## 1 Introduction  
Primary energy consumption in the European Union (EU) of the 27 member states (EU-27) has reduced by 1.1% since 1990, the first year for which official data are available [1], reaching 1351.9 million tonnes of oil equivalent (Mtoe) in 2019 (Figure 1). During the same period, the final energy consumption in the EU-27 increased by 3.2%. In recent years, the highest final energy consumption was recorded in 2006 (990.1 Mtoe). The annual fluctuations are mostly attributed to the prevailing weather conditions (e.g., a sharp increase in heating demand during the cold winter in 2010 or the decrease during the mild winters in 2013 and 2014). Other major influencing factors that drive some of the dropping trends are attributed to the reduced economic activity during the periods of the financial and economic crises (e.g., 2008-2009, 2011-2014) and in part to the improvements in energy efficiency due to new policies. Over the years, the building sector has been dominating the overall final energy balance, increasing from 36.9% in 1990 to 40.1% in 2019. In particular, the residential sector prevails in the existing building stock, representing over 26% of the EU-27 final energy use in 2019 [1]. For this reason, energy efficiency has been and remains the first priority of EU policies moving toward the next challenge of a decarbonized EU building stock and economy by 2050. To support these policies, various European directives are paving the way. For example, the Energy Performance of Buildings Directive (EPBD) [2] that was first introduced in 2002 and is already under its third revision. According to the EPBD, all new constructions are nearly zero energy buildings (nZEB) as of 2021, with an emphasis shifting to the renovation wave of existing buildings, life cycle assessment, embodied energy and smart readiness among other factors. The Energy Efficiency Directive (EED) [3], which was first introduced in 2012, established a set of binding measures to help the EU reach its 20% energy efficiency target by 2020, preparing aspects such as national energy efficiency action plans and minimum energy efficiency standards and labeling. The amended EED in 2018 defined an energy efficiency target for 2030 of at least 32.5% to be achieved collectively for all countries. The savings are set relative to the 2007 modeling projections for 2030. In absolute terms, this translates into a primary energy consumption of no more than 1128 Mtoe for the EU-27 and a final energy consumption of no more than 846 Mtoe in 2030 (Figure 1). Considering the unprecedented events of 2020 in relation to the COVID-19 pandemic and the overall slowdown of European economic activities, it will most likely be possible to finally meet both the primary and final energy targets for 2020. However, given the current trends, it will be very challenging to meet the 2030 targets. The dotted lines in Figure 1 represent a linear trajectory between the year with the highest energy consumption (2006) and the 2030 targets. Under the EED, the EU member states will have to achieve new annual energy savings of 0.8% of the final energy consumption for the period of 2021-2030, except for Cyprus and Malta, which will have to comply with 0.24% annual savings. Details for each EU member state are outlined in the 10-year national energy and climate plans (NECPs). In order to properly prioritize the energy efficiency measures, one needs to consider the grim reality of the European building stock. About 75% of the buildings are energy inefficient, and less than 1% of the buildings are renovated every year, while new buildings in compliance with the EPBD mandates represent less than 8% of the building stock [4]. Residential buildings use 246 Mtoe or over 26% of the EU-27 total final energy consumption [1]. Looking at the breakdown of the annual energy used in the residential sector, the main energy end use is space heating, which represents \(\sim\)63% of the total (Figure 2a).