Residential properties in view of the Canary Wharf in London, UK.Photographer: Hollie Adams/Bloomberg
LONDON: Increasingly hot summer days in London have city dwellers seeking relief. And a new study shows the best bet might not be green roofs but white ones.
Conventional wisdom has long held that roofs adorned with vegetation are a key way to reduce outdoor temperatures by absorbing sunlight and evaporating water into the surrounding air.
However, such roofs had negligible cooling effects when tested in a simulation of London’s two hottest days in summer 2018, according to findings from University College London researchers.
But cool roofs – that is, those covered in white or reflective material – helped reduce temperatures by 1.2°C (2.2°F) on average.
“Statistically a degree or two of difference can lead to a substantial number of lives being saved,” said the report’s lead author Oscar Brousse, a senior research fellow and lecturer at University College.
London is among several cities experimenting with different types of roofs to mitigate the urban heat island effect, which makes densely concentrated locales hotter than surrounding areas.
For a city with relatively few hot-weather adaptations, the benefits of finding the right mix could be manifold: heat-related deaths in the region begin to rise at just 19°C by one estimate, and grow by over 3% with every degree increase above 21.5°C.
To conduct the study, the researchers ran 11 climate simulations investigating the impact of 10 different urban temperature reduction strategies on two July 2018 days when the weather reached 33°C and 37°C, respectively.
During the hottest hours of the day, green roofs were found to reduce temperatures by as much as 0.8°C if implemented citywide, but cool roofs still had an effect two to two-and-a-half times greater during those hours. Green roofs’ impact was even more negligible over the course of the entire day on average.
Water is a critical factor in determining green roofs’ efficacy, according to Becci Taylor, director of sustainable development firm Arup’s Retrofit at Scale programme. — Bloomberg
