Passive cooling refers to a set of design principles and techniques that utilise natural processes to help maintain comfortable indoor temperatures without the need for active, energy consuming, mechanical systems, such as electrically powered air conditioning units, thereby reducing environmental impacts and economic costs. The approach leverages materials and the local environment to achieve cooling and has been employed for centuries in the vernacular architecture of countries that already have hot climates, though in many it has been abandoned in the quest to look modern through the construction of sealed concrete, steel and glass boxes that demand mechanical equipment to cool. Techniques such as considering the orientation of the building’s doors, windows and vents to predominant breezes, creating reflective exterior surfaces through material selection or the application of light-coloured paint, and incorporating thermal mass, insulation, shading, verandas, running water, inner courtyards and vegetation into the design, are all examples of a passive cooling approach.
The fundamental starting point for passive cooling design is to minimise the solar gain of the building in the summer months and thereby reduce the need for internal cooling in the first place. Many methods are available to achieve this goal. For example, one approach applicable to new and existing properties alike, which helps to reduce summer gain whilst ensuring good quality daylight during the winter months, is the installation of adaptable external shades or internal window blinds. Another possible approach is the addition of external shutters, similar to those traditionally used on domestic dwellings and small commercial buildings across continental Europe.
The choice of the facade material and external treatment also impacts solar gain by affecting the reflectivity of the building’s external surfaces. In this regard, large, darkly coloured areas should be avoided in preference for the use of white and/or other light colours. Heat-reflective surface paints, or smart tiles which change emissivity with temperature, could both be selected to reduce unwanted internal heat gains[1]. The structure of the building itself in terms of its resulting thermal mass can also have a significant influence on the internal heat gain and increasing this by using thicker walls and roofs, denser construction materials, and/or thermal insulation, will help to reduce or shift the timing of peak cooling requirements. However, care needs to be taken with this approach to ensure that embedded carbon emissions are not significantly increased by wall and roof thickening, and overall sustainability degraded, by the use of greater volumes of concrete, thermal insulation, etc.
Shading, in the form of overhanging roofs, tree canopies, verandas and internal courtyards, as well as roof mounted solar panels, can also help. The latter has the added benefit of providing local renewable energy whilst contributing to heat gain reduction through providing shade to the area of roof under the panel. The use of solar panels for the provision of electrical power or hot water can not only deliver benefits for GHG emissions reduction, and thereby climate change mitigation, but also help with the adaptation of buildings to higher ambient temperatures and extreme heat events.
Research has shown that roof mounted panels can reduce internal heat gains during the day as well as act as an insulator against heat loss overnight. In this regard, a recent study carried out in India found that the impact of tilted roof PV panel shading varied on a case-by-case basis but consistently resulted in a reduction of the need for internal cooling[2]. In the 11 different city locations evaluated the annual cooling load of the building was reduced. However, in each case this was offset to some degree by an increase in the demand for heating due to the shaded area reducing the beneficial solar gain in the winter months. Overall, energy savings were recorded between 21.3% and 25.1%, with the highest benefit being in hot and dry climatic areas. Other studies have found similar results, for example in the use of PV panels on uninsulated roofs in Jordan[3]. The latter investigation revealed a 10.9% reduction in heat gain during the summer months and a winter increase in heat loss of 3.8%.
Beyond minimising solar heat gain to reduce the need for cooling, when external temperatures allow, the use of natural ventilation to remove heat from internal spaces is the primary tool of a passive design approach. Techniques include capturing local predominant breezes through the orientation of openings such as doors, windows and vents (for example clerestory vents, etc.) and the use of wind chimneys or windcatchers, a ducted design feature commonly employed in the vernacular architecture of Arab nations. The latter are traditionally used to catch cooler air from the external environment, either from predominant breezes or at night, and direct it into the building while ensuring that the hot air is allowed to rise-up and exit the internal space through the same wind inlet/outlet. Commercial windcatchers can decrease indoor temperature by up to 14°C[4], with the cooling efficiency ranging between 22 and 70 per cent depending upon the time of the year and the external temperature variation, etc[5].
The variation of temperatures throughout the day can also be exploited by emerging Phase Change Materials (PCMs)[6] to provide cooling through the storage of ‘coolth’ at times when temperatures are low (e.g. during the night) and its subsequent use for absorbing heat when temperatures are high. PCM technologies are based on substances that can store and release thermal energy during the process of melting and solidifying. They have the potential to revolutionise cooling by providing efficient and passive cooling solutions and can be integrated into building materials or thermal storage units to reduce the need for active approaches.
However, despite the advantages of passive cooling techniques, and their traditional use in the vernacular architecture of many hot countries, they have become less accessible to building designers and occupants in modern times. Globalisation and increased urbanisation have led to the proliferation of uniform building designs worldwide that often disregard the local climate and issues of solar gain in preference for large expanses of sealed glass uninterrupted by openings or systems of blinds; uncluttered external facades devoid of adjustable shades or shutters; low thermal mass; inappropriate construction material; and the use of active cooling in the form of HVAC (Heating, Ventilation, and Air Conditioning) systems to deal with subsequent overheating. This approach results in the unnecessary consumption of energy and, if fossil fuel based, GHG emissions, along with the exacerbation of urban heat island effects in cities and conurbations, which creates a vicious cycle of overheating and growing cooling demand.
To encourage greater uptake of passive cooling measures as a component of a strategy to adapt to higher temperatures, the benefits of their use need to be better quantified and disseminated, not only to professional architects, designers and planners, but also building users. The latter is particularly important in the Global South, where in the decades ahead the majority of the world’s future new building stock will be constructed and, in many countries, new builds, modifications and retrofits are commonly undertaken by end-users with minimal or no intervention from buildings professionals. Cataloguing passive cooling measures and their benefits, in terms of temperature reduction, thermal comfort, environmental performance, health impacts and financial savings, for every climate and representative buildings/regional archetypes, would be a helpful first step in encouraging their wider uptake in countries that are becoming hotter, as well as their revival in those that have lost the practice of their vernacular architecture.