This study investigated the effects of orientation and weather on the thermal performance of testing rooms with and without vertical greenery systems (VGSs) in humid subtropical Hong Kong. The wall temperatures, ambient air temperatures and indoor air temperatures of both rooms were measured and compared. The effects of mean air temperature, solar radiation, total bright sunshine, relative humidity, mean cloud cover and wind speed on the thermal indicators of VGSs were also explored. Orientation and weather have shown significant effects on the daily maximum temperatures of the walls with and without VGSs. A north-facing VGS showed the highest capacity of reducing ambient temperature (10.1 degrees C), while the west-facing VGS gave the greatest wall temperature reduction (6.1 degrees C). The maximum temperature difference between the indoor air temperature of the two testing rooms was 3.6 degrees C, attributed to the VGS. Solar radiation, total bright sunshine and relative humidity showed significant correlations with the thermal performance of the rooms. Based on simulations, the VGSs could reduce the steady-state cooling load of the building envelopes by 12% to 42%, depending on their orientations. The highest reduction was found to happen when putting the VGS on the north wall. Overall, the use of VGSs in built environment could bring remarkable passive cooling benefits to both outdoor and indoor environments. (C) 2018 Elsevier B.V. All rights reserved.