## 5 Discussion  
The noticeable consequences of climate change make it necessary for our built environment to be adapted so that comfortable living conditions can prevail in the future. Especially in densely populated urban environments, adequate approaches must be found to mitigate the adverse effects of climate change and to reduce resource consumption. Integrating green infrastructure as well as renewable energy systems into urban structures are both key aspects in this context. Building envelopes, and in particular roof surfaces, can provide suitable areas for both applications. In addition, they add recreational space for building inhabitants. In the research project described in this paper, a solution for the efficient use of roof surfaces was found that allows each square meter of a flat roof to be used for multiple purposes: a pleasant recreational area for humans, space for plants, and energy production. The developed design is particularly suitable for the retrofitting of existing buildings, as its lightweight structure can be applicable in most cases, since structural requirements will be met. Furthermore, the additional green infrastructure within an urban environment can increase biodiversity by creating different habitats that are often urgently needed in the mostly densely sealed cities. The modular system allows for adaptability that reacts flexibly to different requirements and framework conditions and can provide numerous ecological advantages in a single application. For the green roof system, a variety of plant species can be selected. In general, a high number of species grew successfully in the PV testing facility, even though it was noted that some species had a lower growth rate and bloomed less in zones with more shading. However, in some cases, the shade might have increased the vitality of the plants. This effect was not specifically analyzed in the testing  
Figure 18: Plant species and aspect calendar for the exemplary case study of the open-plan office type. facility, as no test field with direct sun exposure was set up as a comparison. Some plants also react adversely to high sun intensity, with sunburns forming due to high temperatures [28]. The results from the testing facility at the BOKU are only representative for areas with the same local characteristics. The planting area of the testing facility was limited to 36 m\({}^{2}\). Due to the relatively small size, the influence from border areas and diffuse light on the plants was rather large, and consequently merely a small range of plants could be tested. Some plants that were also used in the two exemplary case studies were selected based on their having plant properties similar to the tested species, but they were not specifically tested. A careful selection of species and proper planting design had to be carried out for every project during the planning and implementation process to ensure that adequate plants were chosen under particular framework conditions. The two examples of possible \"PV Rooftop Garden\" designs highlight the variability of uses and planting concepts for intensive green roofs in combination with energy production. However, an equally important aspect lies in the recreational benefits and the extended function of a simple flat roof. A study of the microclimatic conditions inside and outside the test stand at the BOKU showed the importance of sunny and shaded areas in the rooftop garden in increasing thermal comfort: on a hot, windless, sunny summer day, the heat stress for users could be significantly reduced under the PV canopy. During cloudy days, on the other hand, it was slightly warmer under the photovoltaic shelter than out on the part of the roof not shaded by the PVs. On a cold but sunny day, in contrast, thermal comfort was higher outside the PV canopy, where radiation could directly affect the human body [48]. Even though the use of the developed system in buildings is highly suitable for dense urban environments, there are a series of other functions where the system could equally be applied.