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 released Thursday.

But cool roofs — that is, those covered in white or reflective material — helped reduce temperatures by 1.2C (2.2F) 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 19C by one estimate, and grow by over 3% with every degree increase above 21.5C.

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 33C and 37C respectively.

During the hottest hours of the day, green roofs were found to reduce temperatures by as much as 0.8C 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 program. “If a green roof becomes very dry, it won’t be cooling anymore, it needs to be watered,” Taylor said. “You need the evaporative effects or else it becomes essentially a dry arid park with no trees.”

Taylor is leading an initiative to retrofit more London buildings with cool roofs, similar to projects carried out in New York and Singapore. She said cool roofs also hold an advantage over green roofs because they’re easier to install and maintain, and cheaper. Adding reflective material to a roof costs between £13 ($17) and £89 per square meter, depending on the setting. The cost of green roofs, while higher generally, is difficult to estimate because of the variety of soil and vegetation costs and upkeep.

Nonetheless, Taylor noted that green roofs carry benefits that cool roofs lack, including boosting biodiversity, reducing air pollution, absorbing and filtering water and providing green space for communities. She said there is room in the city for both options. Whereas cool roofs are simpler for retrofits, green roofs can be suitable in new construction, where roof design and structural load capacity can be taken into account early on.

While the new research is telling, it comes with some caveats. Modeling can provide widespread insight, but more research is needed to understand green roofs’ cooling impact in the real world on days similar to those studied.

The lack of data on green roofs’ other benefits is a weakness of the report, said Dusty Gedge, founder of Livingroofs.org and president of the European Federation of Green Roof Associations. In addition to referencing previous research that confirms green roofs’ outdoor cooling effects, Gedge highlighted their indoor cooling effect and energy savings. He said their outsized benefits make up for their higher upfront costs.

When weighing different heat mitigation options, Gedge said policymakers and communities should avoid leaning on a single solution or isolating the effects of one metric over all others. “We need a diversity of solutions,” he said.