Cool materials in the urban built environment to mitigate heat islands: potential consequences for building ventilation

Abstract

Urban warming, commonly referred to as the ‘Urban Heat Island’ phenomenon (UHI), is a well-established effect that affects cities all over the world. This occurs due to urban physical characteristics such as urban canyon geometry and vegetation, but mainly to its typical materials. The thermal properties of the materials used for the external walls and roofs of buildings, as well as pavements, can have a major influence on the surface temperature. As a consequence of increased temperature, the UHI has an effect on energy consumption for heating and cooling urban buildings. Cool materials are a cost effective, environmentally friendly and passive technique that uses a coating with high thermal emissivity and solar reflectance properties. At building scale, this technique is recognized for decreasing the amount of heat conducted through the surface and the solar thermal load of the building, reducing its energy requirements for cooling. At urban scale, this strategy contributes to improving the urban microclimate by lowering surface and air temperatures which, in turn, increases the potential for ventilative cooling in the buildings. The goal of this paper is to evaluate the impact of the use of cool materials on the thermal environment of urban spaces and how this can affect ventilative cooling for buildings. The cool materials were evaluated considering the application on roofs and pavements, and the Federal University of Mato Grosso campus, located in Cuiabá, Brazil was used as case study. The study was performed through computer simulations where the 3 scenarios (cool roof, cool pavement and reference scenario) were simulated for the climate of Cuiabá (Aw2 Köppen classification – Tropical wet and dry), considering winter and summer conditions. The methodology consists of three steps: a. preparatory stage (acquisition and compilation of climatic data and physical characteristics of the study area), b. numerical simulation and c. validation of the model and data calibration, for further comparative analysis. As a result, in the scenarios where cool materials were applied, significant differences were found both in the surface temperature and air temperature (height of the pedestrians), up to 7.02°C. The difference was more evident when used as cool pavement than when used as cool roof and this tendency varies in amplitude, considering the locality within the study area and the season analysed. These results allow us to infer that cool materials can increase the potential of the ventilation as strategy for cooling indoor environments, especially by means of stack ventilation which is benefited from greater temperature differences. Thus, it is believed that the wide use of such materials can significantly contribute not only to the mitigation of the heat island effect, but also to reduce overheating risk in buildings by increasing the effectiveness of ventilative cooling and thus reduce the need for air conditioning.