*3.1. Description of Dwellings and Energy Consumption Analysis*

This section shows the analysis regarding frequency of the building main features. Figure 1a depicts the subdivision of the buildings according to the construction year, following the official classification used in the Italian population censuses [58]. A further division compared to the decades is done to account for law modifications in the field of building energy performance (years 2005, 2008, 2010, 2015). The data show that a wide part of the sample buildings belongs to before 1976, exactly the year of the first Italian law about energy saving.

**Figure 1.** Dwellings' subdivision: (**a**) construction year; (**b**) size.

Moreover, Figure 1b shows the building frequency related to their size according to their five different dimensional classes specifically: small <50 m2; small-medium 50–85 m2; medium 85–115 m2; medium-large 115–145 m2; large >145 m2. The average size of the apartments is equal to 112.4 m2 and the most common class is the medium one. These different sizes are connected to the family number of components the dwelling was designed for. It has to be pointed out that a great part of the considered dwellings occupy only one floor, i.e., 360 equal to 86.5% while, among the remaining ones 37, i.e., 8.9%, occupy two floors and 19, i.e., 4.6%, three floors.

The data acquisition procedure does not include the introduction of the characteristics of the walls by the user, but there is the indication concerning the construction year and any refurbishment carried out. Table 4 shows the transmittance values of the buildings, depending on the construction year and according to the climate zone.

Reading the table, it is noteworthy that the current legislation provides a subdivision of the Italian territory into six climatic zones, according to the number of Degree Days (Zone A: DD ≤ 600; Zone B: 600 < DD ≤ 900; Zone C: 900 < DD ≤ 1400; Zone D: 1400 < DD ≤ 2100; Zone E: 2100 < DD ≤ 3000; Zone F: DD > 3000).


**Table 4.** Transmittance values depending on the construction year and the climate zone.

Table 5 points out the number of dwellings that have undergone renovations, divided according to year of construction and type of intervention. The most frequent refurbishment intervention is the replacement of windows, carried out in 197 dwellings (47.0% of the total) [59]. Air leakage entailing heat losses and acoustic discomfort are the main reasons of such intervention [60]. For the new installed building components, a transmittance value was set equal to the most common of the same period of installation.



Regarding the HVAC systems, the analyzed dwellings are all equipped with a heating system and a DHW production system. Most of heating systems are autonomous (73.3%); gas is the most used energy vector in heating systems (98.8%) and in DHW preparation (85.4%). The majority of gas-fed systems provide both heating and DHW as shown in Figure 2.

**Figure 2.** Types of heating systems: (**a**) space heating; (**b**) DHW.

Cooling systems are installed only in 207 dwellings (49.4%). They serve only a few rooms, as depicted by Figure 3a and they show high energy label due to their very recent installation, as Figure 3b reports.

**Figure 3.** Cooling systems: (**a**) dwellings and number of air-conditioned rooms; (**b**) energy label.

In order to validate the simulations' outcomes from collected surveys, a comparison with energy bills was made. As a result of this comparison, a lower correlation between real and simulated gas consumption (*R*<sup>2</sup> = 0.7764) compared to the electricity one (*R*<sup>2</sup> = 0.8977) can be seen. The lower correlation relating to the use of gas depends on the greater uncertainty of occupants' profiles in dwellings and high incidence of this service on energy consumption.

The charts of Figures 4 and 5 show a calculation per unit of surface of the selected indicators: (i) primary energy consumption, (ii) local emissions, (iii) use of renewable energy, and (iv) flexible loads per class of sizes.

**Figure 4.** Key performance indicators: (**a**) specific primary energy; (**b**) specific local emissions.

**Figure 5.** Key performance indicators: (**a**) specific renewable energy; (**b**) specific flexible loads.

At apartment size increases, the average value of primary energy consumption per unit of area decreases, from 215.0 kWh/m2y (small dwelling) to 120.6 kWh/m2y (large dwelling). Even the local emissions, in terms of equivalent CO2, show a decreasing trend from 27.9 kg/m2y (small dwelling) to 16.1 kg/m2y (large dwelling). This is due to the lower ratio between surface and volume in the largest dwelling as well as lower specific occupancy rate. Regarding the medium-large dwellings, a slight variation due to the higher incidence of gas consumption for heating on the overall energy one is found. The larger houses with greater frequency are isolated. They have larger dispersing surfaces and, subsequently, heat losses [61].

Similarly, in Figure 5a, renewable energy consumption per unit of area decreases as the size of the apartment increases, from 24.1 kWh/m2y (small dwelling) to 8.5 kWh/m2y (large dwelling).

The consideration of another parameter is remarkable: the electrification degree. It is the ratio between the electricity consumption and the total primary energy one. It is on average 36.3% for the considered building stock. Next, the consumption of renewable energy is on average equal to 9.0%, depending largely on the renewable share of the power grid where they buy electricity from.

Even for flexible loads per unit of area, a decreasing trend is observed at increasing the dwelling surface, although not continuous, from 12.4 kWh/m2y (small dwelling) to 6.9 kWh/m2y (large dwelling). This is due the fact that different occupancy profiles occur since the small ones have higher rate of occupants' absence compared to the large one. In absolute terms, average flexible electric loads of the analyzed dwellings [54] are equal to 1043 kWh/y and they are largely shiftable loads (667 kWh/y) related to the use of washing machines, dishwashers, and dryers. Storable loads have a lower average magnitude (376 kWh/y), due to the low diffusion of electric heating and DHW systems and as a consequence of the low presence of cooling systems.
