Plant-covered facades cool in the summer and retain heat in the winter

Stefano Boeri Architetti: Bosco Verticale, Milan (2009-14)

Tel Aviv - Facades covered with greenery can reduce indoor temperatures by several degrees Celsius in summer, while slightly increasing them in winter. This is the result of an Israeli study, the first of its kind, reports the news outlet The Times of Israel.

The study by researchers from Bar Ilan University examined the impact of so-called "vertical forests" made up of shrubs and climbing plants grown in balcony pots and living "green walls" composed of smaller plants. In both cases, the greenery cooled the buildings by approximately two degrees, but the green walls reduced thermometer readings slightly more. It also turned out that both types of green coverage equally effectively prevent heat loss from interiors in winter months.

It is known that plants purify the air, absorb carbon dioxide, provide food, and offer protection against adverse weather. Additionally, they also reduce surrounding temperatures through the process of evapotranspiration, in which they release water into the atmosphere in response to warmer and drier air, similar to how the human body cools through sweat. Plants placed along or directly on walls also act as insulation, helping to maintain stable temperatures.

Architects and urban planners in many parts of the world are already utilizing green roofs and walls as a natural solution to urban heat islands, where urban development exhibits significantly higher temperatures than the surrounding area due to the predominance of materials that absorb heat, such as bricks, concrete, and asphalt, and a lack of greenery.

Green walls are not very common in Israel. An exception is the headquarters of Check Point Software Technologies in Tel Aviv, where 2,500 square meters of facade are covered with plants.

The study led by Professor Itamar Lensky from Bar Ilan University and doctoral student Noa Zuckerman examined the effects of green areas on three identical buildings on the university campus near Tel Aviv. Two buildings had plants covering their facades - in one case in the form of a green wall, and in the other, a vertical forest. The third building served as a control, leaving its walls bare. The researchers also examined the temperature on the path between the buildings that were covered with greenery, comparing it with the temperature on the path between bare facades. A total of 800 square meters of facades were greened as part of the study.

Zuckerman monitored changes in humidity and temperatures in the buildings and on the paths between them for 18 months. She also collected data on perceived temperatures from students and academics moving in and out of the buildings.

The findings were presented at the annual scientific and environmental conference, which was held online. According to the results, the temperature inside the building with a vertical forest was 1.9 degrees Celsius lower in summer compared to the control building, while the temperature in the building with a green wall was 2.41 degrees Celsius lower than in the control building. In both cases, measurements were taken as close to the green areas as possible. In winter, the indoor temperature in buildings with vertical forests and green walls was about two degrees Celsius higher than in the control building.

In summer, the outdoor temperature on the path by the building with the bare facade was one degree Celsius higher than on the path between the facades covered with plants. Conversely, in winter, the temperature on the path between the plants was one degree higher than at the control building.

The most noticeable cooling in summer months occurred early in the morning after the plants were watered, according to researchers. Humidity measurements confirmed that the hotter and drier it was, the more the plants reduced temperature.

Lensky noted that a one-degree temperature reduction can save residents of the building up to five percent on their electricity bills. Although green walls could provide greater savings and likely offer better insulation, their maintenance is more expensive and often requires special equipment for high-rise buildings.

According to Lensky, this is the first study that fully examined the impact of living plants on buildings on surrounding temperatures. Previous studies, he claims, were either based on theoretical models or problematic for various reasons, such as having too small a sample, lacking comparisons with a control object, lasting too briefly, or having poorly placed sensors that did not account for temperatures felt by people on the street.

The project, funded by the Ministry of Science and conducted under the auspices of the Smart City Center at Bar Ilan University, is ongoing, and participants hope it will yield even more useful insights. Academics will, for example, focus on other aspects of plant facades, including costs, their ability to absorb carbon dioxide, and their impact on public health. The project is part of a collaboration with Nanjing University in China, which will study the effect of green roofs in an environment with significantly higher humidity.

Last month, researchers in Singapore, a world leader in green walls and roofs, published a study indicating that plant-covered walls reduce stress among urban residents.