## 3 Traditional Calculations  
The thermal insulation of the discussed ceiling systems was described by means of three parameters: thermal resistance, heat transfer coefficient, and heat transfer resistance. \(R_{T}\) thermal resistance is defined as the quotient of the temperature difference and the heat flux density in a steady state, which in the case of a flat homogeneous layer, equals the quotient of the thickness and the thermal conductivity coefficient. The heat transfer coefficient \(U\) for a flat partition is defined as the heat flux in a steady state divided by the surface area and by the difference in ambient (fluids) temperatures on both sides of the partition. The heat transfer resistance \(R_{U}\) of a flat partition is the sum of its thermal resistance and heat transfer resistance. At the same time, this parameter is the inverse of the heat transfer coefficient. First, the thermal conductivity resistance was determined. This parameter was calculated in accordance with the principles given in the EN ISO 6946:2017 standard [28]. Since all considered ceilings are partitions consisting of thermally heterogeneous layers, \(R_{T}\) resistance was calculated from the formula:  
\[R_{T}=\frac{R_{T}^{\prime}+R_{T}^{\prime\prime}}{2}, \tag{1}\]  
where \(R_{T}^{\prime}\)--upper limit of thermal resistance: \(R_{T}^{\prime\prime}\)--lower limit of thermal resistance. This approach therefore requires the division of the partition in question into sections and layers, the sections being parallel to the direction of the heat flow and the layers perpendicular. The division of the ceiling into sections concerns the determination of the upper thermal resistance limit. This is done in such a way that the separated sections consist of homogeneous layers. On this basis, the resistance is calculated as a parallel connection of the resistance of individual sections assuming one-dimensional heat flow perpendicular to the surface of the layers:  
\[\frac{1}{R_{T}^{\prime}}=\frac{f_{1}}{R_{T1}^{\prime}}+\frac{f_{1}}{R_{T1}^{ \prime}}+\cdots+\frac{f_{n}}{R_{Tn}^{\prime}}, \tag{2}\]  
Figure 5: Cross-section of the Teriva Termo thick-ribbed ceiling. 1— prefabricated truss beam, 2—filling expanded clay and concrete hollow brick, 3—concrete laid at the construction site. \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value** \\ \hline height & 240 mm \\ rib span & 600 mm \\ transferred loads depending on the span & from 4.0 kN/m\({}^{2}\) \\ prefabricated concrete class & C20/25 \\ span up to & 7.2 m \\ fire resistance & REI 30 \\ weight & 228 kg/m\({}^{2}\) \\ \hline \end{tabular}
\end{table} 
Table 5: Basic parameters of the Teriva Termo ceiling. where \(R^{\prime}_{Ti}\)--thermal resistance of individual sections (calculated as the sum of resistance of the individual layers of a given section), \(f_{i}\) --relative surface area of a given section (calculated as the quotient of the surface area of a given section to the surface of the entire partition). The division of the ceiling into layers is associated with the calculation of the lower limit of thermal resistance. This resistance is calculated as a series connection of resistances of the individual layers:  
\[R^{\prime\prime}_{T}=R^{\prime\prime}_{T1}+R^{\prime\prime}_{T2}+\cdots+R^{ \prime\prime}_{Tn^{\prime}}, \tag{3}\]  
where \(R^{\prime\prime}_{Ti}\)--the equivalent thermal resistance of each heterogeneous layer, calculated as the parallel connection of the resistances of the elements (homogeneous sections) present in this layer. These quantities are calculated according to the formula:  
\[\frac{1}{R^{\prime\prime}_{Ti}}=\frac{f_{1}}{R^{\prime}_{T1i}}+\frac{f_{1}}{ R^{\prime}_{T1i}}+\cdots+\frac{f_{n}}{R^{\prime}_{Tni}}, \tag{4}\]  
The thermal resistance of a homogeneous ceiling element, with the exception of air layers, is described as the relationship  
\[R_{Ti}=\frac{d_{i}}{k_{i}}, \tag{5}\]  
where: \(d_{i}\)--the dimension of the partition element in the direction of heat flow, \(k_{i}\)--thermal conductivity of the material of this element.