Cooling Cities: Improving urban microclimate with Green Facade Systems by Sanda Lenzholzer & Maricruz Solera Jimenez

As our cities grapple with the ever-growing challenges of urban heat islands and climate change, innovative solutions are emerging to mitigate the impacts of rising temperatures. One such solution gaining momentum is the integration of green facade systems into urban landscapes. These living walls not only add aesthetic value but also offer a range of environmental benefits, including the cooling of urban microclimates.

In a recent interview, we had the privilege of speaking with two esteemed researchers at the forefront of this field: Professor Sanda Lenzholzer, Full Professor at Wageningen University, and Maricruz Solera Jimenez, a PhD candidate and researcher within the European Industrial doctorate project SOLOCLIM at Wageningen University.

Image credit Cool Roof Rating Council

Global Warming and Urban Heat Islands

The rising temperature on Earth poses a dire threat to our ecosystem and human society. As temperatures continue to rise, we witness a troubling increase in extreme weather conditions such as heatwaves, droughts, and intense storms. These changes have a devastating impact on agriculture, biodiversity and can lead to water and food shortages, as well as forced migration of people.

In cities, the phenomenon known as the ‘urban heat island effect’ is exacerbated by global warming. Urbanization, asphalt, and concrete absorb heat during the day and release it slowly at night, resulting in urban areas remaining much warmer than surrounding rural areas. This gives rise to a range of issues, including health risks such as heat-related illnesses, failing infrastructure and increased energy consumption for cooling. Moreover, the urban heat island effect can exacerbate social inequality as vulnerable communities such as the elderly and low-income groups often bear the brunt of the extreme heat.

It is evident that global warming and the urban heat island effect are urgent issues that require serious attention and action, both at the local and global levels. We must commit to sustainable solutions such as greening urban areas, reducing our reliance on fossil fuels, and taking measures to mitigate climate change and adapt to changing conditions.

Unveiling the Cooling Potential of living wall systems

Maricruz Solera Jimenez’s groundbreaking studies highlight the remarkable cooling capabilities of green facade systems, shedding light on their potential to mitigate urban heat islands. Her research reveals that these living walls can significantly lower temperatures, but it’s essential to consider key distinctions between surface temperature and air temperature.

Maricruz’s studies demonstrate that green facade systems can lower surface temperatures by up to 12 degrees Celsius in London. Additionally, green façade systems could lower the ambient temperature by up to 4.1 degrees Celsius in London and 5 degrees Celsius in Germany at short distance from the green wall.

“The larger the surface area of the green facade, the greater the cooling impact. It’s the cumulative effect that matters.” – Sanda

The science behind green facade systems

Plants provide cooling through a process called evapotranspiration. This process involves the release of water vapor from the stomata in the leaves into the surrounding air. The evaporation process requires energy from the surrounding air and this tempers the air temperature. Additionally, plants provide shade, which reduces the amount of direct sunlight reaching surfaces such as pavement and buildings, further lowering temperatures. The combination of evapotranspiration and shading helps to mitigate the urban heat island effect and create a more comfortable microclimate in urban areas.

This effect can be better achieved with a green facade system than with a ground-bound system. This is because of the substrate package on the wall and that the density of plants is usually higher in a green facade system. This higher density and plant diversity maximize the surface area available for evapotranspiration, enhancing the cooling effect. Additionally, the vegetation in green facade systems shades surfaces exposed to sunlight, further contributing to temperature reduction in urban environments.

Maricruz emphasizes that besides the density and variety of plant species, the components (such as planting containers) of the green facade system plays a significant role in temperature regulation.

“The system’s design influences cooling in summer and heating in winter due to its multi-layered structure.” – Maricruz

The year-round thermal benefits of green facade systems

While it’s widely recognized that green facade systems offer cooling benefits during the sweltering summer months, their impact extends far beyond mere seasonal adjustments. Maricruz Solera Jimenez’s study highlights how green facade systems can increase temperatures by an average of 0.4°C compared to a bare wall, at night during winter.

One of the key factors contributing to this year-round functionality is the layered construction of green facade systems. These systems, consisting of structures, planter boxes, and substrate layers, provide insulation for buildings, maintaining indoor temperatures regardless of external weather. This insulation offers a sustainable solution for reducing heating costs and energy consumption by maintaining cooler temperatures in summer and trapping heat in winter.

Maricruz highlights the impact of exterior green facades on indoor temperature, stating:

“Green facades outside also reduce wind speed, which in turn affects the interior of the building. So, it’s not just about the exterior; they have a significant influence on the indoor environment as well.”

Image credit: Prof. Sanda Lenzholzer en Maricruz Solera Jimenez

About Prof. Sanda Lenzholzer and Maricruz Solera Jimenez

Maricruz’s interest in sustainability and green wall systems grew from her diverse experience in architecture. She has worked on various projects, including housing developments and healthcare facilities, and has always been fascinated by the potential of green wall systems to improve urban climates.
Joining her in this conversation is Professor Sanda Lenzholzer, whose illustrious career spans the realms of landscape architecture, urban design, and climate science. As Chair Holder of Landscape Architecture at Wageningen University, Principal Investigator at the Amsterdam Institute for Advanced Metropolitan Solutions (AMS), and Visiting professor at Politecnico di Milano, Sanda brings a wealth of expertise to the discussion. Her pioneering work in climate-oriented urban design, exemplified by her acclaimed book “Weather in the City,” underscores the vital intersection between research and practice in shaping sustainable urban environments.

Together, their insights promise to shed light on the profound effects that green facade systems can have on urban microclimates and the broader implications for sustainable urban development.