Covering the facades of buildings with plants can reduce temperatures indoors by several degrees in the summer and increase them by almost as much in winter, according to an Israeli study of green walls thought to be the first of its kind in the world.
The data, presented Wednesday by researchers from Bar-Ilan University, studied “vertical forests” of bushes and climbers grown out of balcony-based planters, along with denser “green walls,” made up of smaller plants grown in soil in rows of containers.
The results: Both cool buildings by about 2° C (3.6° F), though green walls are better at bringing down the mercury. And the two are equally good at keeping warmth inside.
Aside from purifying the air, absorbing carbon dioxide and providing food and shelter, plants can also cool temperatures via a process known as evapotranspiration, in which they release water into the atmosphere in response to hotter, drier air, much the same way humans sweat to cool themselves down. And plants placed on or beside walls can also act as natural insulators, helping keeping a building cool or warm.
Today, architects and urban planners in many parts of the world are using vegetation on roofs and walls as a nature-based solution to urban heat islands, an observed phenomenon in which cities can be several degrees hotter than surrounding areas due to the prevalence of heat-absorbing brick, concrete and asphalt, with little green space.
Israel has relatively few buildings planted with green walls, one exception being the Check Point Software Technologies company headquarters in Tel Aviv, which has covered approximately 2,500 square meters (27,000 square feet) with plants.
The study, led by Bar-Ilan University Prof. Itamar Lensky and doctoral student Noa Zuckerman, utilized three identical buildings connected by a shared base on the university’s campus, outside of Tel Aviv.
Facades of two of the buildings were covered with plants — one with green walls and one with a vertical forest — and the third building’s sides were left bare, as a control. A walkway between the two-greened buildings was also measured against the control of a walkway between two bare facades. In total, 800 square meters of wall were “greened” — equivalent to 8,600 square feet.
Over 18 months, Zuckerman used sensors to track changes in humidity and temperature in the buildings and in the walkways between the structures. These measured temperatures on the walls themselves, and also gathered data on the temperatures as felt by students and faculty walking along two outdoor paths between the three buildings, or circulating indoors.
According to the results, presented at the annual Science and Environment Conference organized online by the Israel Society of Ecology and Environmental Sciences, general indoor air temperatures during the summer closest to the vertical forest were 1.9° C (3.4° F) cooler than in the control building, while areas nearest to the green wall were 2.41° C (4.3° F) cooler than the control.
During the winter, indoor temperatures close to both the vertical walls and green walls were warmer than the control by around 2° C (3.6° F).
Outside, several meters away from the vegetation, summer temperatures on the path closest to the control building were around 1° C (1.8° F) warmer than they were on the path between the two greenery-covered buildings. In winter, the greened passage was around a degree warmer.
Cooling appeared to be strongest in the mornings in summer, after the plants had been irrigated and were at optimal function. Humidity censors confirmed that plants cooled temperatures more the hotter and drier it was.
According to Lensky, who studies atmospheric science, cooling a space by a single degree Celsius could save around five percent on the electricity bill. While green walls provide the most savings, and may be better insulation, they are also more expensive to maintain and could require special equipment, such as cranes, to tend.
He noted that his study appeared to be the first in the world to fully study live plants’ effect on temperature. Previous studies have either used theoretical models or had various other flaws, such as using too small samples, being too short-lived, lacking proper controls, or having improper censor placement that failed to capture “what the man in the street will feel,” he claimed.
The green walls project, which received funding from the Science Ministry and is being carried out within Bar-Ilan University’s Center for Smart Cities, is continuing to be maintained and monitored, with the hopes it will yield more useful results.
Zuckerman is researching the use of technology to carry out sensing from a distance, while other academics are looking at aspects such as costs, the ability of green walls to absorb carbon dioxide — a global warming gas, and the effects of green walls on public health.
The project is part of a partnership with China’s Nanjing University, which has created a green roof to be studied in its decidedly wetter and muggier climate. Research results from there have not yet been made available.
Last month, researchers from Singapore, a world leader in green walls and roofs, released the results of a study suggesting that green walls reduce stress in an urban environment.
Psychologists from the Nanyang Technological University used virtual reality headsets to create a noisy street with some buildings that were painted green and others that were covered with plants. Participants wore portable electrocardiogram devices so that the researchers could monitor changes in heartbeat that indicate stress.
Stress levels were significantly higher among those only exposed to buildings painted green while they stayed stable among those who saw the buildings covered in plants.