Beat the heat: White roofs can bring down city temperature by 3.6°F

Researchers at the University College of London discovered that applying white or reflective paint to city roofs would best mitigate the heat this summer.

With temperatures on the rise and cities trapping heat, researchers searched for a way to break the cycle. Residents must relieve themselves with air conditioning but that only makes matter worse.

If the practice of painting roofs were adopted citywide, it would reduce the number of air conditioning systems being used inside, which conversely raises temperatures outside, according to a press release.

“One of the main strengths of our study is that it is amongst the first to do such a comprehensive analysis of common interventions for reducing exposure to hot air temperatures,” Michael Lucibella told IE , a researcher involved in the study.

Cool roofs work better

At the University of London, researchers created a 3D climate model of Greater London based on the two hottest days on record in 2018 to evaluate the most effective heating management system: painted cool roofs, green roofs, solar panels, ground-level tree vegetation, or air conditioning.

As planners and designers are currently exploring all options, they applied each across all buildings.

First, they calculated that air conditioning heats up the city center “by as much as 1 degree Celsius.”

Green roofs had an erratic pattern throughout the day. Their effects fluctuated significantly. It decreased temperatures by 0.5 degrees Celsius, but at night, the trapped heat in the organic matter caused temperatures to rise.

Planting deciduous trees in the city’s greenspaces helped manage the heat overnight, but it didn’t produce encouraging daytime results as levels of humidity increased.

Solar panels and street-level vegetation barely made a dent with a reduction rate of 0.3 degrees Celsius on average though they benefit the environment. Green roofs provide drainage and wildlife habitats but have little cooling effects.

However, promisingly, by reflecting the heat rather than absorbing it, cool roofs would balance out the heat inside and outside. They could reduce the heat outside by 1.1 and even 2 degrees Celcius in some areas.

“We comprehensively tested multiple methods that cities like London could use to adapt to and mitigate warming temperatures,” lead author Dr Oscar Brousse stated in a press release .

“We found that cool roofs were the best way to keep temperatures down during extremely hot summer days. Other methods had various important side benefits, but none were able to reduce outdoor urban heat to nearly the same level.”

Temperatures on the rise in cities due to climate change

The research published in Geophysical Researcher Letters presents the problem at hand in major cities. Cities trap heat in what is known as the “Urban Heat Island Effect.” With temperatures soaring in recent years, urban planners are actively implementing active and passive solutions such as central air, vegetation, as well as cool and green roofs.

This study from the University College of London, to support a citywide initiative, studied the real benefits that these various approaches offered. “Our results are provocative more than conclusive,” Lucibella said to IE, “they are conditional to the model and the period we have chosen.”

However, “for reducing air temperature at pedestrian levels during warm summer days in London a fresh coat of paint on the roof would be your best bet.”