Subtropical summer thermal effects of wirerope climber green walls with different air-gap depths

- Green walls in subtropical Hong Kong were studied on summer sunny, cloudy, rainy days.
- In daytime, northeast green wall cooled external surface by 3.5 °C on average.
- At night, only external surface of northwest green wall was cooled significantly.
- Orientation and air gap of northeast green wall enhanced external cooling.
- Deep air gap of 1-m favored external wall surface cooling by subtropical green wall.

Thermal benefits of tropical climber green walls deserve more in-depth studies. A field experiment was designed in humid-subtropical Hong Kong to assess microclimatic effects of plant species, orientation and weather condition. In-situ measurements of air and surface temperatures of the indoor and outdoor environment were conducted respectively on the northeast and northwest oriented wirerope climber green walls with different air-gap depth. Adjacent bare walls were monitored as experimental controls. Days with sunny, cloudy and rainy weather in summer 2016 were chosen based on field-monitored incident solar radiation. The daytime and nighttime mean temperatures of the sampled days were evaluated statistically by t-test, analysis of variance (ANOVA) and Mann-Whitney U test. The best cooling benefits were realized at external building surface at 3.49, 0.52 and 1.19 °C respectively in sunny, cloudy and rainy weather during daytime. The respective values were 0.78, 0.05 and 0.03 °C during nighttime. Sunny weather featured more substantial external air and surface cooling, but indoor warming was recorded in cloudy and rainy weather. The northeast green wall achieved greater external air and surface cooling than the northwest one respectively by 0.38 and 0.77 °C in sunny weather. However, the northwest green wall registered higher external surface cooling by 0.47 °C at night. These results suggested the importance of research-informed green wall design for effective thermal regulation in urban areas. A deep air gap between the vegetation and the exterior building wall would provide more external surface cooling. Walls receiving higher solar exposure could be prioritized for greening.