In both indoor and outdoor settings, south-facing green walls can reduce temperatures.

Have you ever stood in the center of a metropolis and simply felt the heat emanating from its surfaces? Or how it feels hotter within a closed space than outside?

Urban Heat Island effect

The Urban Heat Island (UHI) effect, in which urban regions are noticeably warmer than rural areas, has been exacerbated by climate change and urbanization. As a result, heatwaves and other extreme heat events have become more often, and indoor and outdoor thermal conditions in buildings have deteriorated. This has increased the need for cooling energy, put more strain on power grids, and raised the possibility of power outages.

Prior research on UHI mitigation has mostly concentrated on enhancing outdoor settings, but building envelopes—the material that divides the inside and exterior—allow indoor and outside temperature conditions to interact dynamically. Therefore, it is crucial to assess them together.

Additionally, there hasn’t been enough research done on developing resilience under compounded harsh situations, like heatwaves that may even coincide with power outages.

A new study under extreme conditions

An international research team headed by Associate Professor Jihui Yuan from the Graduate School of Human Life and Ecology at Osaka Metropolitan University assessed the effects of UHI mitigation strategies (UHIMS), including envelope materials, green roofs, and vertical greenery, on both indoor and outdoor thermal environments.

The study concentrated on a school, at a location known for its scorching summers. The researchers employed an integrated simulation method in the analysis, combining an Urban Microclimate Model (UMM) to capture outdoor microclimate dynamics with a Building Energy Model (BEM) to replicate indoor thermal conditions.

In order to assess building performance under realistic and severe conditions, the simulations took into account future climatic forecasts as well as extreme conditions, such as summer heatwaves and power outages, based on meteorological data records. The Physiologically Equivalent Temperature (PET) was used to measure thermal comfort, allowing for a uniform assessment of both indoor and outdoor settings.

The results

According to the findings, the south-facing facade’s green wall lowered inside temperatures by as much as 1.7°C. Furthermore, thermal comfort was significantly impacted by albedo, or the quantity of light reflected by a surface. High albedo exterior surfaces were shown to be more successful in lowering indoor temperatures, while low albedo exterior surfaces increased outdoor thermal absorption, absorbing about 1.5°C. Furthermore, it was discovered that external materials’ radiative qualities have a greater impact on thermal settings than their heat capacity.

Albedo is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects all incident radiation).

“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.”