Thermal performance of a vegetated cladding system on facade walls C.Y. Cheng a , Ken K.S. Cheung b , L.M. Chu a, * a Department of Biology, The Chinese University of Hong Kong, Science Center, Shatin, NT, Hong Kong, China b Housing Department, Hong Kong SAR Government, Kowloon, Hong Kong, China article info Article history: Received 25 November 2009 Received in revised form 20 January 2010 Accepted 8 February 2010 Keywords: Vertical green panel Zoysia japonica Heat flux Temperature reduction Thermal comfort Watering regime abstract An experimental approach is used to assess the effect of vegetation on the thermal performance of a vertical greening system, which comprised of turf-based vertical planting modules, on an elevated facade wall of a public housing apartment. Despite temperature fluctuations in the various compart- ments external and internal to a concrete wall, the vegetated cladding reduced interior temperatures and delayed the transfer of solar heat, which consequently reduced power consumption in air-conditioning compared with a building envelope with bare concrete. Vegetation cover led to a different pattern of temperature fluctuations on wall surfaces, which may affect the comfort of occupants even after sunset. The cooling effect which was closely associated with the area covered by living plants and moisture in the growth medium, demonstrated the value of maintaining a healthy vegetation cover beyond visual amenity. Marked variation in moisture distribution along the vertical profile of the growth medium highlighted a concern rarely addressed in planting on ground. Substrate moisture measured at randomly selected locations would underestimate the water stress in some plants and impair their survival. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Hong Kong, like other metropolitan cities, is running out of space for greening. The increase in building and paved surfaces leads to the heat island effect characterized by higher humidity and night time temperature, polluted air and increased concentrations of particulates because on restricted airflow [1]. With the increasing concern on limited ground area for planting, there is a pressing need for an innovative way of greenery to overcome the constraints of open space deficit in urban housing development. The beneficial values of roof gardens have drawn substantial attention since the late 1990s. A vegetation cover on the rooftop provides a cooler interior environment [2e5] and a milder microclimate under the urban canyon [6]. Unfortunately, land scarcity and dense population have sent the residential buildings to go high rise, which limited the effectiveness of roof garden as an urban greening measure. Large proportion of rooftop is occupied by devices such as water tank, elevator machinery and solar panels, resulting in very little space for greening. Recently, the popularity of vertical greenery is growing in the context of urban landscaping because of its smaller footprint, aesthetic value and heat island mitigation impact. Vertical planting is certainly an alternative to roof greenery in a city composed of tower blocks which have high wall to roof ratio, and consequently large potential surface area for greening. Moreover, facade walls, unlike rooftops, usually have no insulation layer against solar heat. The community will benefit if a vegetated cladding on building can exhibit the efficacy of a green roof. Empirical data on the thermal performance of green wall are scarce, though emerging. A metropolitan scale survey in Tokyo suggests temperature reduction by 5e8  C at facade wall surface [7]. Eumorfopoulou and Kontoleon [8] even reported a better cooling effect of up to 10.8  C in another study under Mediterra- nean climate. Some manufacturers of proprietary products demonstrated a cooling capability of about 10  C [9]. However, not much is known about the functionality of the vegetation layer. Most research focuses on the engineering aspects of the system and their contribution to cooling and energy saving. The role of plants on their thermal performance is rarely addressed. Thermal and energy data were obtained by computer simulation [6,10], but the situation on facade wall is much more complicated compared with roof garden as aspect and height seem to have marked influence on Abbreviations: Amb, Outdoor air temperature (  C); Cloud, Average amount of cloud (%); Ext B , Temperature of the exterior surface of bare concrete wall (  C); Ext C , Temperature of the exterior surface of panel-covered wall (  C); Hf B , Heat flux from bare wall (W m 2 ); Hf C , Heat flux from panel-mounted wall (W m 2 ); Int B , Temperature of the interior surface of bare wall (  C); Int C , Temperature of the interior surface of panel-covered wall (  C); P C , Power consumption for air-condi- tioning in room with panel-covered wall (kWh); P B , Power consumption for air- conditioning in room with bare concrete wall (kWh); Rad, Global solar radiation (MJ m 2 ); Sub, Temperature of substrate (  C); Sunshine, Time with bright sunshine (h); Veg, Temperature of grass surface (  C); Wind, Average wind speed (km h 1 ); DT, Temperature difference between panel and ambient air (  C). * Corresponding author. Tel.: þ852 26096378; fax: þ852 26035745. E-mail address: leemanchu@cuhk.edu.hk (L.M. Chu). Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv 0360-1323/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2010.02.005 Building and Environment 45 (2010) 1779e1787