Roof materials on Chicago buildings can substantially lower temperatures during heat waves

CHICAGO — A new city-wide model showed that updating all roofs in Chicago from traditional materials to white, green or solar panel-covered roofs would lower air temperatures by as much as 2 degrees Celsius and reduce energy consumption more than 16%. This research will be presented on Wednesday, 14 December at AGU Fall Meeting 2022.

“Previous studies have shown the advantages of implementation of cool roofs and green roofs in urban areas at a building scale or neighborhood scale,” said Haochen Tan, an atmospheric scientist at Argonne National Laboratory.  “However, such advantages cannot simply transfer to city-scale benefits because of the differences of each building and the local climate.”

Tan and colleagues wanted to scale up those previous studies and look at the effectiveness of heat-mitigating roofs over an entire city.

The study focused on the Chicago metropolitan area during a 6-day heatwave in August 2021. The team used numerical weather prediction models to see what would happen if the roof of every building in the city were changed to either cool (white, reflective coating), green (vegetation), or solar panel-covered roofs.

“Results show that the deployment of the cool roof can reduce the air temperature by about 2 degrees [Celsius] over their most urban areas,” said Tan. “Cool roofs can reduce the hours of high temperatures during a heat event day.”

With lower temperatures, there is less need for air conditioning. Tan says at the city-wide scale, cooling energy consumption can be reduced by 16.6% when cool roofs cover buildings. Green roofs and solar panel roofs were also effective at reducing energy consumption, creating drops of 14.0% and 7.6%, respectively.

There are trade-offs for each of these roof materials, said Tan. For example, large-scale deployment of cool roofs has the best potential for cooling, but the performance can be misleading because the reflective material can degrade over time because of weathering or dirt accumulation. On the other hand, solar panels have a lower reduction in energy consumption and can be expensive to install in maintain, but the roofing material is not passive.

“If we assume that all electricity generated by the solar panel can be applied to the cooling demand, we can expect savings of almost half (46.7%), of cooling energy consumption,” said Tan.

The researchers, who have recently published their findings in Science of The Total Environment, note that their study can help city planners decide what roof materials would be best when trying to mitigate heat waves.