Assessment of thermal performance of cool versus green roofs. Prediction of their impact over urban temperatures in arid cities.

CORREA, ERICA NORMA; CANTÓN, MARÍA ALICIA; ALCHAPAR, NOELIA LILIANA; FLORES ASIN, JUAN EMILIANO

Rapperswil

Congreso; EuroSun 2018 - The 12th edition of the International Conference on Solar Energy for Buildings and Industry; 2018

International Solar Energy Society (ISES)

The cities growth increases the outdoor air and surface temperatures of the urban-building envelope contributing to urban warming. Mitigation strategies aim to restore the thermal balance cities, favoring losses and decreasing profits. Internationally, the most efficient strategies are the application of cold and green roof (Santamouris et al., 2007; Akbari and Levinson, 2008; Sfakianaki et al., 2009; Synnefa and Santamouris, 2012). Both can reduce the surface temperature, the sensible heat flow delivered to the atmosphere (Zinzi and Agnoli, 2012) and the heat flow into the buildings when they are not isolated. In Mendoza city, Argentina, city of arid zone where the hydric resource is a limiting one -Index of aridity = 1.7; Annual rainfall = 200 mm (González Loyarte, Menenti and Diblasi, 2009)-, the application of green roofs is discouraged associated with their water consumption. However, this technology offers ecosystem services that exceed those of cold materials; since they not only lower the air temperature by means of evapotranspiration and shading associated with vegetation, but also purify the air by means of carbon fixation, the retention of atmospheric particulate material and the production of oxygen; they control the excess of runoff and the consequent problems of erosion and floods and have the capacity to maintain ecological cycles (Vásquez, 2016). The objective of this research is to evaluate and compare the thermal behavior of four roofing technologies, two vegetated roofs and two cold roofing materials.In this frame, this research aim to generate reliable predictive tools that describe the microclimatic changes of urban environments in an arid context. Methodologically, three stages were carried out: (1) Experimental test: evaluation of the thermal and optical performance of cold roofing materials of greater local application -aluminum and geotextile membranes- and vegetated roofs with greater hygro-thermal efficiency in the region with Nassella tenuissima and Sedum spectabile species incorporated. (2) Databases: generation of input data of materials and technologies of roof for urban climate forecasting software, ENVI-met, adjusted to the regional arid environment through optical and thermal characterization. (3) ENVI-met theoretical models: comparison of the thermal behavior of 4 scenarios that differ in their cover technology with respect to a base case. Results demonstrate that in both roof cover there are technological alternatives that allows diminish the surface temperature. The better thermal response corresponds to a green roof vegetated with Nassella sp. In green roofs vegetated with Nassella sp. -native species- the surface temperature is up to 10 ºC lower than green roofs vegetated with Sedum sp. ?worldwide recommended species-. In relation to cool roof the white geotextile membrane has proved to be better than traditional aluminum membrane. White geotextile shows surface temperatures 20 ºC lesser than aluminum membrane. Both cool and green roofs technologies incorporation, improves the outdoor spaces habitability. However, the efficiency of each roof covers depends on the suitable selection of technology composition. The final goal is detect what materials allow us to reach maximum efficiency to lower the urban temperature with the least consumption of resources.