Plea2004 - The 21 th Conference on Passive and Low Energy Architecture. Eindhoven, The Netherlands, 19 - 22 September 2004 Page 1 of 6 The Thermal Effects of Green Roofs and Green Façades on an Urban Canyon Eleftheria Alexandri and Prof. Phil Jones Welsh School of Architecture, Cardiff University, Cardiff, Wales, UK ABSTRACT: This paper investigates quantitatively the thermal effect of plants in the formation of the urban canyon microclimate. Through this theoretic work an attempt is done to indicate whether covering building surfaces (roofs and walls) in existing urban environments with vegetation could be a realistic proposal for the mitigation of raised urban temperatures . A two-dimensional, dynamic heat and mass transfer model has been developed, describing the thermal exchanges in an urban canyon, due to both temperature and humidity concentration changes. A parametric analysis is done for diverse climates (dry, humid, and arid) and for different urban canyon geometries . The surface materials of walls and roofs are changed from construction ones to plants (grass and creepers) parametrically. Conference Topic: 3 Comfort and well-being in urban spaces Keywords: green roofs, green walls, urban canyon, microclimate, heat and mass transfer models INTRODUCTION Existing cities tend to have raised temperatures, compared to their surrounding rural areas [1]. This is mainly due to the use of construction materials with high thermal capacities, small porosity, low albedos, and the lack of vegetation in urban spaces. These raised temperatures can be uncomfortably high for the urban inhabitants, especially during hot seasons. In conjunction with urban pollution they can even be responsible for the loss of lives [2]. A way of lowering these raised temperatures in the whole urban scale is by covering urban surfaces (such as building walls and roofs ) with vegetation. Thus urban temperatures can be decreased by the evapotranspiration of plants , the change of surface albedo and the shading of construction materials . The extend of this reduction depends on the climatic conditions of the area (air temperature and humidity, irradiation, wind speed and direction), the characteristics of the urban materials (including their albedos) and the urban geometry. This paper presents the thermal effects which vegetation has on the built environment, when placed on building surfaces . An analysis is carried out for diverse geometries and climates. This calculation is done with a dynamical, two-dimensional heat and mass transfer micro-scale model, developed in Cardiff University, which describes the temperature and humidity distributions in an urban canyon. 2. METHODOLOGY 2.1 Method of Calculating Urban Temperatures Urban temperatures and humidities (both air and surface ones) are calculated dynamically with the use of heat and mass transfer equations. The impact of mass transfer on heat transfer has been considered to be crucial in the rational estimation of the impact of vegetation on temperatures for diverse humidity concentrations. This set of algorithms for building materials, soil, vegetation and air has been presented in [3]. The model formed by these algorithms has been programmed in C++. It is a two-dimensional one, describing the typical geometry of an urban canyon. Its outputs are temperature and humidity for each of its nodes, for both nodes in the air and nodes in the materials (including building materials, soil asphalt, sand and loam (underneath the asphalt l ayer), and vegetation). Wind distributions within and over the canyon are calculated with the CFD code winAir4 [4]. Shading in the canyon is calculated with the software Ecotect [5]. A visual representation of the model is given in figure 1. Tin RHin RHin Tin RHurban Turban Turban RHurban Turban RHurban Tground Hground asphalt sand loam Figure 1 : Representation of the two-dimensional urban canyon model.