Argonne modeled Chicago’s August 2021 heatwave with different roof types; white roofs lowered air temperature by nearly 2°C and cooling energy use by 16.6%

Chicago summers are getting harder to sit through, and a six-day heatwave in August 2021 gave scientists a real-world test case. In a new study published in Science of the Total Environment, researchers at Argonne National Laboratory modeled the effects of replacing all rooftops in the Chicago metropolitan area with a cooler surface. The results were notable. Using white cool roofs lowered near-surface air temperature by an average of 1.5 degrees Celsius across the metro area, and cut cooling energy demand by 16.6 percent on average.

How scientists tested a fix for an entire city

Most previous cool roof studies have been limited to a single building or small neighborhood. Haochen Tan, an atmospheric scientist at Argonne, and his team wanted to see what would happen if the concept were scaled up to an entire metro area. The study notes that they used a Weather Research and Forecasting model, a tool climate scientists use to simulate weather in great detail for specific regions, and applied it to the real August 2021 heatwave. Instead of just imagining the effect, they simulated three real scenarios across every building in the region: roofs that were white and reflective, roofs covered in vegetation, and roofs fitted with solar panels.


White roofs came out ahead

Cool roofs proved to be the most effective option by a long shot. The air around them stayed cooler through the day because white surfaces reflect more sunlight instead of absorbing it as heat. That was a reduction of about 1.5 degrees Celsius in near-surface air temperature and a 16.6 percent reduction in the energy needed to cool buildings, averaged across the metro area, per the Argonne research summary. However, there is a limitation. Cool roofs can lose some of their shine, literally, as dirt and weathering build up over the years, which can chip away at their performance if not maintained, the research notes.

Green roofs and solar panels were not far behind

Vegetated green roofs performed nearly as well, cutting cooling energy use by 14 percent, the paper says. Solar panel roofs by themselves reduced cooling-energy demand by 7.6 percent, mainly through the shade they cast on the roof surface. That figure could rise to almost 47 percent if all the electricity the panels generated were also used to run air conditioners, a separate benefit from actively offsetting grid electricity, distinct from the passive cooling the panels provide structurally. Each option has its own trade-off. Green roofs are more expensive to install and maintain. Solar panels have a higher up-front cost, but pay some of that back with the electricity they produce.

Why it matters beyond a single city

The EPA explains that cool roofing can help reduce indoor and outdoor temperatures in cities and mitigate health risks associated with long periods of heat exposure, including heat exhaustion, dizziness, and heat-related illness. The agency also cites a 2019 study by researchers Heather Macintyre and Clare Heaviside, published in Environment International, which modeled a UK city and found that citywide cool roof adoption could offset nearly 18 percent of seasonal heat-related deaths tied to the urban heat island effect. In a city like Chicago with a long history of extreme heat, that kind of number matters. The 1995 Chicago heatwave remains the deadliest weather event in the city's history, with an estimated 739 deaths, a grim benchmark that still informs how the city plans for hotter summers today.

A blueprint city planners can actually use

The numbers are useful, but more than that, this study gives city planners a basis for making the choice. The Argonne researchers found cool roofs to be one of the most cost-effective ways to reduce temperature, since they're cheaper to install than green or solar roofs and don't require additional water. Green roofs also have the added benefit of controlling stormwater and providing usable green space on top of buildings. The upfront cost of solar roofs is higher, but it is paid off with clean energy and the cooling benefit. None of these is a silver bullet on its own, and the researchers are careful to note that real-world performance depends on maintenance, budget, and local building density. But together, the three strategies give city planners concrete, comparable options rather than guesswork.