Climate model of Greater London finds white or reflective roofs are significantly more effective at reducing temperatures than both vegetation-covered roofs and deciduous tree cover
Architects will increasingly be looking to incorporate green roofs and street level vegetation to urban projects to satisfy new Biodiversity Net Gain regulations, but according to the results of a new study, the boost for habitats would not be matched by a significant reduction in air temperatures on hot summer days.
An international team of researchers, led by University College London (UCL), used a three-dimensional urban climate model of Greater London to test the effects of nine different urban heat management systems on street-level air temperatures during two of the hottest days on record, in the summer of 2018. The results were published in the scientific journal Geophysical Research Letters.
The study found that, if adopted widely throughout London, green roofs would have a negligible net cooling effect on the city, while extensive street-level vegetation, or solar panel installation, would provide a net cooling effect of just 0.3°C.
Highly reflective cool roofs were found to have the greatest impact, able to cut outdoor temperatures by around 1.2°C, on average, and by up to 2°C in some locations.
Air conditioning systems are known to increase outdoor temperature and were found by the study to warm the air by 0.15°C across the city, increasing to 1°C in central London.
Oscar Brousse, the study’s lead author and lecturer at UCL Bartlett School Environment, Energy & Resources, said he was ‘disillusioned’ to learn that green roofs would essentially have no effect on temperatures at city scale. ‘Although maximum cooling reaches about 0.5°C during the hottest hours, green roofs accumulate heat during the day, then release it at night in the form of infrared, reducing the average cooling effect,’ he said.
And while converting city greenspaces from grass to deciduous tree cover would cool temperatures overnight, researchers said this would have mixed net effects during the day and probably increase air humidity, affecting residents’ thermal comfort.
Urban planners and designers should now be seriously considering cool roofs as part of schemes, said Brousse: ‘I have a hard time understanding why we would not do it, especially on roofs that we know are complicated to retrofit or where other interventions would be tricky to implement, such as on sloping roofs where the structural load is already heavy. Whitening a roof is not complicated or costly and is really effective.’
However, the cooling effects would only be noticeable if rolled out on at least a neighbourhood scale, and should be considered as part of a systemic approach, taking into account other factors, he added. ‘You need to define where you have more risks of water runoff,or where you would like to preserve more biodiversity, etc. These are societal and political questions,’ said Brousse.
The study was carried out alongside researchers at Royal Netherlands Meteorological Institute, the Centre for Energy, Environmental and Technological Research in Spain, and Finland’s Tampere University, and supported by Wellcome and NERC.
Finding ways to adapt or mitigate urban heat islands has become a high priority for city planners and designers, who are exploring passive and active cooling methods to reduce discomfort and mortality rates among residents. In 2023, the UK reported 2,295 deaths that could be attributed to heatwaves during the English summer.