Urban street canyons – impact of different materials and colours of facades and ground and different positions of persons on outdoor thermal comfort

Abstract

Outdoor thermal comfort in urban street canyons has become, in times of adaptation to climate change, an important field of research also for cities in moderate climates. Many investigations can be found dealing with measurements of façade and ground temperatures, solar radiation, wind, etc. The impact of different materials, colors, low emissivity paints, green facades, etc. is widely discussed. To have the possibility to compare and assess these different measures a simple but holistic model based on standard VDI 3787 (‘Klima-Michel model’) was developed. It is extended by a model for the dynamic behavior of materials using only known physical parameters of the materials. This model shows good accordance with measurements but can be used independently of measurements and for all materials and locations. The assessment is done with the Universal Thermal Comfort Index (UTCI). The whole model is described in this paper. In the second part of the paper a street canyon with height to width ratio 1:1 was investigated for a moderate / Mediterranean climate (Bern). Different measures for the reduction of heat stress are investigated, compared and could be finally classified in a list of priorities regarding their impact. Shading has highest impact, less of the persons (sun umbrellas, awnings, trees) but more of the whole canyon (fabric, big trees). Green facades and low emissivity paints reduce heat stress remarkably. Very bright materials increase heat stress because of their high reflection of solar radiation from facades and ground to the persons in the canyon (this effect was found as dominating the corresponding reduction of IR radiation from the absorbing material to the persons). The influence of massive versus light materials is quite low.