Air-conditioning isn’t just expensive; it’s also terrible for the environment. Accounting for 10% of global energy consumption today, space cooling in 2016 alone was responsible for 1045 metric tons of CO2 emissions. This number is only expected to increase, with the International Energy Agency estimating that cooling will reach 37% of the world’s total energy demand by 2050.
Air-conditioning units are in particular harmful because they rely on a refrigerant called Hydrofluorocarbon (HFC). While HFC only accounts for 1% of all greenhouse gas emissions, it is also thousands of times more potent than carbon dioxide.
Designing with materials that are naturally cooling can help mitigate these environmental effects by reducing the need for air conditioning. Below, we compile some material and structural solutions to passive cooling that may help designers regulate building temperatures energy-efficiently.
Thermal Mass and Insulation
Dense materials such as stone, concrete, and earth each have a number of properties that allow them to act as good insulation from heat. These alternately include good thermal conductivity (ability to rerelease passive cooling), thermal lag (slow heat transmission), low reflectivity (lower redistribution of heat), and high volumetric heat capacity (elevated ability to store heat). When such materials are used in bulk, their insular qualities become especially potent, exemplified by unique ‘cave homes’ such as Kapsimalis Architects’ Summer Cave House in Santorini. Other projects, like A-cero’s Concrete House II, rely on thick concrete walls to achieve similar effects.
More traditional homes may not use such bulky materials but rely on effective thermal insulation instead. Typically, the thermal resistance of insulation is measured by what is called the ‘R-factor’ or ‘R-value.’ The higher this value, the more thermally resistant the material, and the more effective of an insulator it is. Materials such as polystyrene, polyurethane foam, and phenolic foam are examples of thermal insulators that have phenomenally high R-values.
Besides its thick concrete walls, A-cero’s Concrete House II and a multitude of similarly heat-conscious designs utilize natural elements such as green roofs or ivy walls. Green roofs are not only aesthetically pleasing, but they also provide shade, remove heat from the air, and reduce the temperature of the roofs. Some notable examples include Renzo Piano’s California Academy of Sciences, CPG’s Nanyang School of Art, and Enric Ruiz-Geli’s Villa Bio.
Incorporating water into a building can likewise cool a home through evaporation and air flow, depending on the climate. This methodology was recognized as early as the Romans, who often designed their homes around a central courtyard pool.
Window Material and Placement
Green roofs and water features may seem excessive to the average homeowner or designer, but passive cooling can also be as easy as choosing the right glass for a building’s windows. The lower the solar heat gain coefficient (SHGC) of the glass, the less heat it transmits and the cooler the building. These benefits can be increased with external blinds, which prevent sunlight from hitting the windows at all and thus reduce the amount of heat or glare that reaches the interior. Even the positioning of these windows can have passive cooling effects through cross-ventilation, or the aligning of windows to facilitate air circulation. Notable examples of cross-ventilation include Louisiana shotgun houses, which minimize interior walls that may obstruct horizontal drafts.
Lastly, light-colored reflective roofs, another alternative to green roofs, can effectively cool interiors by redirecting sun rays and decreasing heat absorption. Examples include roofs with sheet coverings, reflective tiles or shingles, or reflective paint. While standard or dark roofs can reach 150 degrees Fahrenheit in intense heat, ‘cool roofs’ may only reach 50 under the same conditions.
High roofs and cupolas may also allow existing heat to rise and escape areas in use. Similarly, covered porches and awnings can protect interiors from sunlight and glare. Altogether, material considerations and structural design go hand in hand to create effective alternatives to air-conditioning and mechanical cooling, decreasing HFC use and detrimental greenhouse gas emissions.