## 2 Building Energy Retrofitting  
### Current Status  
Despite significant efforts by most countries to promote the decarbonization of buildings, the global implementation of energy efficiency measures in existing buildings is lacking and falls far short of the necessary actions to achieve net-zero carbon dioxide emissions by 2050. This is evident from the fact that existing buildings still make up the majority, accounting for over 97% of the building stock [9]. The EU's building stock, for instance, consists of more than 220 million units, with 85% of them constructed before 2001. It is projected that around 85-95% of these buildings will still be in existence by 2050 [10]. The majority of these buildings are energy inefficient, relying on fossil fuels for heating and cooling and employing outdated technologies and wasteful appliances. Therefore, it is advisable to focus on the existing building stock by implementing comprehensive energy efficiency improvements through well-planned renovation and retrofit processes. These actions should consider the specific characteristics, specifications, and various economic, technical, and functional limitations associated with each building. Building energy renovation refers to a comprehensive process aimed at improving the energy efficiency and indoor thermal comfort of existing buildings. This is achieved by implementing cost-effective measures and techniques that target various building components, including energy production and supply systems, building envelope and constructions, building management and control strategies, as well as renewable and alternative energy units. Building energy retrofitting is a well-established and widely adopted approach that is proven to have positive technical, economic, and societal impacts. In addition to reducing energy consumption, energy losses, and carbon emissions, building energy retrofits also contribute to better indoor air quality, improved thermal comfort, increased productivity, enhanced asset value, and an overall better quality of life [11; 12]. Despite these favorable outcomes, the annual rate of energy renovation in the EU is currently low, accounting for only around 1% of the building stock per year. When it comes to deep energy renovations, the average annual rate is even lower at 0.2% in the EU [1]. At this pace, achieving carbon neutrality and decarbonizing the building stock would take centuries. To meet the EU's 2030 climate target and achieve climate neutrality by 2050, there is an urgent need to significantly increase these renovation rates by a factor of 15. Specifically, the annual rate should reach 3% by 2030 and be sustained until 2050 [13]. Therefore, it is crucial to take action and scale up efforts and innovations in this area. In line with this objective, the EU Commission introduced a strategy to initiate a Renovation Wave for Europe [14]. The aim is to at least double the annual energy renovation rate for both residential and non-residential buildings by 2030 and promote deep energy renovations. By pursuing these goals, it is projected that 35 million building units will be renovated by 2030. Over the past two decades, Denmark witnessed significant activity in energy retrofitting and renovation of its existing building stock, particularly considering that more than 75% of the current buildings were constructed before 1980. The Danish government implemented an extensive buildings renovation strategy as part of the 2012 energy agreement, titled \"Strategy for the energy renovation of the existing building stock\" [15]. This strategy comprises 21 initiatives aimed at accelerating energy-efficient and cost-effective renovation projects in Denmark, with a target of achieving a 35% reduction in heating demand by 2050. Furthermore, Denmark, in line with its ambitious goal of reducing emissions by 70% by 2030, committed to new energy-saving measures for government-owned buildings through a circular letter that came into effect on 1 January 2021 [16].