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Highlights • Calculation of life cycle costs and payback periods of building intervention packages. • Calculation of financial gap between nZEB and cost optimality. • Study covers single and multi-family house buildings for 2 periods and climate zones. • Ranking of intervention packages based on cost effectiveness and energy efficiency. Abstract Nearly zero energy buildings (NZEBs) constitute one of the main pillars of the energy policy of European Union (EU). Greece has recently established a clear definition for NZEB, however the interdependence between cost-optimality and the energy performance of NZEB has not been adequately studied. The present work includes a cost effectiveness analysis among different energy efficiency interventions for the existing residential Greek building stock. To this end, the calculation of the investment as well as the operating and maintenance cost for a wide range of energy efficiency measures were carried out. All calculations were based on the methodology defined in the Directive 2010/31/EU on the energy performance of buildings, while the economic evaluation assumptions were based on the EN 15459-1 standard. The goal is the determination of the most cost effective (cost-optimal) measures that are applicable for two different building types; Single Family House (SFH) buildings and Multi-family House (MFH) apartment blocks, taking into account the period of their construction and the climate zone they belong to. Furthermore, the scope of the paper includes also the assessment of the gap between these measures and the NZEB thresholds. The analysis showed that in various cases and depending on the exact national NZEB definition, NZEBs are located beyond cost optimality. [ABSTRACT FROM AUTHOR]

• In 2017, renewable heat used in residential buildings in AT and CH was 48% and 30% • Renewable heat must be increased by 131% in AT by 2040 and 210% in CH by 2050. • Public subsidies contributed to 51% and 36% of total heat savings in AT and CH. • AT relies more on public subsidies and has a lower cost-effectiveness of policies. • CH obtains similar heat savings with eight times lesser public subsidies. Reducing heat demand and decarbonising heat supply are essential if countries have to meet net zero emission targets. This paper presents a comparative assessment of the impact of energy efficiency polices for heat demand reduction and heat decarbonisation in the buildings sector (2000–2017) in Austria (AT) and Switzerland (CH). It assesses the heat demand, heat supply, effect of different factors on changes in total heat consumption, impact of energy policies that provide public subsidies for heating and compares the effectiveness of different policies and measures. The assessment shows that renewable heat supply in residential buildings has to be increased by about 150 PJ (130% increase from 2017) in AT by 2040 and by 120 PJ (210%) in CH by 2050, which is challenging. Energy savings from improvements in energy efficiency from 2000 to 2017 are 83 PJ in AT and 97 PJ in CH, but these are largely offset by increase in activity and wealth effects thereby resulting in small changes in final heat consumption. The average (2014–2017) annual public subsidies for heating were about 2500 million Euros and 310 million CHF and contributed to 51% and 36% share of energy saving in AT and CH respectively. Both countries achieved similar energy savings, albeit following different policy approaches. Public subsidies for heating are an important policy instrument in AT but have lower cost effectiveness. On the other hand, CH minimised the use of public spending by deploying other policies and funding measures that leveraged private investment to achieve similar energy savings. [ABSTRACT FROM AUTHOR]

• Linking top-down energy efficiency indicator analysis with bottom-up policy analysis. • Explaining the contribution of energy efficiency policies in Germany and Switzerland. • Analysing the counteracting effects to energy policy of rebound and non-compliance. This paper studies the factors underlying the evolution of the energy consumption for space heating in residential buildings by linking top-down analysis, based on meso -indicators, and bottom-up analysis, based on policy evaluations. The top-down analysis (i.e. the perspective from energy statistics) allows one to separate the change in total energy use into activity level, societal factors and energy efficiency gains. The explanatory power of the resulting meso-(statistical) indicators is often limited, if the underlying factors are not examined (e.g. changes in heating levels and patterns, weather effects, cost of energy and policies regarding insulation and heating system standards). We overcome most of these drawbacks by conducting a bottom-up analysis (i.e the perspective from single policy measures), which enables us to discern the contribution of energy efficiency policies to the changes observed with the meso -analysis. We focus on space heating consumption in the residential sectors for Germany and Switzerland. A major aim of this analysis is to show the contribution of energy efficiency policies (such as thermal building regulation, subsidy programmes, fiscal measures etc.) towards the changes in this indicator. The results show that the progress in energy efficiency (both autonomous and policy induced) in both countries had the greatest effect (-776 PJ for Germany, −42 PJ for Switzerland) regarding the change in energy consumption for space heating in the period from 2000 to 2016. However, the impacts of "technical and comfort" rebounds (+436 PJ for Germany, N/A for Switzerland) and other developments such as societal changes (+316 PJ for Germany, +35.5 PJ for Switzerland) were found to compensate for a significant part of the energy efficiency gains. In both countries, it was possible to link physical energy efficiency indicators to policy evaluation, but limitations were also identified which are primarily related to data gaps. [ABSTRACT FROM AUTHOR]

• A replicable methodology for identifying reference buildings is presented. • The k -means clustering method is applied to the energy performance certificate database. • To illustrate the methodology, an EPC database including 13,701 offices is used. • Sixteen clusters and corresponding representative offices are identified. • Representative offices can be used for cost-optimal retrofitting analysis of the entire stock. A large number of buildings must be evaluated to formulate energy retrofitting policies for existing building stock. In this context, it is crucial to identify reference buildings that can effectively represent the entire stock, since such buildings can then be used to assess the individualized cost-effectiveness of retrofitting measures. This paper presents a novel approach for identifying and defining a set of reference buildings by applying the k -means clustering method to energy performance certificate databases. To this end, a four-step methodology has been envisaged. First, an energy performance certificate database is prepared and variables that have an impact on energy consumption are pre-selected. Selected data are then pre-processed. Next, the k -means clustering method is applied. Finally, the resulting cluster centroids are used to identify the closest energy performance certificates in the database, in other words, the representative buildings that will then be used for cost-optimal retrofitting analysis. The methodology is illustrated using the energy performance certificate database managed by the Catalan Institute of Energy (ICAEN), which includes a sample of 13,701 offices. Due to the large number of missing values in the database, the k -means clustering algorithm was finally performed over 6,083 energy performance certificates. Seven representative office blocks and offices in industrial buildings and nine representative offices in residential buildings were identified. The results establish the basis for supporting strategic decision-making for energy saving retrofit interventions in existing Spanish offices. [ABSTRACT FROM AUTHOR]