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

Abstract: Brazil had a significant quantitative housing deficit of 7.7 million residences in 2015, especially in low-income housing. Traditionally, these projects have low levels of thermal insulation which can result in discomfort caused by warmth or cold. The residential sector is responsible for a significant part of the energy use in Brazil. A green roof, a rooftop system with the addition of vegetation in the upper layer, is a way of reducing heat gain in the summer and heat loss in the winter. That is why green roofs are considered as a passive technique worldwide – that is, a technique not using energy for heating or cooling – to improve the indoor thermal comfort. There is a growing use of green roofs in Brazil. A more comprehensive understanding of the effect of green roofs on thermal comfort will help to identify key design aspects to improve their efficiency. This thesis investigates the effect of the thinner and most common type of green roof, called extensive, on the indoor thermal comfort in Porto Alegre, a city in Southern Brazil with a subtropical climate. The indoor thermal comfort in naturally ventilated low-income houses was calculated using computer simulations. Three groups of factors influencing the thermal performance of green roofs were analysed: the first was related to the building envelope (level of insulation), the second was associated with the green roof composition (soil thickness and vegetation density), and, finally, the third was related to the outdoor environment (presence of trees near the houses). For the last group of factors, calculations of outdoor microclimate and indoor thermal comfort were linked in a process called coupled simulation. The results confirmed the suitability of green roofs to increase the indoor thermal comfort in low-income housing projects in Porto Alegre, especially in one-story buildings. Green roofs would be beneficial to new projects, complying with the minimum level of insulation requirements in the Brazilian Standards, and to the existing poorly insulated building stock. Extra insulation in the building envelope would increase the benefits provided by the green roofs in the winter season even more. For the whole year, the best combination of features was low-density plants and a thicker soil layer (150mm thick). The vegetation around the building influences the microclimate and also affects the indoor climate. However, the shade provided by trees is helpful in the summer but disadvantageous in the winter. Both the green roof and the garden landscape should not consist of too dense vegetation in the climate of Porto Alegre, because it reduces the solar heat gain which is beneficial in the winter. The effect of green roofs on the microclimate was also evaluated. Results from this investigation revealed a minimal and local cooling effect from the green roofs on the outdoor temperature. Green roofs alone are thus not effective in reducing the high temperatures during the summer season in Porto Alegre. This work contributes to meeting the goals of the 2030 Agenda for Sustainable Development from the United Nations General Assembly (2015), especially the 11th Sustainable Development Goal: “Make cities and human settlements inclusive, safe, resilient and sustainable”. The findings of this study that the use of green roofs increases indoor thermal comfort meets the aim of adopting local and affordable resources, being relevant to low-income housing projects.