Understanding solar shading and passive cooling

As the UK experiences warmer summers and more frequent periods of extreme heat, overheating is becoming an increasingly important consideration in building design. The Climate Change Committee (CCC), in its recent A Well Adapted UK report, identifies overheating as a growing climate risk and highlights the important role that passive measures can play in reducing its impact. 

One of the most effective passive cooling strategies is dynamic solar shading. By controlling the amount of solar radiation entering a building, solar shading can help reduce overheating risk, improve thermal comfort and limit the demand for mechanical cooling. And being dynamic can be retracted when not needed. 

Understanding solar heat gain

The science behind solar shading is simple. Solar radiation passes easily through glazing and is absorbed by objects and surfaces, which then re-emit heat that is less able to escape back through the glass. This process, known as solar heat gain, causes the indoor environment to heat up increasing internal temperatures, particularly in buildings with large, glazed areas exposed to direct or reflected sunlight. As temperatures rise, without the correct shading, cooling systems may be required to maintain comfortable conditions, increasing energy consumption and associated carbon emissions. 

Solar shading devices are designed to intercept or manage sunlight before it contributes to this unwanted solar heat gain. External shading devices, including blinds and awnings, are particularly effective because they prevent up to 98 per cent of solar radiation (in the case of external Venetian blinds when closed) from reaching the glazing in the first place. In contrast, internal shading is less effective because, once solar radiation has passed through the glazing it is more difficult to direct it back out, and much of it is absorbed and released within the space. However, some internal shading products have special reflective coatings applied to the rear face making them more effective than typical internal shading products and having some internal shading is better than nothing at all.   

What do g-values and shading factors mean?

For architects and specifiers, understanding performance measures such as g-value for glazing and Gtot for glazing and shading combined is important when evaluating solar shading solutions. 

The g-value, sometimes referred to as the solar factor, indicates the proportion of total solar energy transmitted through glazing. Lower g-values correspond to lower solar heat gain. 

Gtot represents the total amount of solar energy transmitted through the shading and glazing combined. Again, the lower the Gtot the lower the solar heat gain. Together, these metrics help assess the effectiveness of a solar shading system as part of the overall building design and can help inform specification decisions during the design process. 

Balancing thermal comfort and natural daylight

Well-designed solar control strategies can contribute to thermal comfort while maintaining useful levels of natural daylight too. This balance is important because excessive sunlight can create both overheating and visual discomfort, while insufficient daylight may increase reliance on artificial lighting. 

Effective solar shading strategies seek to manage these competing requirements by controlling solar gain without unnecessarily reducing daylight levels. The result can be a more comfortable indoor environment which supports occupants’ thermal and visual comfort throughout the day. 

The role of dynamic solar shading

Dynamic solar shading can provide the ideal solution. Automated systems can respond to factors such as the time of day, seasonal conditions and solar intensity, helping to optimise daylight and solar control throughout the year. 

Dynamic shading systems can be integrated into building management systems, enabling shading devices to respond automatically to changing conditions and support wider energy efficiency objectives. 

Supporting passive cooling and energy efficiency

The environmental case for solar shading is becoming increasingly relevant. Research cited by the CCC has found that external shading can significantly reduce indoor operative temperatures. 

By limiting heat build-up at the point of entry (i.e. glazing), solar shading can reduce cooling demand and contribute to improved building energy efficiency. When considered alongside other passive design measures, such as ventilation and thermal mass, solar shading can form part of a broader strategy for managing overheating risk. 

Designing for a warmer future

As the built environment adapts to climate change, passive cooling strategies are likely to play an increasingly important role. Solar shading has long been used on building facades across different climates and continues to offer a practical means of managing solar heat gain in both residential and commercial buildings. 

For designers seeking to balance occupant comfort, energy performance and climate resilience, solar shading remains an important component of a considered passive cooling strategy. 

• To learn more about specifying effective solar shading systems, visit www.bbsa.org.uk and explore the technical resources available from the BBSA.