Effect of vegetation biomass structure on thermal performance of tropical green roof

Abstract

The passive cooling effect of green roofs in humid, tropical Hong Kong was investigated with reference to three vegetated plots, grass, groundcover herb, and shrub, with contrasting growth form and biomass structure and a bare control plot. Temperature was monitored at 15-min intervals for a year at seven levels: high (H) at 200 cm, middle (M) at 60 cm, low (L) at 20 cm, surface, soil, rockwool (water storage), and roof-tile surface. The findings indicated the crucial roles played by biomass quantity and structural complexity in passive cooling functions. Temperature variations of vegetated roofs occurred mainly during the day, with lower maximum and minimum than the control, but they did not cool air at night better than the control. Control and grass surfaces were warmed above the ambient temperature, but groundcover and shrub surfaces followed the ambient. Despite complex biomass structure, shrub created the most extreme diurnal air temperature regime. Despite simple biomass structure, grass cooled air more effectively than groundcover and shrub. Four anomalies in the vertical temperature profile were detected. First, the grass roof cooled daytime nearground air to create a suspended temperature inversion. Second, the stagnant air within the shrub biomass trapped heat to generate a daytime canopy temperature inversion. Third, the elevated branch-foliage biomass of groundcover and shrub brought passive cooling to form a perched thermal discontinuity. Fourth, the air gap of the plastic drainage layer arrested downward heat transmission in all vegetated plots to form a subsurface thermal discontinuity.The findings provide hints on species choice and design of green roofs.

Green-Roof Effects on Neighborhood Microclimate and Human Thermal Sensation

“…Previous field and modeling studies in various climatic zones indicate that an individual green roof can reduce roof surface temperature by 15-45 °C, near-surface air temperature by 2-5 °C and building energy consumption by up to 80% [3][4][5][6][7][8][9][10][11][12]. In an extremely compact tropical city such as Hong Kong with severe shortage of ground-level green spaces and intense UHI effects [13,14], green roof could bring significant benefits.…”

Plant cover and floristic composition effect on thermal behaviour of extensive green roofs

“…9a the evolution of substrate temperature at three different depths (T À2 cm, T À4 cm, T À8 cm), the volumetric water content of the substrate (VWC) and external air temperature (Tout), during a representative week of summer 2012 are shown. The substrate displays a characteristic bell-shaped curve, indicating accumulation of solar heat after sunrise, gradually raising soil temperature to the maximum peak in the early afternoon and then reaching a minimum value at early morning [15].…”

“…The results of their study revealed that a 100-mm-thick substrate layer of the roof garden can decrease the temperature fluctuations significantly in summer periods. Jim [15] investigated the passive cooling effect of green roofs in humid, tropical Hong Kong with reference to three vegetated plots and a bare control plot. The thermal performance of the three vegetation types demonstrated pronounced variations in air temperatures at different heights, surface temperature, and material temperature at different depths.…”

Water Through Green Roofs

“…Because of their tolerance of extreme temperature and light conditions, warm season turf grasses have been used on green roofs in sub-tropical and tropical climates, although typically supplemental irrigation is provided (Jim 2012;Ju et al 2012;Sutton et al 2012;Chen 2013). Some wetland species have broad habitat tolerances or some ability to withstand short periods of dry conditions.…”