According to the research, published in Nature Cities, green roofs could cool South Korea’s capital city, Seoul, by about 1°C during summer and slash the energy demand associated with cooling by almost 8 per cent if there is 90 per cent rooftop greenery coverage.

Led by Indira Adilkhanova and Professor Geun Young Yun from Kyung Hee University, and co-authored by UNSW Sydney Scientia Professor Mattheos Santamouris, the study is one of the first to analyse the effect of green roofs on urban-scale energy consumption and climate conditions.

“Previously, we have only looked at the energy impact of green roofs for singular buildings, but this is the first study to evaluate the real climatic and energy impacts of green roofs at the city scale,” said Prof. Santamouris, the Anita Lawrence Chair in High-Performance Architecture at UNSW Arts, Design & Architecture. “Our findings demonstrate the tremendous potential of green roofs to substantially decrease the peak temperature of a city and increase energy savings.”

The researchers ran large-scale cooling climatic and building energy simulations under three greenery coverage scenarios to evaluate the potential of green roofs to lower the temperature and energy needs of Seoul during August, the hottest summer month. They focused on non-irrigated green roofs, described as lightweight with large-scale implementation potential and lower maintenance costs.

They found that the higher the coverage of green roof systems, the more significant the falls in temperatures and energy demand. When 90 per cent of the buildings were covered with green roofs, the city’s air temperature and surface temperature dropped by up to 0.54°C and 2.17 °C, respectively. At the same time, building energy use dropped by about 7.7 per cent.

“Green roofs help cool temperatures by facilitating evaporation through plant transpiration, which cools the surrounding air and reduces the need for mechanical cooling systems (air conditioning), lowering the overall energy demand,” Prof. Santamouris explained. “Additionally, the layer of soil and vegetation on green roofs provides insulation, reducing heat transfer into buildings and further lowering energy costs.”

Although there is limited green roof coverage in Seoul currently, it could rise to 60 per cent, spurred by local policies. Prof. Santamouris, however, believes that 90 per cent coverage could be achieved with the right incentives.

“Green roofs are one of the more expensive heat mitigation technologies with initial capital costs and ongoing maintenance, so it’s not always suitable,” he noted. “But for cities and businesses in developed countries that can afford it, green roofs are an important social contribution [and] policymakers can create the conditions to encourage widespread adoption.”

Meteorological conditions

The efficacy of green roofs is also impacted by meteorological conditions, prompting the professor to call for more detailed analysis to fully reflect the annual cooling and energy-saving potential of green roof systems over the long term.

“The performance of green roofs is very much affected by several parameters, mainly humidity, precipitation, solar radiation, and temperature,” said Prof. Santamouris. “But green roofs also have important climatic benefits, as well as many other important benefits, such as absorbing rainwater, increasing biodiversity, and improving the aesthetic quality of the city.”

There will also be severe health consequences if urban heat mitigation strategies are not implemented at scale. On the current trajectory, he warned, night temperatures in cities could jump by up to 5°C by 2050, resulting in a significant rise in heat-related illnesses and deaths.

“There is an urgent need to implement a combination of heat mitigation techniques and technologies in our cities to decrease urban temperatures,” he cautioned. “If we do not, the cost in the coming decades will be catastrophic – not just for the economy but on quality of life, particularly for low-income populations who will suffer the most.”