Modeling shows green roofs can cool cities and save energy

Extensive green cover on building roofs could significantly reduce citywide temperatures and lower energy costs, a new study suggests.

The research, led by Indira Adilkhanova and Professor Geun Young Yun of Kyung Hee University and co-authored by Professor Mattheos (Mat) Santamouris of UNSW Sydney Scientia, found that green roofs could cool the city’s capital. South Korea by about 1°C in summer and reduce the ambient temperature. the energy demand associated with cooling of almost 8% under maximum coverage.

The study, published in Nature Citiesis the first to analyze the transformative effect of green roofs on urban-scale energy consumption and climate conditions and could provide a blueprint for modeling their potential in other cities around the world.

“Previously, we had only looked at the energy impact of green roofs for individual buildings, but this is now the first study assessing the real climate and energy impacts of green roofs on a city scale,” says Professor Santamouris, Professor Anita Lawrence. Chair in High Performance Architecture at UNSW Arts, Design & Architecture. “Our results demonstrate the enormous potential of green roofs to significantly reduce a city’s maximum temperature and increase energy savings.”

Professor Santamouris specializes in developing heat mitigation technologies and strategies to cool cities affected by urban overheating. Seoul, South Korea, is one such city where climate change and rapid urbanization are impacting the scale of overheating problems, increasing energy consumption needs and harming health.

“One of the major problems in the world’s built environment is severe urban overheating,” says Professor Santamouris. “And as our cities get warmer, so do thermal discomfort, heat-related illnesses and deaths.”

Green roofs are a promising strategy for reducing heat and energy consumption in urban areas. Able to be installed on new and renovated buildings, they constitute a scalable nature-based solution to meet the challenges of urban heat.

City-wide impact of green roofs

For the study, the research team performed large-scale cooling simulations of building climate and energy under three vegetation cover scenarios to assess the potential of green roofs to reduce temperature and cooling needs. from Seoul during the hottest summer month, August. They specifically focused on non-irrigated extensive green roofs, a type of lightweight green roof with potential for large-scale implementation and lower maintenance costs.

Modeling revealed that the higher the coverage of green roof systems, the greater the reductions in temperatures and energy demand. When 90% of buildings were covered with green roofs, the city’s air temperature and surface temperature decreased by up to 0.54°C and 2.17°C, respectively. At the same time, the energy consumption of buildings decreased by approximately 7.7% (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), thereby reducing overall energy demand,” explains the Professor Santamouris. “Additionally, the layer of soil and vegetation on green roofs provides insulation, thereby reducing heat transfer into buildings and further reducing energy costs.”

Current green roof coverage in Seoul is modest but is expected to expand to between 30 and 60 percent depending on local policies over the coming decades. But Professor Santamouris says 90% coverage – the highest potential for green roof concentration – could be achieved with the right incentives.

“Green roofs are one of the most expensive heat mitigation technologies, with initial investment costs and ongoing maintenance, so they are not always suitable,” says Professor Santamouris. “But for cities and businesses in developed countries that can afford it, green roofs provide an important social contribution that policymakers can create the conditions for widespread adoption.”

Professor Santamouris says the effectiveness of green roofs depends on weather conditions which must be taken into account during design and implementation. More detailed analysis is also needed to fully reflect the annual cooling and energy saving potential of green roof systems in the long term.

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

Professor Santamouris warns that failure to implement urban heat mitigation strategies like green roofs will have serious consequences. On the current trajectory, nighttime temperatures in cities could rise by up to 5°C by 2050. Many people, even in developed countries, could struggle to afford electricity costs for cooling, and heat-related illnesses and deaths will increase significantly. .

“There is an urgent need to implement a combination of heat mitigation techniques and technologies in our cities to reduce urban temperatures,” says Professor Santamouris. “If we do not do this, the cost in the decades to come will be catastrophic, not only for the economy, but also for quality of life, especially for low-income populations who will suffer the most.”