## 2 Materials and Methods
### A Description of the Residential Apartment Unit for the Energy Retrofitting Project  
Improving energy efficiency in older residential constructions contributes to the reduction of greenhouse gas emissions. The energy classification status of a building often  
Figure 2: Layout of the design and analysis of supersaturated data. dictates the direction that subsequent retrofitting actions will take to make it greener. It is not a simple task. Successful energy-efficient retrofitting attempts often rely on a complicated 'optimal-energy' analysis that engages integrated information from an energy conservation standpoint as well as from the building's inherent engineering layout [84; 85; 86]. Enhancing the energy performance of a residential apartment unit, by considering its indoor and outdoor thermal-environment design, requires a screening/optimization analysis on a score of quantity and quality determinants [87; 88; 89]. Prioritization of energy effective tactics is imperative to improving the energy classification status of a residential unit. A fruitful approach to discovering the essential improvement opportunities and matching them to effective energy options is through benchmarking, which is supported by modern data analysis techniques [90]. However, it is not always accessible. Efficient solutions to retrofitting older buildings are based on modeling physical systems that should economically harmonize greener existing materials to contemporary technology practices [91]. Building energy system designs usually need to confront complexity issues. Resolving the complicated details may be expedited by the utilization of advanced computational aids [92]. While examining a residential architecture, an energy efficiency analysis should list and quantify as many implicated effects as possible; it reduces the energy consumption uncertainty and determines its optimal resource usage [93]. Specifically, in the European Union, the energy consumption trends with respect to the performance of residential buildings have been a focal matter. The European Commission declared the '20/20/20 rule' through its European Energy Policy which conjectured--by the year 2020--a 20% energy production from renewable sources, a 20% reduction in released pollutants, and 20% in energy savings [94]. The pertinent European Directives 2002/91/EC and 2010/31/EC on the energy performance of buildings, now flourish to the 'Renovation Wave for Europe' [8]. Thus, the objective of greening-through-retrofitting the older European residential building stock has been now communicated; it urges (at least) a doubling of the annual \"deep-energy\" renovation rate by 2030. On the way to EU-wide climate neutrality, the message is coherent: \"Mobilizing forces at all levels towards these goals will result in 35 million building units renovated by 2030\" [8]. In this case study, we exemplify the use of a supersaturated design in a lean (and green) screening of a real apartment that has residential use. Geographically, it is located in Attica prefecture (central Greece). The location plays an important role through its influence--via the climatic phenomena--on the amount of energy, which is necessary to attain ideal conditions for a comfortable living. The specific apartment unit is located in Climate Zone B, where the winter is considered mild and the summer is intense [95]. It was built in the year 1994 and, therefore, it belongs to the building stock category that is primed for deep modernization. Therefore, it is not known whether it conforms or not to the latest regulations of energy-efficient buildings that were published in the year 2010. It is situated on the fifth floor of a five-floor residential building. This condition maximizes its environmental exposure and, hence, its energy losses to surroundings--mainly, due to the peripheral heat exchange. The'shell' of the building is discontinuous and heterogeneous. Thus, it exhibits utterly insufficient insulation, which is to be solely found in the walls and not in the structural columns.