## 1 Introduction
The estimated total operational energy use intensity (EUI) averaged 169.5 kWh/m\({}^{2}\)[7] and has decreased over the past two decades by \(\sim\)15.5% (Table 1). The data are scaled to the European average climate in order to alleviate the influence of a cold or mild winter on increasing or reducing the heating energy consumption compared with a normal winter. The climatic corrections on the indicators are derived by correcting the space heating component of the final energy consumption using heating degree days (HDD) to correspond to normal winter conditions [8]. For example, the values in Europe range from 700 to 800 HDD in Cyprus and Malta and up to 4000-5000 HDD in the Nordic and Baltic  
Figure 1: Annual energy uses in the European Union (EU-27). Data source: Eurostat [1]. countries, averaging -2800 HDD for the normal EU-27 climate. The EUIs for space heating have decreased by 26.5% (Table 1) from 148.4 kWh/m\({}^{2}\) in 2000 down to 109.0 kWh/m\({}^{2}\) in 2018 as a result of building and equipment energy efficiency improvements and different living standards, among other factors. Considering the divergence of climate conditions across Europe, it is more relevant to use normalized indicators to compare data from different countries that have different space heating demands. Since 2000, energy efficiency progress has been significant in the residential sector according to the ODEX index [8], which aggregates unit consumption changes. The ODEX for the residential sector is calculated as a weighted average of unit consumption trends at the level of the different end uses for households. Since 2000, the European residential sector achieved the largest energy gains among all the different sectors [9], averaging 1.5% per year to reach a technical energy efficiency index of 72.5% in 2018 (Table 1). On the other hand, the share of the population at risk of poverty or social exclusion is 21.9%, resulting in 7.4% of the inhabitants being unable to keep their homes adequately warm [10]. In this direction, the installation of thermal insulation on the opaque building elements is among the popular energy conservation measures (ECMs) to reduce heating energy demand [11]. As buildings move into the era of electrification, efforts will focus on further reducing the primary energy use by moving toward improving the energy mix with more renewables for green electricity generation. This will further lower the primary energy factor (PEF), which currently averages \(\sim\)2.3 in the EU-27 and is calculated by dividing the primary energy demand of electricity generation by the electricity delivered to final uses, including transmission and distribution losses. In the building sector, having placed the foundations of nZEBs for new buildings, the next big challenge is renovating the existing building stock. The priority is serving the well-being of occupants and improving the indoor environmental quality (IEQ) during building renovations while improving the energy performance of existing buildings. Minimizing the loads for heating, cooling, ventilation, lighting and appliances, using energy efficient equipment to serve the low demands and coupling \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Households**} & \multirow{2}{*}{\begin{tabular}{c} **Total Energy Use** \\ **(kWh/m\({}^{2}\))** \\ \end{tabular} } & \multicolumn{2}{c}{\
\begin{tabular}{c} **Space Heating** \\ **Energy Use** \\ **(kWh/m\({}^{2}\))** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **ODEX Index** \\ **(\%)** \\ \end{tabular} } \\ \hline  & **2000** & **2018** & **2000** & **2018** & **2000** & **2018** \\ \hline European (EU average climate) & 214.1 & 169.5 & 148.4 & 109.0 & 100 & 72.5 \\ Hellenic (normal climate) & 164.8 & 133.3 & 114.8 & 87.8 & 100 & 71.3 \\ Hellenic (EU average climate) & 261.0 & 201.0 & 206.9 & 158.3 & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Key indicators for final energy use and efficiency progress in residential buildings at normal EU and national climate conditions [7].