The Issue:
IEN Consultants was approached by a client who had retrofitted a building façade with greenery and wanted to quantify the temperature impact. Understanding the thermal behavior of the green wall was crucial to the client, as they wanted to evaluate the effectiveness of this eco-friendly feature in reducing the wall's temperature and minimising the urban heat island effect.
Our Approach:
For this study, we captured thermographic images of the client's building at three different times during a sunny afternoon. These images were then compared with thermographic pictures of a neighboring building that shared the same orientation along the same street. To ensure accurate comparisons, the pictures were taken minutes apart.
By studying the façade of a building using a thermographic camera, we could gain valuable insights into its thermal performance and its contribution to the heat island effect. The thermographic camera captured infrared heat radiation emitted by the façade, allowing the façade temperature to become visible. The thermographic software allowed us to identify the average surface temperature within a user-defined area.
For this study, a FLIR thermograph camera was used to capture the pictures, which were taken at 12:30 PM, 3 PM, and 5 PM for the west facing façade of the two buildings located in downtown Kuala Lumpur. The sky condition was sunny, except for 12:30 PM, when it was partially sunny and at which time the sun had not yet swung around to shine directly at the West facing façade.
A comparison of the thermographic photos of the two buildings are shown below using a linear temperature scale from 25 - 50°C:
The thermographic images show that the façade treatment has a significant effect on the façade temperature. While highest temperature is observed for the green-coloured facade panels, a significantly lower temperature is observed for the white-coloured façade and even lower for the façade with greenery.
Our Findings:
In order to quantify the temperature difference, the following two façade area segments were chosen for analysis:
The temperature analysis showed that the façade with greenery had a 4.1 – 9.4°C cooler temperature than the neighbouring façade without greenery. The temperature difference was seen to increase throughout the afternoon, as the incident solar radiation on the facade increased from the setting sun. The more sun exposure, the bigger the temperature drop for the facade with greenery.
During the afternoon, we observed distinct thermal behaviors among the different facades. The green-coloured wall consistently increased in temperature, gradually getting warmer as time passed. In contrast, the facade with greenery maintained a relatively cool temperature, demonstrating its ability to resist excessive heat absorption. The white-coloured facade warmed up but showed less temperature rise compared to the green-coloured facade. These observations highlighted the contrasting thermal properties of the facades, with the façade with greenery proving particularly effective in maintaining a cooler surface temperature. The thermographic images showed that the façade with greenery maintained a temperature below 33°C, which was lower than both the white-coloured façade (37°C) and the green-coloured façade (43°C).
From this thermographic investigation, it can be concluded that the type of façade chosen for a building plays a crucial role in determining its temperature, as different materials possess distinct thermal and physical properties. Each material interacts with the surrounding environment and solar radiation in its own way, leading to significantly different variations in the thermal behavior of the building and its effect on the surroundings.
The reflective nature of a white wall allows it to effectively bounce back a significant portion of the solar heat that reaches its surface. Instead of absorbing the incoming heat, the white facade reflects it into the surrounding environment. This reflective property helps to minimize heat transfer into the building, resulting in lower temperatures on the wall and inside the building, but will increase heat reflection to the surroundings and potentially cause visual glare to people.
In contrast, a dark-coloured wall has a higher capacity for heat absorption. When exposed to sunlight, the dark-colored surface of the wall absorbs a larger amount of solar heat energy. This absorption results in increased the wall's temperature and can transfer heat to the interior of the building and the urban surroundings, contributing to the urban heat island effect and adversely affecting the thermal comfort of the occupants inside and outside of the building.
A façade with greenery effectively limits the heat transfer between the building and the external environment thanks to the process of evapotranspiration of plants, where much of the solar energy is spent on evaporating water. While greenery keeps temperatures low, they will add humidity to the air. In terms of thermal comfort improvement, the temperature reduction outweighs the humidity increase, so greenery is a net benefit to thermal comfort in tropical cities. By acting as a thermal barrier, the green wall contributes to maintaining a more stable and comfortable indoor and outdoor environment while reducing the reliance on cooling systems - as well as helping to create a more natural biophilic urban environment.