## 1 Introduction
Figure 2: Breakdown of the final energy consumption in (**a**) European residential buildings (data source: Odyssee [5]) and (**b**) Hellenic residential buildings (data source: ELSTAT [6]). with systems that exploit renewables will make it possible to move toward the ambitious decarbonization goals. Normative calculations are used to estimate energy savings and the abatement of carbon emissions that shape European policies and integrated national energy and climate plans. However, it has been well documented that there is a significant gap between the calculations and actual energy use in dwellings, with similar trends in various European countries like the United Kingdom, France, the Netherlands and Germany [12]. These differences may be the result of different occupant behaviors, actual equipment performance or thermophysical envelope properties, among other factors. As a result, these differences influence the analysis of available information from energy performance certificates (EPCs) and the estimation of the expected energy savings that motivate different action plans, and they help assess the cost-effectiveness of different policies. Similar results have also been observed in Greece [13]. Normative calculations, according to European standard EN 13790 and national specifications for assessing and labeling the energy performance of buildings, are performed using predefined conditions [14]. For example, the regulations prescribe specific operating periods for the heating systems in the calculations, consider that the entire dwelling is heated and set the indoor temperature at a constant temperature. These are logical assumptions, and a model can be calibrated for converging to the actual building performance. This is important prerequisite information for performing accurate energy assessment of new buildings or estimating energy savings as a result of implementing ECMs and for performing realistic technoeconomic studies. However, it can be cumbersome to assess large renovation programs and policies for a large pool of buildings or practically impossible for large portfolios or regional or national building stocks. Accordingly, the interpretation of dropping trends of energy use in the residential sector or a very low EUI for a dwelling is not by default a positive or a desirable characteristic unless the IEQ issues are properly addressed. This is one of the major concerns when looking at actual energy use data and, from that alone, trying to derive an assessment about a building's performance. Furthermore, calculations or even detailed building simulations may also predict energy consumptions that exhibit significant deviations from the actual energy consumption as a result of, for example, diverging default values from the actual operating conditions, the prevailing indoor conditions and the role of the occupants in the operation of the installations [13; 15; 16; 17]. More realistic estimates of the actual energy use can be made by gaining a better understanding of how some determinant calculation assumptions may deviate in practice. For example, occupants interact with their indoor environment and control the operation of the building's technical installations in ways that do not comply with standard or other prescribed conditions. Furthermore, the inelasticity of heating needs combined with increasing energy prices, an economic crisis or fuel poverty may significantly impact heating demand and result in adverse indoor conditions [18]. Insights on how occupants actually use the heating systems in their dwellings can support a more realistic assessment of energy consumption for space heating, which is the biggest end use in residential buildings. Apparently, there are also implications on indoor thermal comfort conditions that occupant surveys can shed light on for the interpretation of indoor conditions.