## 1 Introduction  
The building sector is responsible for about 40% of total global final energy use in some developed countries as well as 40% of total greenhouse gas emissions [1]. Additionally, in the Republic of Serbia, residential households have a 35% share in the total amount of energy consumption [2]. It is therefore clear that significant improvement in energy savings can be achieved by the increase in energy efficiency in this sector. Despite the unnatural environment of modern cities, they should provide comfort to their inhabitants. The goal of optimizing energy consumption and protecting the environment should also be accomplished. New solutions have emerged in the construction strategy as a result of growing populations in cities [3]. Their aim is to reduce the impact of considerable energy needs while maintaining or improving citizens' quality of life. Due to the constant increase in urbanization, it is necessary to optimize building areas and their use. Fulfilling these demands can generally be achieved through various approaches. Retrofitting measures in residential buildings with different aspects and challenges with the importance of cost-effectiveness are discussed in references [4, 5]. However, greenery systems provide additional social and environmental value, with great potential to mitigate some negative aspects of urban housing. An overview of different greenery systems and the effectiveness of their application in different climates are presented in reference [6]. Green roofs, the practical tool of the urban greening concept, are structures that are covered with a vegetation layer. The growth of different forms of vegetation on top of buildings thus provides aesthetic, environmental, and economic benefits [7; 8]. Additionally, green roofs have a positive effect on improving the global microclimate in cities, reducing noise levels and purifying rainwater and air inside the building. They are also a living space for various animals and insects, and they can be used for food production. It is important to emphasize that green roofs can improve the thermal performance of buildings, so they are a great option for potential energy savings [9] through reducing energy consumption for heating and cooling [10]. Even though validation of the results of the simulation model [11] is valuable confirmation in research on green roofs, reference [12] argued that there is a lack of experimental results in terms of energy savings in the literature. Data for measuring the effects of green roofs are further discussed in the Section 2. There are three types of green roofs, depending on usage, maintenance requirements, and structural considerations: intensive, semi-intensive, and extensive. Green roofs consist of the following layers (from bottom to top of the construction): protective board, waterproof membrane, drainage layer, filter, vegetation substrate, and plants. Besides the essential layers, green roofs can include additional filters [7], root barriers, thermal insulation, and irrigation systems (above or inside the soil layer). Since growing medium represents a relatively thin layer in the case of extensive green roofs, they are easily feasible and lightweight. Consequently, they serve aesthetic, remedial, or technical purposes, and, compared to other types of green roofs, their maintenance and construction costs are lower. In addition to traditional green areas, intensive green roofs can provide similar amenities, usually arranged as parks or gardens. However, their initial and maintenance costs are considerably higher due to their significant weight and irrigation requirements. Furthermore, their waterproofing layers are more reliable, and the energy efficiency of the adjacent spaces of building is substantially improved. Besides all the above advantages, an important aspect of the applying of green roofs is reflected in the economic benefits. Many studies, such as [7; 13; 14], claim that the probability of profit of a green roof is much higher than its potential financial losses.