## 2 Materials and Methods
### A Description of the Residential Apartment Unit for the Energy Retrofitting Project
The model that has been selected to predict the building energy performance in Greece is based on the '_semi-steady-state-with-monthly-step_' mode. It is in accord with the requirements as they are outlined in EN ISO 13790 [96], as well as other accompanying standards. This permits the calculation of the primary energy demands while taking into account the comfortable-living level of the apartment unit inhabitants. It also incorporates a primary energy differential in relation to a reference building unit. The classification categories assign grades to apartment units ranging from A to G. Excellent performers receive an 'A+' grade. This translates to consuming an energy amount of 33% less than the reference apartment unit. On the other end, the worst possible rating, a 'G' grade, signifies consuming an energy amount 273% higher than that of the reference apartment unit. Ingeneral, incrementally-rated building apartments reach 'Green' status (A\({}^{+}\), A, B\({}^{+}\)) as long as they consume less than 75% of the energy demand of the reference unit. To facilitate the energy classification process for different types of building usages, the term specific energy consumption is introduced, which denotes the annual average consumption per unit area of the apartment [97]. It is measured in kWh/m\({}^{2}\) per year. The behavior of the inhabitants is crucial in assessing the total energy consumption. Several variables that might contribute to the estimation, such as: (1) the total time of occupation of the apartment unit, (2) the routine internal thermostat settings, and (3) the type and variety of rated electromechanical and thermal devices in use. Their temporal trends could generate enough variability to make estimations of energy consumption difficult to interpret. A different approach is to realize that there are generally three energy-controlling categories, which are associated with: (1) the building shell, (2) the network of electromechanical devices, and (3) the renewable energy production systems. Each of the three stated categories undergoes an examination. They aid in identifying opportunities for appropriate interventions in case the building energy classification status is not found to be favorable. To dimensionalize the potential effects of the various combinations of the electromechanical systems, as well as those in relation to heating, cooling, and air conditioning, it is imperative to delineate the unit design conditions. To achieve meaningful estimations, the monthly standard energy demands are correlated to the proper climate zone data as they are furnished by the National Meteorological Service in Greece and in accordance with the energy efficiency regulation [97]. In the influence group of the particulars of the architectural design, its custom external elements are also included. Furthermore, the optimal design of the electromechanical systems plays a crucial role in classifying and improving an apartment unit. The architectural design may indicate how to achieve optimal insulation as well as how to configure the optimal selection and arrangement of the electromechanical systems. Both aspects also contribute to the minimization of thermal and cooling loads. Optimal insulation is related to the minimization of energy losses by entrapping warmness or coolness in the respective seasons. Optimal electromechanical systems maintain the desired room conditions by minimizing the consumption of energy and other resources. From an energy inspection perspective, there are three types of electromechanical systems that deserve separate attention. Wintertime heating systems are based on energy resources, such as oil, gas, electricity, and solar energy. They may be centrally or locally controlled through different kinds of distribution systems that are distinguished by: (1) their structure (single or double piped), (2) their flow medium (air, water, etc. ), (3) their mode of distribution (internal or external), and (4) their insulation coverage area [98].