# Experimental Evaluation and Numerical Simulation of the Thermal Performance of a Green Roof
## 1 Introduction  
In recent years, urban areas' growth and the consequent sources of pollution have led to an increase in terms of global warming. In addition, the so-called Urban Heat Island (UHI) phenomenon has developed. UHI is represented by the temperature rises in areas characterized by a high urban fabric, if associated to the surrounding rural areas [1]. The evaluation of the UHI phenomenon is crucial to design plants effectively and to evaluate the buildings' energy needs. It is noteworthy to suggest interventions for the mitigation of this phenomenon, lowering its intensity. Achieving this goal is fundamental to reducing the increasing building energy consumption, especially during the warmer months [2; 3]. If, on the one hand, the internal temperatures of the buildings can be set lower, the same cannot be done for the external environment, with the exception of countermeasures finalized for the reduction in UHI effects. Furthermore, the high temperatures that occur in cities during the warmer months can involve substantial and damaging effects on daily life [4; 5; 6; 7]. Therefore, the UHI phenomenon and its countermeasures are topics of interest in the scientific literature. Several works aimed at assessing UHI intensity suggest strategies to reduce its impacts, such as building design approaches [8; 9; 10]. Several mitigation measures were recommended in order to mitigate UHI impacts on environmental, energy, economic and social aspects. Some of them are based on the existing correlation between the UHI and the polluting gases in the atmosphere. Consequently, the reduction in pollutant emissions has a direct influence on the UHI, and the UHI reduction has a direct influence on building energy consumption [11; 12]. All this can be achieved by means ofa rational buildings project, but also considering the emissions reduction induced by transport or industrial sources. Further mitigation solutions can be reached by employing construction materials with a high albedo. This research focused on the assessment of the effectiveness of a green roof thermal behavior. The goal was to analyze the thermophysical properties and thermal behavior to estimate the real effect on the building's energy performance optimization. Several measuring instruments were installed for observing heat transfer phenomena across the green roof. The experimental campaign consisted of acquiring data from both the green roof and a nearby conventional roof, in order to compare the thermal performance. Moreover, the roof-lawn system, characterized by a multilayer structure, was reproduced using a Finite Element Model (FEM) code for obtaining equivalent thermophysical properties. It is worthy to mention that the compositions of the roofs were known but the thermophysical properties of each layer were undetermined. In addition, the green roof is made of five layers, of which the roof-lawn is non-homogeneous, and consists of a layer of grass and the underlying substrate. On the other hand, the original roof is made by only three layers (explained in the following section), characterized by conventional materials. This approach was applied for achieving suitable information for creating building energy models for the simulation of yearly energy needs. The roof-lawn system showed its advantages, pointing out a higher thermal inertia with no overheating during the warmer months and a lower thermal transmittance. This resulted in lower energy demands and, consequently, better internal environmental conditions. Consequently, it is possible to assert that a green roof can enhance the thermal inertia of a roof, thus increasing the internal comfort, reducing energy needs [13; 14; 15; 16; 17; 18; 19]. In addition, if installed in high-density urban areas, a green roof can offer positive contribution against the UHI phenomenon, also absorbing polluting gases.