Extensive green roofs in Porto Alegre, Brazil : effect on indoor thermal comfort in residential buildings

Abstract

This research aims to develop further understanding of the effect of extensive green roofs on the indoor thermal comfort in naturally ventilated low-income houses, in the Brazilian subtropical city of Porto Alegre. This is achieved by studying the influence of a set of variables, to comprehend the best combinations of elements to a climate responsive green roof design. Moreover, in order to create more realistic results, different scenarios with green roofs and other elements (as grass on the ground and trees) are created, to evaluate the influence of the local microclimate on the indoor thermal comfort. Additionally, understanding how the extensive green roofs affect the outdoor microclimate in Porto Alegre is also a part of the aim. Evaluations are made for the whole year as well as for the summer and winter seasons. The study estimates the effect of extensive green roofs on the indoor thermal comfort and on the outdoor microclimate by calculations through computer simulations. The simulations are performed with EnergyPlus for the indoor thermal comfort and with ENVI-met for the microclimate simulations. The studied houses are conceived for urban or suburban areas, to be implemented in any Brazilian city, as long as some adjustments are made to better respond to the climate, complying with the Brazilian Standards 15.575 and 15.220. The indoor thermal comfort is evaluated primarily with the Adaptive thermal comfort model of the ASHRAE Standard 55, and the calculation of heating and cooling degree-hours. For the outdoor microclimate, the variables air temperature and mean radiant temperature are evaluated. The research shows that green roofs increase the indoor thermal comfort in the studied one-storey houses. For the whole year, the best combination of features is low-density plants and a thick soil layer (150mm). For the winter season, extra-insulation in the building envelope would improve indoor thermal comfort. The extensive green roofs would be beneficial both to new projects, complying with the minimum level of insulation from the Brazilian Standards, and to the existing building stock, which is typically poorly insulated. The microclimate around the buildings affects the indoor thermal comfort, and this investigation shows that, when the green roofs are integrated in a green strategy at the site (with trees and grass on the ground), their cooling effect increases compared to when they are the only green strategy. This study also confirms the advantages of performing coupled simulations, where the results of the microclimate simulations are used as input to the building simulations, to have more realistic predictions of the indoor thermal comfort. Concerning the effect of the extensive green roofs on the microclimate, a minor and local cooling effect is observed, especially at pedestrian level. That indicates that the effect of extensive green roofs as a strategy for reducing the outdoor air temperature in an urban context might be overestimated. The study showed that in the climate of Porto Alegre, a trade-off is needed: the best configuration of vegetation for the indoor thermal comfort over the whole year (trees with a lower density) is not the best configuration for providing a cooling effect on the microclimate. Since the use of green roofs are advantageous to low-income housing projects, this work contributes to meeting the Sustainable Development Goals of the United Nations, especially goal 11: “Make cities and human settlements inclusive, safe, resilient and sustainable”.