The world is heating up, but our cities are heating faster.
Paved with asphalt and packed with concrete, urban centres radiate heat, turning cities into sweltering heat islands that can get up to 10C warmer than the surrounding countryside.
As climate change accelerates, more frequent and intense heatwaves are causing demand for cooling to skyrocket its on track to triple by 2050. If this cooling comes in the form of standard air conditioning, cooling-related greenhouse gas emissions will nearly double.
During a heat wave, air conditioning can save lives but it uses tremendous amounts of energy, and the emissions it releases are making the world even hotter, says Honpeng Lei, Chief of the Climate Mitigation branch of the United Nations Environment Programme (UNEP). Its also unaffordable for billions of people, leaving them vulnerable on dangerously hot days.
But there are alternatives: passive and low-energy cooling techniques can keep rooms comfortable when temperatures peak. When strategically combined these methods can cool a building by 6-9C, reducing our reliance on air conditioning or eliminating it altogether though the most effective combinations will change for different climates around the world.
Here are five ways to keep buildings comfortable without cranking up the AC.
Painting roofs white can lower surface temperatures by up to 20C. Credit: Barry Davis/Robert Harding via AFP ">
1. Cool roofs and walls
The colour and material of a building can have a big impact on indoor comfort. Highly reflective roofs covered in light-coloured coatings or specialized paints also known as cool roofs reflect heat away from the building, lowering surface temperatures by up to 20C. A UNEP-backed project in Delhi, India which involved adding reflective paint to the roof of one of Indias busiest bus terminals, managed to cool the building by 2-3C.
Incorporated into thicker walls and partitions, certain concretes and earthen bricks can stabilize indoor temperatures as they absorb and slowly release heat, thus providing critical insulation. Keeping a building airtight can prevent outside heat from seeping in. And reducing the window-to-wall ratio from 40 to 20 per cent can cut the cooling requirements of commercial buildings by up to 12 per cent.
2. External shading
Extended overhangs, shutters and awnings prevent the suns rays from directly heating windows and walls. Horizontal fins on a buildings faade are also very effective at blocking the midday sun; a case study found that combined with overhangs, these fins could reduce the demand for cooling by more than 15 per cent per year.
Natural shading also plays an important role. Leafy trees planted near windows and walls do double duty, cooling the immediate environment while protecting nearby buildings from direct solar exposure.
3. Natural ventilation
Even with the best insulation and shading, buildings heat up on a hot day. A well-ventilated building provides for passive heat removal, and can cut air-conditioning use by up to 55 per cent.
Cross-ventilation, ventilated roofs, and other features designed to promote airflow allow excess heat to leave the building. This is helpful in all climate zones; but its especially effective in hot and dry climates, where nighttime temperatures drop significantly and ventilation has been shown to extend thermal comfort hours by nearly a quarter.
4. Expanding green and blue spaces
The heat island effect is exacerbated by a citys surfaces as well as its walls; replacing pavement with greenery or water features can dramatically reduce the ambient temperature of entire neighbourhoods.
Trees, parks, leafy courtyards and ponds represent an affordable and scalable approach to combatting extreme heat, lowering temperatures through a process called evapotranspiration. The same applies to green roofs and walls, which reduce a buildings surface temperature while insulating it from the sun.
Examples of heat-conscious urban greening include Singapore, where the citys gardens save an estimated US$ 36.8 million in energy costs every year. Another is Melbourne, Australia, where urban greening in a new suburb was valued at nearly US$ 5 million due to electricity savings, reduced mortality and gains in productivity.
5. Low-energy mechanical cooling
When passive cooling systems require a mechanical boost, consider the humble fan. Products like fans and evaporative coolers use far less energy than air conditioners, providing comfort at a fraction of the monetary and environmental cost.
Fans dont cool the air but by moving it around they take advantage of the bodys natural ability to cool itself, especially important in hot and humid climates where sweat doesnt evaporate as easily. The effect can be dramatic, reducing perceived indoor temperatures by as much as 4C often enough to cut or eliminate air conditioning use altogether.
Evaporative coolers take the cooling power of evaporation one step further, using fans to transform water into vapor; as it evaporates, water absorbs heat from the air, cooling a room while using 80 to 90 per cent less power than an air conditioning unit. In hot, dry conditions, evaporative coolers can reduce temperatures by up to 15C.
By combining high-efficiency fans with passive cooling methods, the Green School Programme in Indias Tamil Nadu province, supported by UNEP, is lowering indoor temperatures by around 3C in 300 schools. UNEP has similar projects are underway in Brazil and the Philippines.
In some cases these solutions arent enough, and air conditioning really is the best way to keep people safe, says UNEPs Lei. But passive and low-energy cooling solutions like these present a pathway towards smarter, more sustainable and more affordable cooling. Not only do they reduce emissions they build global climate resilience, preparing our cities for the hotter future that lies ahead.
Author: Hilary Heuler
Reviewed by: Honpeng Lei, Gulnara Roll, Lily Riahi, Marisofi Giannouli and Benjamin Hickman
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