Objective  The construction land in densely populated cities is becoming increasingly rare, and green space is scarce on the ground. The green building planting (GBP) may make full use of the idle area of building roof and wall to contribute to urban greening, and relieve the thermal environment strain. By affecting ventilation and shading, the building space can produce cold and heat exchanges, and variation in solar radiation, thus further differentiating the cooling potential of GBP. However, the particular cooling effects of GBP in different design heights are unclear, and the divergence of the thermal effects of different types of GBP in 3D space cannot be adequately compared. Therefore, this research focuses on extracting the size and height features of typical single buildings from urban vector data, and simulating the spatial and temporal characteristics of the cooling intensity of both roof greening and vertical greening, while putting more emphasis on the difference in thermal effects caused by the design height of greening.

Methods  The research adopts the "green facade" module of ENVI-met to perform microclimate simulation and spatial-temporal analysis of cooling potential before and after GBP in typical single buildings in Shanghai. Based on the unified setting of climate backdrop and built environment conditions, the research establishes a combination of ten unique building heights (design height, h), four GBP forms, and non-greening reference model, involving 50 simulation scenarios in total. To represent the cooling potential of GBP, the research takes the difference in outdoor ambient temperature between the greening (T_green) and non-greening (T_bare) scenarios of the same single building as the cooling intensity (T_bare−T_green), and the area occupied by the remaining "cooling grid" left upon the removal of building as the cooling coverage, to indicate the average difference of the thermal effect in each greening scenario.

Results  The findings of this research reveal that GBP cooling phase is concentrated after midday, and cooling intensity peaks when the cumulative intensity of solar radiation is at its maximum. Specifically, the cooling intensity of roof greening diminishes with the increase of design height, while the cooling coverage thereof varies little, generally staying in the roof space. The cooling intensity of vertical greening is less influenced by design height, while the cooling coverage thereof is significantly expanded with the increase of design height, mostly affecting pedestrian space near the ground. 1) Throughout the day, the major cooling period of GBP is 11:00–17:00, with the peak cooling intensity occurring at approximately 16:00 when the cumulative intensity of solar radiation reaches its maximum. Yet, the air temperature drops indistinctively at night, and even local warming occurs. The maximum cooling intensity of roof greening is about 0.05 ℃ higher than that of vertical greening; however, the period of thermal effect in vertical greening is extended by around 2 hours. The cooling intensity of vertical greening is highly influenced by the orientation (direct exposure to sunlight or not), and reaches the peak when the south building facade is directly exposed to sunlight. 2) Under the action of convection and gravity, the cold air forms a 3D spatial distribution difference. The highest cooling intensity of roof greening remains at 3 m above the rooftop, which can cover an area of approximately 5, 100 m³ near the building surface; when the design height is less than 12 m, it is feasible to improve the thermal environment near the ground. The cooling area of vertical greening is concentrated in the space where design height is located. The cooling area of fully covered vertical greening is mostly located below 5 m on the building's windward side, with a maximum cooling coverage of around 14, 000 m³. 3) The cooling intensity of the roof greening and partial vertical greening shows a decreasing trend with the increase of design height. The maximum difference of cooling intensity between design heights is 0.12 ℃. The fully covered vertical greening is less influenced by design height in the adjacent building space, and the average cooling intensity is roughly 0.10 ℃. The cooling coverage of roof greening grows and then drops with the increase of design height, reaching a maximum when the h reaches 21 or 24 m; the cooling intensity of partial vertical greening only decreases with the increase of design height, while that of fully covered vertical greening only increases with the increase of design height.

Conclusion  Roof greening and vertical greening both offer distinct advantages. Without regard for greening funds or construction conditions, fully covered vertical greening may best expand the cooling coverage. Roof greening should be prioritized in mixed-use buildings where large-scale GBP can hardly be implemented due to the high cooling intensity per unit area. Meanwhile, given that the cooling potential of partial vertical greening is limited in low-rise buildings or the bottom area of high-rise buildings, the aesthetic landscape benefits thereof should be mainly considered when determining whether partial vertical greening should be adopted. This research is conducive to deepening the understanding of the cooling mechanism of various GBP strategies, and serves as a reference for the practice of GBP design from the standpoint of environmental benefits.