## 4 Conclusions  
An accurate sky temperature used in CFD simulations or another buildings energy assessment program (e.g., EnergyPlus) is necessary to reflect the real performance of the buildings. Otherwise, this may lead to incorrect assessment of the existing or new buildings' thermal performance, operating energy, and GHG emission over the entire life-span.  
In this paper, the effect of the different sky temperatures on buildings was compared between the real data for four real-sized test buildings and the CFD simulation results. The monthly sky  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **CB** & **InsCB** & **InsBV** & **InsRBV** \\ \hline Higher sky temperature by +10 \({}^{\circ}\)C & 16.5\% & 17.5\% & 17.1\% & 16.8\% \\ \hline Higher sky temperature by +5 \({}^{\circ}\)C & 10.2\% & 10.4\% & 9.4\% & 9.3\% \\ \hline Higher sky temperature by +2 \({}^{\circ}\)C & 7.2\% & 7.4\% & 6.9\% & 6.8\% \\ \hline Exact sky temperature & 6.5\% & 7.1\% & 6.2\% & 6.4\% \\ \hline Lower sky temperature by \(-2\) \({}^{\circ}\)C & 8.3\% & 9.1\% & 8.7\% & 8.9\% \\ \hline Lower sky temperature by \(-5\) \({}^{\circ}\)C & 13.3\% & 15.1\% & 14.7\% & 13.9\% \\ \hline Lower sky temperature by \(-10\) \({}^{\circ}\)C & 19.3\% & 22.6\% & 21.9\% & 19.1\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Errors between CFD simulations and real data using different sky temperatures.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Month** & **CFD Average (\({}^{\circ}\)C)** & **Calculated (\({}^{\circ}\)C)** & **Difference (\({}^{\circ}\)C)** \\ \hline January & 0 & 0.6 & 0.6 \\ \hline February & \(-1\) & \(-1.1\) & 0.1 \\ \hline March & \(-2\) & \(-2.3\) & 0.3 \\ \hline April & \(-3\) & \(-3.7\) & 0.7 \\ \hline May & \(-4.5\) & \(-5.3\) & 0.8 \\ \hline June & \(-5.5\) & \(-6.6\) & 1.1 \\ \hline July & \(-5.5\) & \(-6.6\) & 1.1 \\ \hline August & \(-4.5\) & \(-5.3\) & 0.8 \\ \hline September & \(-3\) & \(-3.7\) & 0.7 \\ \hline October & \(-2\) & \(-2.3\) & 0.3 \\ \hline November & \(-1\) & \(-1.1\) & 0.1 \\ \hline December & 0 & 0.6 & 0.6 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Difference in the sky temperatures between CFD analysis and calculated values.  
temperature was determined by two approaches; i.e., numerically and using CFD simulation with less than 1.1 \({}^{\circ}\)C difference. The sky temperature varies throughout the year but it was found that monthly sky temperature provides a reasonable degree of accuracy for building thermal simulation. It seems to be unnecessary to input a weekly or even daily sky temperature which would require 52,365 separate calculations.  
It was found that inaccurate sky temperature (10 \({}^{\circ}\)C higher or lower than the exact sky temperature) will lead to an average error of more than 20% (3 times more compared with the accurate value for the sky temperature) of internal air temperature for a building. However, the error decreases to about 6% (mainly due to simulations errors) when the exact temperature is applied.  
Conceptualization, D.A.; Formal analysis, A.A.; Investigation, A.A. Project administration, B.M.; Supervision, A.P. and B.M.; Writing--original draft, A.A.; Writing--review & editing, A.P. All authors have read and agreed to the published version of the manuscript. This research received no external funding. The authors declare no conflict of interest.