Going up against the heat: Vertical greenery keeps cities cool

Ever stood in the middle of a city and just felt the heat radiating off its surfaces? Or entered a closed room and wondered how it could feel hotter than outside?

Climate change and urbanization have intensified the Urban Heat Island (UHI) effect, where urban areas are significantly warmer than rural areas. This has in turn increased the frequency of extreme heat events, such as heatwaves, and deteriorated both outdoor environments and indoor thermal conditions in buildings, leading to higher cooling energy demands, greater strain on power grids, and the elevated risk of power outages. Previous studies on UHI mitigation have primarily focused on improving outdoor environments, but indoor and outdoor thermal conditions interact dynamically through building envelopes, the material separating the interior and exterior. So, it is essential to evaluate them in an integrated manner. Furthermore, building resilience under compounded extreme conditions, such as heatwaves coinciding with power outages, has not been sufficiently investigated.

To address this, an international research team led by Associate Professor Jihui Yuan from Osaka Metropolitan University’s Graduate School of Human Life and Ecology evaluated the impacts of UHI mitigation strategies (UHIMS), such as green roofs, vertical greenery, and envelope materials, on both indoor and outdoor thermal environments. The study focused on an educational facility in Shahrud, Iran, a city characterized by extremely hot summers. In the analysis, the researchers used an integrated simulation approach that combines a Building Energy Model (BEM), which reproduces indoor thermal conditions, with an Urban Microclimate Model (UMM), which captures outdoor microclimate dynamics. Based on weather data records, the simulations considered future climate scenarios as well as extreme conditions, including summer heatwaves and power outages, to evaluate building performance under realistic and severe conditions. Thermal comfort was assessed using the Physiologically Equivalent Temperature (PET), enabling consistent evaluation of both indoor and outdoor environments.

The results revealed that a green wall installed on the south-facing facade improved indoor thermal conditions by up to 1.7°C. In addition, albedo, the amount of light reflected by a surface, showed significant effects on thermal comfort. Low albedo exterior surfaces improved outdoor thermal comfort by approximately 1.5°C, while high albedo exterior surfaces were found to be particularly effective in reducing indoor temperatures. Additionally, it was found that the radiative properties of exterior materials have a stronger influence on thermal environments than their heat capacity.

“This study could function as an initial guide for resilient buildings that can maintain acceptable thermal conditions even under extreme conditions,” said Yuan. “It could also contribute to the advancement of urban heat island mitigation strategies that integrate both urban- and building-scale approaches, while helping to achieve both reduced energy consumption and improved thermal comfort.”

The findings were published in Energy and Buildings.

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