How sloping facades can help reduce overheating

Overheating in buildings is becoming an increasing concern. The established focus on maintaining heat needs now to be balanced with strategies for heat rejection to ensure thermal comfort. One strategy that is rarely used is sloping, self-shading buildings. I set out to study this and its wider impacts. But first a bit of background.

The UK is projected to encounter temperatures in the mid-century comparable to those currently experienced in southern Europe. The architecture of that region has historically incorporated additional shading strategies to prevent overheating, such as shutters, overhangs and urban morphology as prominent features. The UK has also benefited from these historic lessons to adapt to the changing climate. Another option, which is spatial and inherently passive is using building form as shade. I set out to investigate how shading can become an integrated part of a building’s form. Can a slope be a new overhang?

In recent years, post-occupancy evaluation of highly-insulated buildings has revealed a conflict: larger south-facing windows aim to reduce heating demand but also increase the risk of overheating. Shading walls and windows have traditionally been key design factors influencing indoor conditions and preventing overheating. Most overheating studies focus on building fabric, services, and occupant impact. However, less research has explored how a building’s geometric form as a passive design strategy could control solar gain. So we ask this question again: can a slope be a new overhang?

During the design phase, a building’s form is typically shaped by considerations such as function, cost and aesthetics. Studies of form suggest that buildings with different external envelope area but similar internal volumes or floor area have different energy demands (see reading list below). And that geometric strategies aimed at enhancing the energy efficiency of a building may possess architectural characteristics and significantly affect the overall appearance of structures. 

It is sensible to consider thermal performance from conceptual design, emphasising the proactive assessment of building envelope geometry’s impact on its thermal performance – to contribute to a more thermally comfortable and sustainable building design. My study showed the potential impacts of strategies using sloping forms.

Studying sloping forms

To answer this question, a pilot study in the form of a simple box was modelled, adopting the fabric specifications characteristic of a typical UK Passivhaus. The research focused on adjusting the tilt angle of the south façade, exploring its impact on balancing the solar gain. The variations were tested in 5-degree intervals, ranging from 90 degrees (a vertical wall) to 145 degrees (using DesignBuilder engine).

The findings of the study proposed that an inclined wall within the range of 105 degrees to 120 degrees could serve as an effective shading strategy depending on the location and different UK weather projections. A tilt angle of between 110 and 115 degrees emerged as a nearly optimal configuration. This conclusion takes into account potential future climate scenarios in London, particularly addressing the heightened risk of overheating by the mid-century. The incline of the facade, serving as a shading mechanism, may be adjusted to a steeper angle in hotter climates with intense sunlight or to a gentler slope in regions experiencing milder summers (see sketch). This is because, in hotter climates, the building has a reduced capacity for heat gain.

The study demonstrated the benefits of self-shading in mitigating overheating. Implementing sloped walls can prove advantageous for various facades, such as the west-facing side, enabling the exploration of more unconventional architectural forms through this concept.

Geometric implications, such as sloped walls, pose construction challenges and economic feasibility considerations. Nevertheless, recent advances in construction methods have simplified the construction of more complex designs. We also note the small concurrent increase in heating demand due to the inclined façade. However, this trade-off is acceptable for reduced summer overheating and improved thermal comfort. This approach especially appears sensible under future extreme summer scenarios and is also applicable to large-scale buildings with extensive glazing.

Further impacts of sloping and self-shading buildings

Recognising geometry’s potential for aesthetic and spatial delight, sustainable architecture can extend beyond solely technical solutions. The study suggests that the building form, as a design concept, can be shaped to adapt to the climate since adjusting the building facade induces additional environmental responses such as air movement and different daylight distribution.

Computational fluid dynamics (CFD) results from the pilot study indicate that, when the inclined wall faces the wind, there is a decrease in the airflow rate entering the space. The simulation conducted by ANSYS Fluid Dynamics shows a reduction in pressure distribution on the exterior surface of the tilted facade. Conversely, for the case of a tilted facade on the leeward side, ventilation rates noticeably increase. External CFD calculates a stronger negative pressure on the exterior of the tilted facade, extracting additional airflow from the building. Although the prevailing wind in the UK is from the south-west, this means that when wind comes from the north, east or west, ventilation increases with a sloped south facade. The CFD analysis below shows the pressure distribution on and around different facades. The lighter shade shows positive pressure whle the darker shade shows negative pressure distribution.

What about light? Daylighting analysis reveals a reduction in illuminance inside a building with a sloped facade. This could potentially reduce unnecessarily high levels of daylighting in extensively glazed buildings. And, of course, other design strategies for reducing overheating might also lead to reduced illuminance.

Overheating is primarily a result of high solar gain, coupled with a lack of sufficient ventilation. My study focused on exploring a form-based shading strategy to mitigate overheating, particularly emphasising the optimal inclination of a south-facing facade for self-protection in a highly airtight building. Through it, external shading has emerged as a particularly effective single intervention to lowering overheating, especially in a warming climate. It suggests that a self-shading strategy with a 110-115-degree tilted south facade effectively reduces overheating risk in London, especially for future climate scenarios.

These findings could serve as a wellspring of inspiration for architects, offering a pathway to derive architectural solutions for so-called technical problems (in this case overheating). By aligning design principles with energy considerations, this study provides architects with a practical resource to creatively address and resolve building physics problems within the broader context of architectural form and design.

Yahya Lavaf-Pour is a senior lecturer in architecture at the University of the West of England