With cities around the world scrambling to cope with record heat, and households facing higher power bills to run air conditioning, access to cooling is emerging as a crucial issue in climate change resilience. And passive cooling methods that bring down indoor temperatures without mechanical systems are having a moment.
“Cooling is increasingly in demand in buildings, especially as the climate gets hotter,” Omar Dhia Al-Hassawi, assistant professor in the School of Design and Construction at Washington State University, told Machine Design earlier this year. “There might be inclusion of mechanical systems, but how can we cool buildings to begin with—before relying on the mechanical systems?”
Cue the return of age-old techniques that can make buildings less reliant on air conditioning systems, make it easier for occupants to get through the next massive heat wave, and reduce energy costs—whether or not they fully replace AC. “With energy demand for active mechanical space cooling projected to double by 2050, researchers are investigating alternative means, such as passive cooling systems, to help reduce demand,” Machine Design writes.
“Rather than relying on refrigerant, this approach uses nature—including cross breezes, reflective surfaces, shade, and water evaporation—to reduce the indoor temperature,” Business Insider explains, citing Tim David, owners of a heating, ventilation, and air conditioning (HVAC) company in Alabama. “While some of the changes can be costly to implement, incorporating passive cooling can reduce your energy costs in the long term,” and many jurisdictions have loans or tax credits available to help cover the up-front cost.
Six Passive Cooling Techniques
Passive cooling means incorporating a half-dozen tenets into building design and operation, Business Insider says:
• Windows and doors are placed to maximize natural air flow, capturing outdoor breezes and funnelling through the space in the straightest line possible.
• Taking advantage of convection—the basic principle that hot air rises—to draw cooler air into a building as heat escapes through roof vents. “This creates airflow, even when there isn’t a natural breeze,” Business Insider explains.
• White or light-coloured paint and building materials, reflective tiles, or heat-reflecting paint on roofs can deflect daytime sunlight that would otherwise heat the building.
• Trees, shrubs, and indoor shades can all be used to keep the heat out when a window is getting full sun.
• Better insulation in walls, floors, and ceilings keeps heat from reaching living areas, just as it blocks the cold in winter.
• Water cools the air around it when it evaporates. “I really like adding water features outside, especially near the natural airflow intake windows, as this can really help cool the air entering the house,” David told Business Insider.
“In passive cooling, you design your building to become the air conditioner,” said Pablo La Roche, a passive cooling and carbon-neutral design specialist at California State Polytechnic University Pomona, in a Q&A with the state’s Public Policy Institute. “A passively cooled building uses the building’s materials and design to transfer heat to heat sinks, such as the air around the building, in the same way air conditioning does—but without using electricity.”
Passive cooling begins with properly siting a building, because “you can’t just lift and rotate it,” La Roche added. But the techniques are practical for existing buildings as well as new ones.
“If you’re in an old building with no insulation, single pane glass, and unshaded windows getting lots of sun, the air conditioner is either working full time or you are exceedingly and sometimes dangerously hot,” he said. “And not everyone has air conditioners—which is a problem and an equity issue. You want to reduce the heat coming into the building so that the AC doesn’t have to get as much as heat out of the building.”
Energy Savings Cover the Cost
La Roche said the cost of added insulation, shading, and new windows can be paid back through energy savings. But while Tim David said the financial gains begin accumulating right away in a new building, a retrofit can take four years or more to pay off.
“If you’re renovating to try to reduce your reliance on air conditioning, you’re more likely to spend up front in order to get long-term savings,” Business Insider says. “Blackout shades can cost hundreds of dollars, while painting your home can easily cost US$5,000 or more. A new tile roof—one of the most effective passive-cooling strategies—can cost $20,000 or more.”
None of those estimates captured the incremental cost of the passive cooling features—how much more an owner would pay for a tile roof, for example, compared to shingles, if the roof replacement was already taking place. Either way, the savings would weigh against the cost of buying, maintaining, and replacing an air conditioning system, and operating it at a cost the news story places at $150 to $180 per month.
“It might take a few years to recoup the initial investment, especially if major renovations are involved,” David said, though the dollars would come back much faster in the southern and southwestern United States, where air conditioners run through most of the year.