## 1 Introduction
### Energy Retrofit of Danish Buildings  
Aiming to attain a green and fossil fuel-free country, the Danish government has set very ambitious and strict energy and environmental goals by 2050 [29]. Thus, in the recent decades, a large number of energy frameworks, legislations, and plans have been highlighted in order to improve the overall energy sector efficiency. Under the new Danish plan, the potential of improving the energy performance of newly built and exiting buildings was highlighted as a major milestone to reach the objectives. In this regard, energy standards for new buildings have been dramatically tightened in the recent years in terms of building physical envelope design and energy systems operation and management [30]. Figure 2 provides an overview on the evolution of the maximum allowed yearly primary energy consumption for a 150 m\({}^{2}\) Danish residential building, as set by the successive Danish building regulations from the 60's until now [31]. It is clear from Figure 2 that large energy savings could be attained with newly energy efficient buildings design and operation when compared to old buildings. In addition to new buildings, efforts have targeted exiting buildings, the largest being the \"Strategy for energy renovation of the existing building stock\" [32]. The strategy, which was issued in 2014, comprises a list of initiatives aiming to establish the route to energy-efficient buildings in tomorrow's Denmark through systematic and cost-effective retrofit projects throughout the country. This strategy was driven by the fact that around 80% of the Danish buildings were built before 1980 and the majority of these buildings have been retrofitted or will require retrofitting in the very near future. The ultimate aim of this energy retrofit strategy is to reduce energy consumption in existing buildings by 35% in 2050. Consequently, large retrofit projects were launched all around the country in the last decade, mainly in the big cities [33]. However, the current trend in retrofitting existing buildings is that building energy retrofit projects are driven by the need for a whole or part-modification and change in a certain aspect or specification of the building [34]. Therefore, it is led by a reactive decision-making and, thus, missing out on a cost-effective and energy efficient planning along with the absence of any systematic screening and selection process. For example, although additional insulation to the external walls and roofs of the building could lead to substantial heating and cooling energy savings, this solution is not a general best solution for all buildings of different types and systems [35]. In recent decades, a large block of theoretical investigations and practical applications in the field of building energy retrofit has been heavily presented in the literature, targeting different aspects, including energy policies, developed methodologies, business models, economic and environmental assessments, in addition to technical energy retrofit measures investigation, assessment, and demonstration [36; 37]. Among these, residential sector and private houses investigations have the ultimate share with few studies targeting commercial and non- residential buildings. [PERSON] et al. [38] has reported the energy retrofit projects of four non-residential buildings in Denmark, with two office buildings, cultural building, and a daycare center. The set of implemented measures were highlighted and the impact of carrying out the retrofit projects in the real case buildings was presented and evaluated. As a result, the authors reported an average of 65% savings on the overall energy consumption due to retrofitting the four buildings. [PERSON] et al. [39] targeted old Danish buildings that were built  
Figure 2: Maximum allowed annual energy demand per m\({}^{2}\) of heated floor area in a standard new 150 m\({}^{2}\) building [31]. before 1930, and the investigation concentrated on improving the building's physical envelope and components constructions.