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New study reveals green roofs can cool cities and save energy

New study reveals green roofs can cool cities and save energy

A recent study has unveiled the significant potential of green roofs in reducing temperatures across major cities and lowering energy costs.

Published in Nature Cities, the study is ground-breaking as it is reportedly the first to analyse the transformative impact of rooftop greenery coverage on urban-scale energy consumption and climate conditions. Its findings could provide a blueprint for modelling the effectiveness of green roofs in cities globally.

Led by Indira Adilkhanova and Professor Geun Young Yun from Kyung Hee University and co-authored by UNSW Sydney Scientia Professor Mattheos (Mat) Santamouris, the research found that the implementation of green roofs could potentially lower the summer temperatures of South Korea’s capital city by approximately 1°C. Additionally, it could reduce the energy demand associated with cooling by nearly 8 per cent under maximum coverage.

“Previously, we have only looked at the energy impact of green roofs for singular buildings, but now this is the first study to evaluate the real climatic and energy impacts of green roofs at the city scale,” says Professor 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.”

Specialising in developing heat mitigation technologies and strategies for cooling cities affected by urban overheating, Professor Santamouris highlights the challenges faced by Seoul, South Korea, where climate change and rapid urbanisation contribute to overheating issues, increased energy consumption and adversely affecting health.

“One of the major problems in the built environment worldwide is severe urban overheating,” he says. “And as our cities heat up, thermal discomfort and heat-related illness and death also rise.”

The research indicates that green roofs offer a promising strategy for mitigating urban heat and energy consumption. They can be installed on both new and existing buildings, making them a scalable, nature-based solution to address the challenges of urban overheating.

Large-scale impact

The research team conducted large-scale cooling climatic and building energy simulations under three greenery coverage scenarios, evaluating the potential of green roofs to lower the temperature and cooling needs of Seoul during the hottest summer month, August. They specifically focused on non-irrigated extensive green roofs – a lightweight variant with large-scale implementation potential and lower maintenance costs.

The modelling found that the higher the coverage of green roof systems, the more significant the decreases in temperatures and energy demand. When 90 per cent of the buildings were covered with greenery, the city’s air temperature and surface temperature decreased by up to 0.54°C and 2.17 °C, respectively. Meanwhile, building energy use dropped by approximately 7.7 per cent (0.64 kWh/m2).

“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,” says Professor Santamouris. “Additionally, the layer of soil and vegetation on green roofs provides insulation, reducing heat transfer into buildings and further lowering energy costs.”

While the current green roof coverage in Seoul is modest, local policies aim to boost it to 30-60 per cent in the next few decades. Professor Santamouris suggests that with the right incentives, coverage of 90 per cent could be achieved.

“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 says. “But for cities and businesses in developed countries that can afford it, green roofs are an important social contribution that policymakers can create the conditions to encourage widespread adoption.”

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Professor Santamouris says the efficacy of green roofs is impacted by meteorological conditions that must be considered in the design and implementation. More detailed analysis is also needed 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,” he says. “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.”

Professor Santamouris emphasises the urgent need for adopting urban heat mitigation strategies, such as green roofs, to avert severe consequences. If the current trajectory persists, night temperatures in cities may surge by up to 5°C by 2050. Many people, even those in developed countries, may struggle to afford the costs of electricity for cooling, and heat-related illnesses and deaths will rise significantly.

“There is an urgent need to implement a combination of heat mitigation techniques and technologies in our cities to decrease urban temperatures,” says Professor Santamouris. “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.”

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