The influence of facade properties on the canopy layer microclimate within city blocks

The application of high albedo surfaces and highly reflective or 'cool' materials is often advocated as a means to mitigate urban heat island and to decrease indoor temperatures and thus to reduce cooling loads in warm weather. Besides the horizontal surfaces of roofs and roads, the application of reflective coatings is increasingly recommended for walls as well. In case of facades, the influence of surface reflectivity is shown to be greater for lightweight structures. The effect is most pronounced during daytime, under clear and calm conditions. The current paper investigates the role of facade albedo, heat transmission coefficient and fenestration ratio on the canopy layer microclimate though a numerical simulation study. To consider the interplay between built form and facade properties, the study takes four metropolitan urban block typologies from Budapest as models. The examination focuses on the airspace within these urban blocks. Considering that mean radiant temperature and air temperature govern human thermal comfort during clear and calm weather, the analysis is performed on the basis of these two parameters. The study utilizes ENVI-met for microclimate modeling and MATLAB for data analysis. The results indicate that among the three facade parameters albedo drives the canopy layer microclimate. Changes in facade albedo are found to be directly proportional with changes in air and radiant temperatures: higher values increased both air and radiant temperatures. The impact of fenestration ratio is primarily exerted though the albedo indirectly, as increasing fenestration ratio decreases the albedo of walls. The influence of heat transmission coefficient is found to be marginal.