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As the mercury ticked upward in Portland, Ore., last month, I braced for my apartment to become unbearable.

Normally, my un-air-conditioned basement unit would be fine for the Pacific Northwest’s temperate summers. But these are not normal times. Climate change has lengthened and intensified heat waves, pushing temperatures to unheard-of extremes. In Portland, a new all-time high was set three days in a row: First, 108 degrees. Then 113 degrees. <a href="https://www.washingtonpost.com/national/extreme-heat-wave-porland-seattle/2021/06/28/c88790d6-d82d-11eb-bb9e-70fda8c37057_story.html" target=_blank>Then 116</a>.

To my astonishment, the apartment stayed tolerable all weekend. The tile floors seemed to emanate coolness. The greenery surrounding my windows blocked direct sunlight and helped bring down the temperature of the outside air. I didn’t have a thermometer, but my guess is that the temperature inside never got above 80 degrees.

“You saw for yourself the power of passive cooling,” buildings scientist Alexandra Rempel told me. “It really can be amazingly, amazingly effective.”

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Rempel, an assistant professor in the environmental studies program at the University of Oregon, studies how to design buildings that can stay cool “passively,” without relying on air conditioning. The techniques that helped my apartment beat the heat — shade, building materials, strategic ventilation — can be used in almost any home, she explained.

On a warming planet, passive cooling can help protect people without access to air conditioning and lighten the load on the electrical grid from those who do. It can also help reduce greenhouse gas emissions produced by burning fossil fuels for power — a necessary step for tackling climate change and the only hope we have for avoiding an even hotter future.

Nathan Howard

It’s important to understand why buildings get hot. During the day, heat comes from solar radiation — the sunlight that streams through windows and beats down on roofs and walls. At night, the big problem is environmental radiation — the energy emanating from asphalt, concrete and any other surfaces that had been absorbing sunlight all day.

Passive cooling is about effectively managing these sources of radiation. And timing is everything.

How to stay cool and safe during a heat wave

As soon as the sun rises, window shades should come down. Window glass is “one of the weakest links” in a building’s defense against solar radiation, Rempel said, because it readily transmits heat. The best way to prevent this is to install exterior window coverings, like shutters or retractable awnings. If those aren’t an option, inside curtains or blinds are a good alternative. You can even cover a piece of cardboard in aluminum foil and press it into the window frame.

Having vegetation around your building can prevent the walls from heating up as well. Trees not only provide shade, they can bring down the surrounding air temperature through a process called <a href="https://www.epa.gov/heatislands/using-trees-and-vegetation-reduce-heat-islands" target="_blank">evaporative cooling</a>. As leaves release water into the air, energy is used to turn the liquid into vapor — which means it doesn’t go into heating up the environment. The same phenomenon explains why sweating helps cool you off.

“Cool roofs” also make a big difference, Rempel said. Topping a building with light-colored, highly reflective materials prevents it from soaking up the sun’s heat. Even better: Build a rooftop garden. According to the <a href="https://www.epa.gov/heatislands/using-green-roofs-reduce-heat-islands" target="_blank">Environmental Protection Agency</a>, vegetation can lower a roof’s temperature by 30 to 40 degrees. Though more expensive than simply painting the roof white, a green roof can also handle storm-water runoff, reduce air pollution and provide mental health benefits.

Melina Mara

If you can limit the amount of solar radiation your home absorbs during the day, you will have less environmental radiation to worry about once the sun sets. As night falls and the outdoor air temperature drops, it’s time to open up your windows. Create cross ventilation by opening windows and doors on opposite sides of rooms. If your home has more than one story, make sure rising hot air can flow out through windows on upper floors or openings in the roof.

“Cool night air is really the best free cooling resource we have,” Rempel said.

Earth is now trapping an 'unprecedented' amount of heat

This is also the best time to turn on a fan. Remember that fans don’t cool air; they just move it around. Having a fan blow air that is hotter than your body temperature can actually make it harder for your body to shed heat by sweating.

But if indoor air temperatures are below about 95 degrees, it is safe to turn on <a href="https://www.energy.gov/energysaver/home-cooling-systems/fans-cooling" target="_blank">ceiling or window fans</a>. If you are using a window fan, make sure to place it where it will draw in the coolest air — a unit in the window overlooking a leafy backyard is preferable to one that pulls in hot air and car exhaust from a busy roadway.

A cool nighttime breeze not only lowers indoor air temperature, it also can take heat out of the materials from which your home is built. Certain materials, such as tile and drywall, are especially good for this. They have a high “specific heat capacity” — it takes a lot of energy to raise their temperatures by just a degree. This is why your bathroom’s tile floors are always cold, even when the house is warm.

“Materials are really the invisible player in all of this,” Rempel said. “They can remain quite cool and be a cool buffer” once the day starts heating up.

ALISHA JUCEVIC

Studies show that passive cooling can be quite effective at reducing the need for air conditioning; one analysis in Albany, N.Y., found that these techniques reduced summer cooling loads by 50 percent. For those who don’t have access to air conditioning, or who can’t afford it, passive cooling can prevent homes from becoming unsafe to stay in. And amid record-setting heat waves <a href="https://www.washingtonpost.com/nation/2021/06/17/california-heat-wave-power-grid/" target=_blank>that strain energy systems</a>, it can reduce AC use among those who do have it, averting the need for blackouts.

Reducing reliance on air conditioners is essential, Rempel said, because the devices are terrible for the planet. They require a lot of energy to run, accounting for about <a href="https://www.energy.gov/energysaver/home-cooling-systems/air-conditioning" target="_blank">6 percent</a> of all electricity use in the United States and 117 million metric tons of carbon dioxide each year. The main refrigerants used in air conditioners, a class of chemicals known as hydrofluorocarbons, are some of the most potent greenhouse gases on the planet, trapping thousands of times more heat than an equivalent amount of carbon dioxide.

“Running an air conditioner is just trying to solve a problem that it is also worsening,” Rempel said. Passive cooling offers a way to handle heat that doesn’t risk raising temperatures further.

Climate change has gotten deadly. It will get worse.

But, much like climate change, the problem of deadly temperatures is too big to be addressed person by person, home by home. To beat the heat, experts say, people have to advocate for policy changes as well.

Extreme temperatures can be made worse by the way cities are designed, a phenomenon called the “urban heat island effect.” Tall buildings create canyons that trap heat close to the ground. Dark surfaces like asphalt absorb the sun’s energy and radiate it back into the environment, keeping temperatures high well after dark. Human activities, like operating factories, driving cars and even running air conditioners, create “waste heat” that makes the problem worse.

These problems are disproportionately likely to affect those who can least afford to deal with them, said David Hondula, who helps lead the urban climate research center at Arizona State University. When I spoke to him last year for a <a href="https://www.washingtonpost.com/graphics/2020/climate-solutions/phoenix-climate-change-heat/" target=_blank>story about heat islands in Phoenix</a>, Hondula explained that low-income people of color are more likely to live in neighborhoods with little vegetation that are next to highways and industrial areas. At night, the temperature in one of Phoenix’s poorest neighborhoods is as much as 10 degrees hotter than in wealthier communities. At the same time, residents of urban heat islands are less likely to have air conditioning. Even if they do, they may avoid turning it on to save money on their electricity bills.

But you can advocate for your city to adopt policies that help keep all homes cool in a heat wave. Creating more parks and planting vegetation in public spaces, especially in heat island neighborhoods, helps to bring temperatures down, Hondula said. Improving walkways, bike lanes and access to public transit can reduce car use and eliminate some exhaust heat.

Enhanced regulation can also ensure that passive cooling tactics are accessible to those who most need them. Although states like Oregon have heating requirements that rental properties must meet in the winter, landlords are not required to ensure that units stay cool in the summer. There is no requirement that residents of affordable housing units be able to safely open their windows.

“We really need to be intervening with buildings and neighborhoods to make them more survivable,” Rempel said.

2019 Walter Sullivan Award for Excellence in Science Journalism–Features; 2020 AAAS Kavli Science Journalism Award - Silver; 2023 Evert Clark/Seth Payne Award for a young science journalist

Georgetown University, B.S. in International Culture and Politics

Sarah Kaplan is a climate reporter covering humanity's response to a warming world. She previously reported on Earth science and the universe.  