Extensive greenery coverage on building rooftops could help to cool summertime temperatures and reduce energy demand across entire cities, new research shows.

In their latest study published in the Journal Nature, researchers from Kyung Hee University in Seoul and the University of New South Wales (UNSW) ran large-scale cooling climatic building and energy simulations.

The aim was to evaluate the potential of widespread adoption of green roofs to lower the temperature and cooling requirements across the city of the South Korean capital of Seoul during its hottest summer month of August.

It found that higher levels of green roof coverage led to lower air and surface temperatures across the city.

When as much as 90 percent of buildings were covered in green roofs,  the simulations predicted that the city’s air temperature would decrease by 0.54 degrees Celsius whilst surface temperatures would be reduced by 2.17 degrees Celsius.

At that same level of greenery, the city’s energy consumption was reduced by 8 percent.

The study is the first to evaluate the cooling and energy consumption impacts of green roofs at a city-wide scale.

Previous studies have focused on the energy impact of individual buildings.

UNSW Sydney Scientia Professor Mattheos Santamouris, a co-author of the report, said the significance of the study and its findings should not be underestimated.

“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,” Santamouris said.

“Our findings demonstrate the tremendous potential of green roofs to substantially decrease the peak temperature of a city and increase energy savings.”

“One of the major problems in the built environment worldwide is severe urban overheating.

“And as our cities heat up, thermal discomfort and heat-related illness and death also rise.

“Green roofs are a promising strategy for mitigating urban heat and energy consumption.

“With the ability to be installed on new buildings and retrofitted, they’re a scalable nature-based solution to address the challenges of urban heat.”

The latest research comes amid growing concern about the impact which climate change may have upon cities and major urban centres in coming decades.

Already, as many as 450 cities around the world are impacted by extreme urban heat.

This not only leads to higher cooling requirements but can also affect quality of life and human health. This is because higher urban temperatures being associated with more heat related illness and death.

The effect is likely to increase further in the future on account of global warming and rapid urbanisation.

In addition to Santamouris, the research was led by Indira Adilkhanova and Professor Geun Young Yun from Kyung Hee University in Seoul.

The study was completed in the South Korean capital of Seoul, where green roof coverage is modest at present but is expected to increase to between 30 and 60 percent over coming decades on account of local policies.

With the right incentives, the researchers say that coverage could be increase to 90 percent – the uppermost potential of green roof concentration.

For the study, the research team ran large-scale cooling climatic and building energy simulations under three greenery coverage scenarios.

The research focused specifically on non-irrigated extensive green roofs – a type of lightweight green roof which is suitable for large-scale implementation and has lower maintenance costs.

According to Santamouris, green roofs can help to cool temperatures by facilitation evaporation which occurs through plant transpiration.

This cools the surrounding air and reduces the need for air conditioning through mechanical cooling systems – thus helping to lower the overall requirements for energy consumption.

In addition, the layer of soil and vegetation on green roofs helps to provide insulation by reducing heat transfer into buildings.

This further helps to reduce energy consumption and costs.

As well as climactic benefits, Santamouris says that green roofs also deliver other benefits.

These include absorption of rainwater, greater biodiversity and better aesthetic quality of cities.

However, he cautions that green roofs may not always be the most suitable option for every city.

Because of costs associated with initial capital and ongoing maintenance, green roofs are one of the more expensive types of urban heat mitigation technologies.

This makes them better suited to wealthier nations.

Beyond this, the performance of green roofs is impacted by meteorological conditions. These include humidity, rainfall, solar radiation and temperature.

Such conditions need to be considered when designing and implementing a green roof strategy at city scale.

Finally, more detailed analysis is needed to fully reflect the annual cooling and energy saving potential of green roofs over the longer term.

Santamouris says the importance of urban heat mitigation in cities should not be underestimated.

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

“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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