## 4 Research Methodology
### Energy Performance Calculations--Thermal Bridge Incorporation Approach  
Regarding the energy performance of the whole building, heating and cooling energy needs were calculated according to the simple hourly method described in standard EN ISO 13790 [43]. This calculation method was selected because it is the official calculation method in Croatia, prescribed by national legislation. Calculations were performed considering the primary function of the MUZA and with the interior air temperature \(T_{i}=20~{}^{\circ}\)C for the case of 4 climate conditions in Croatia (Gospic, Zagreb Maksimir, Split Marjan, and Hvar). All of the materials and building envelope characteristics were taken as shown earlier in this paper (Figure 6, Tables 1 and 2). All the elements of the building envelope were taken as having direct contact with external air (meaning no heat losses towards the ground). Geometric characteristics for all building elements were calculated using the overall external dimensions. Geometric characteristics of MUZA are shown in Table 1. Technical systems installed in MUZA are described in Section 3.3. Energy generated on an installed PV system was calculated according to the Croatian algorithm for determining energy requirements and efficiency of thermotechnical systems in buildings [44], adopted by the Croatian Ministry of Physical Planning, Construction and State Assets. All energy performance calculations were conducted with the assumption that the MUZA is not shaded by neighbouring buildings.  
The climates of Gospic and Zagreb Maksimir can be classified as Cfb and Cfa climates, respectively, according to the Koppen-Geiger climate classification, while on the other hand, the climates of Split Marjan and Hvar are both classified as Csa climates according to the Koppen-Geiger climate classification. In order to quantify the demand for heating energy for a building, it may be better to use heating degree days (HDD) for the observed cities. HDD's are as follows: Gospic HDD = 3486.0, Zagreb Maksimir HDD = 3045.2, Split Marjan HDD = 1437.7, Hvar HDD = 1173.0. Only days with a daily mean air temperature equal to or below 15\({}^{\circ}\)C are considered for HDD.  
In terms of energy performance, the specific heating (\(Q^{\prime\prime}{}_{H,nd}\)) and cooling energy (\(Q^{\prime\prime}{}_{C,nd}\)) needs were calculated, as well as the specific delivered energy (\(E^{\prime\prime}{}_{del}\)) and specific primary energy (\(E^{\prime\prime}{}_{prim}\)). Delivered energy (\(E^{\prime\prime}{}_{del}\)) included heating, and cooling energy, as well as energy generated on the PV system. The \(E^{\prime\prime}{}_{prim}\) was calculated using the primary energy factor of\(f_{p}=1.61\) for electrical energy.  
During the analysis, the impact of thermal bridges was evaluated, where, firstly, thermal bridges were taken into account by using the default \(U\)-value increase (\(\Delta U\)) prescribed in Croatian technical regulations [14]. This \(\Delta U\) was taken with the value \(\Delta U=0.05\) W/(m\({}^{2}\)K), which can also be found in the regulations of other EU member states for well insulated buildings. In the second variant, thermal bridges are taken into account by using the \(\psi\)-values listed in Table 6 with the respective lengths of the thermal bridges and the \(U_{eff}\)-values of the opaque envelope elements.