## 4 Research Methodology
### Calculation of Effective Thermal Transmittance  
The building's thermal envelope may have thermally homogeneous or heterogeneous layers, and the accurate estimation of heat losses of structures with studs and thermal insulation is of vital importance for reliable calculation of the building's energy needs [20]. The European standard EN ISO 14683 [36] does not consider the contribution of thermal bridges in LSF walls, making it impossible to estimate the contribution of thermal bridges to the heat losses using the tabulated values. Even though the EN ISO 6946 [34] standard provides a simplified method for calculating thermal resistance of a building component consisting of homogeneous and heterogeneous layers, the situations with steel studs cannot be calculated using this simplified method, due to the significant difference between the upper and lower resistance values [16], and only a numerical calculation is admissible [37]. Approaches to an estimation of thermal transmittance (\(U\)-value) or thermal resistance (\(R\)-value) of heterogeneous layers are described in detail in [20], where the analytical method according to EN ISO 6946 [34], or its adaptation of the method by [PERSON] [38], can be used. Furthermore, ASHRAE Zone Method [39] and its numerical approach could be used to assess these thermal bridges [35] using 2D or 3D models for point thermal bridges (i.e., joints between horizontal and vertical elements, etc.). In addition to the evaluation of heat losses, critical temperatures in the critical positions can also be checked using 2D and/or 3D numerical models. In this paper, two-dimensional (2D) models are used to perform a calculation of the effective \(U\)-value of the MUZA's opaque elements, which represent its characteristic thermal envelope components with integrated steel studs. Four cases, as shown in Table 5 and Figure 9, were analysed in total--walls with steel studs spacing between 60 cm and 30 cm, the roof with steel studs spacing of 48 cm, and the floor with steel studs spacing of 38 cm. Steel stud spacings are defined based on static calculations. The \(U\)-values predicted by the numerical calculations were calculated by employing the approach described in EN ISO 10211-1 [35]. Cross-sections of the representative wall, floor, and roof are shown in Figures 6-9. The layers and their properties are shown in Figure 6 and Table 2, respectively. Each wall, floor, and roof comprises a steel structure containing cold-formed C-shaped steel studs (150 \(\times\) 50 \(\times\) 20 \(\times\) 1.5 mm) welded to a steel frame which is formed by rectangular hollow steel sections (180 \(\times\) 100 \(\times\) 5 mm). Two-dimensional numerical models resulted in the thermal transmittance of the overall structure, and an effective \(U\)-value (\(U_{\mathit{eff}}\)). \(U_{\mathit{eff}}\)[35] was derived from the two-dimensional thermal coupling coefficient \(L_{\mathit{2D}}\) [W/m\(\cdot\)K], which essentially represents the overall heat transferred through the building element. If one determines the \(L_{\mathit{2D}}\) from the numerical model according to the [35] and divides it with the length of the geometrical model \(l\), one can derive the \(U_{\mathit{eff}}\) value (Equation (1)). \[U_{\mathit{eff}}=\frac{L_{\mathit{2D}}}{l} \tag{1}\]  
where the \(L_{\mathit{2D}}\) can be calculated as shown in Equation (2) [35], where \(\phi\) [W/m] is the heat flow rate resulting from the 2D calculation, and \(T_{i}\) and \(T_{e}\) are the internal and external environment temperature, respectively. \[L_{\mathit{2D}}=\frac{\phi}{(T_{i}-T_{e})} \tag{2}\]  
The difference between the effective thermal transmission \(U_{\mathit{eff}}\) and the one-directional \(U\)-value is calculated according to Equation (3):  
\[\Delta U=\frac{U_{\mathit{eff}}-U}{U}\cdot\] 100 Here, \(U\) is the thermal transmittance calculated according to the EN ISO 6946 [34] (Equation (4)).