entering through the façade should improve building performance. But as glazing technology becomes increasingly sophisticated, another question is becoming harder to ignore. Are we actually choosing the right solar factor for the building, or simply choosing a lower one?
The solar factor, or g-value, represents the fraction of incident solar radiation that enters a building as heat through the glazing. In North American terminology, the related Solar Heat Gain Coefficient (SHGC) is commonly used. But the number cannot be considered in isolation. Its effect depends on climate, orientation, glazing ratio, shading, occupancy, ventilation and the building’s heating and cooling strategy. The same g-value can therefore produce very different results in two buildings.
The solar energy nobody sees in the specification
A glazing datasheet can make façade design appear deceptively simple: U-value, g-value, visible light transmittance and reflection. But the building is much more complicated. A glass unit with a g-value of 0.30 does not behave identically on a south-facing curtain wall, a west-facing office, a residential building or a heavily shaded façade. Solar energy is not simply something to eliminate. During summer, solar gains can contribute to overheating and cooling demand. During winter, the same solar gains can provide useful passive heating. 
This creates a fundamental balance: keep unwanted solar heat out when it causes overheating, while allowing useful solar energy in when it can contribute to building performance. The objective, therefore, cannot simply be the lowest possible g-value. It has to be the appropriate g-value.
When lower g-value becomes the wrong answer
The instinct to specify a lower g-value is understandable. But reducing solar gains does not automatically solve every building-performance problem. A lower g-value can reduce cooling loads and peak solar gains. At the same time, it can reduce useful winter solar gains and influence daylight and comfort. Research into glazing performance has shown that glazing ratio, orientation, solar factor and shading interact significantly in determining building heat loads. The optimum therefore changes according to the building.
This becomes much clearer when looking at real projects.
1. Southampton University: when highly glazed offices became too hot
At Southampton University in the UK, a new campus combined renovated Victorian buildings with modern, highly glazed office extensions. The modern extensions had relatively low thermal mass and a high level of glazing. The combination led to excessive office temperatures and occupant discomfort, with users requesting individual air-conditioning units. The James-Parkes Building studied by researchers had an east-facing elevation oriented approximately 26° from east towards south. Simulations examined a range of responses, including forced-air convection, louvres, façade changes, electrochromic glazing and holographic solar-control elements.
The important point is that the problem could not be reduced to a single glass parameter. Glazing area, orientation, thermal mass, glare and solar control were interacting simultaneously.
2. Polo Eustachio, Italy: when solar gains overwhelmed the façade
A particularly well-documented case is Polo Eustachio, part of the University Polyclinic in Ancona, Italy. The six-storey complex, constructed in the 1990s, has fully glazed façades on its north and south sides. Researchers selected a south-facing third-floor office as the worst-case environment because of its unobstructed exposure to the sky and prolonged solar radiation. The glazing had a reported g-value of 0.504.
The office suffered from serious overheating caused by abundant solar gains. Monitoring showed operative temperatures approaching 28°C under free-running conditions. Researchers then tested predictive and fuzzy control strategies for the air-conditioning system. With model-predictive control, the 24°C setpoint was kept within 2°C, with the offset within ±0.5°C for 95.8% of the time on the sunnier test day. The case is important because it demonstrates that even a defined solar factor cannot be evaluated separately from façade exposure, building geometry and mechanical systems.
3. Salford Quays: when changing the glazing was not enough
A 20-storey residential tower containing 191 dwellings at Salford Quays, Manchester, provides another example. The building had a large percentage of glazing, creating a significant overheating risk when assessed using the TM59 methodology. The initial assessment showed that the building was failing the overheating criteria by a considerable margin.
The design team considered mechanical ventilation at 8 air changes per hour, but this created problems with cost, noise and available space. Changes to the glass specification were then considered. Reducing the g-value allowed the required ventilation rate to be reduced from 8 ACH to 4 ACH. However, even this was still considered too expensive and noisy and insufficient for comfort during the hottest periods. The project illustrates a critical point: changing the g-value can improve one part of the calculation without solving the entire building-performance problem.
4. Swiss International Scientific School Dubai: when advanced glazing produced an unexpected result
The Swiss International Scientific School Dubai provides perhaps the most striking example of the difference between glazing performance and façade performance. Research into the school’s adaptive façade found that its electrochromic glazing could reduce the g-value from approximately 0.60 to 0.25, while visible transmittance also fell significantly. On paper, this represented a substantial improvement in solar control.
But the façade had an extremely high 85% window-to-wall ratio.
Post-occupancy evaluation reported that energy consumption increased by 27% compared with the conventional façade. The research identified the very high WWR as contradicting the intended energy-efficiency benefits of the electrochromic glazing. It also reported winter discomfort among occupants because the low g-value restricted passive solar heating while the building lacked a heating system.
This is a particularly important lesson for high-performance glazing. The technology itself was capable of dramatically reducing solar gains. But excellent glass performance did not automatically produce excellent whole-building performance. The problem was the relationship between the glazing technology, window-to-wall ratio, climate, building operation and energy strategy.
5. ABC Building, White Rose Park: when the façade had two seasonal problems
The ABC Building at White Rose Park in Leeds provides a different perspective: an existing glazed building being upgraded rather than a new building being designed. The building, originally constructed in 1982, is undergoing a major refurbishment that includes fenestration improvements. The project has retained the existing MAG Presslock curtain-wall system while replacing approximately 748 m² of glazing.
The replacement glazing uses Guardian SunGuard Silver 35/26, reducing the reported g-value from 0.63 to 0.266, approximately 58% less solar energy transmission through the glass. The centre-pane U-value is also reported to improve from approximately 2.8 to 1.3 W/m²K. The significance of this case is that the façade had to address more than summer solar gain. The refurbishment sought to improve the building’s broader thermal performance while retaining the existing curtain-wall system. It illustrates an increasingly important challenge for Europe’s existing building stock: the same façade can be too hot in summer and insufficiently insulating in winter.
The g-value is not an isolated number
These five projects point toward the same conclusion. The g-value belongs to the glazing system, but its consequences belong to the building. Orientation matters. Glazing ratio matters. Shading matters. Climate matters. Occupancy matters. Ventilation matters. Heating and cooling systems matter.
The Southampton case demonstrates the interaction between glazing, thermal mass and solar exposure. Polo Eustachio demonstrates what can happen when a fully glazed, south-facing façade receives persistent solar gains. Salford Quays shows that reducing g-value does not necessarily eliminate overheating. Dubai demonstrates that sophisticated adaptive glazing can still underperform when combined with an extremely high WWR. And White Rose Park shows that an existing façade may need to balance solar control with winter thermal performance.
None of these cases means that a lower g-value is inherently wrong. The lesson is more subtle. A lower g-value is not automatically a better g-value.
So what should the industry optimise?
Perhaps the question is no longer: “How low can we make the g-value?” The better question may be: “What g-value does this building actually need?” That means considering the complete chain:
Climate → orientation → glazing ratio → shading → daylight → heating → cooling → occupancy → comfort.
The solar factor should be one part of that calculation, not the entire strategy.
The uncomfortable conclusion
The glass industry has become exceptionally good at controlling solar radiation. But the next challenge may not be controlling more of it. It may be controlling the right amount. A g-value of 0.25 is not automatically better than 0.35. A g-value of 0.35 is not automatically better than 0.45.
The appropriate value depends on what the building is trying to achieve, where it is located, how it is oriented, how much glass it contains and what other strategies are working alongside the glazing. The future of solar-control glazing may therefore not be about making the g-value ever lower. It may be about making the decision behind the g-value more intelligent. Because the most efficient façade may not be the one that blocks the most solar energy. It may be the one that knows exactly how much solar energy to let in.
Source: Glass Balkan
