London modelling shows ‘cool roofs’ best at beating heat

White or reflective coverings on roofs are the best way to combat increasing urban heat, according to a new study from University College London.

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Led by Dr Oscar Brousse from UCL’s Bartlett School Environment, Energy & Resources, the study modelled the impact of various cooling approaches for the buildings and streets of London. The team used a three-dimensional urban climate model of Greater London to test the thermal effects of different passive and active urban heat management systems, including painted “cool roofs,” rooftop solar panels, green roofs, ground level tree vegetation and air conditioning. The modelling was based on the two hottest days of the summer of 2018.

It was found that cool roofs could reduce average temperatures across London by 1.2°C, with reductions of up to 2° in some locations. Solar panels and street level vegetation were each found to have an average cooling effect of around 0.3°C, though both of course provide additional environmental benefits. Increasingly popular green roofs - consisting of vegetation that offer benefits like water drainage and wildlife habitats – were found to have a negligible cooling effect on average across the modelled area. The work is published in Geophysical Research Letters.

“We comprehensively tested multiple methods that cities like London could use to adapt to and mitigate warming temperatures, and found that cool roofs were the best way to keep temperatures down during extremely hot summer days,” said lead author Dr Brousse.

“Other methods had various important side benefits, but none were able to reduce outdoor urban heat to nearly the same level.”

The study also found that air conditioning, while benefiting users within buildings, could warm the outside temperature in dense central London by as much as 1° C, moving warm internal air outside to the city streets. As climate change raises average temperatures across the globe, cities are particularly vulnerable due to the ‘urban heat island’ effect, where concrete pavements and buildings trap heat. Many buildings with HVAC systems in place will naturally be tempted to increase their air conditioning, potentially adding to the heat island effect on the streets outside.

The study found that cool roofs have the dual benefit of reducing temperatures inside buildings, as well as the surrounding temperatures in the city at large. The passive cooling method could therefore reduce internal building temperature and reduce air conditioning demand, which would in turn have an additional external cooling effect.