# Data-Driven Decision Support for Smart and Efficient Building Energy Retrofits: A Review
## 2 Building Energy Retrofitting: An Overview  
Building energy retrofitting focuses on improving energy efficiency and reducing energy consumption in existing buildings, addressing the significant energy use and greenhouse gas emissions from the building sector. This is particularly important in regions like the EU, where over 220 million building units exist, 75% of which are energy inefficient, with more than 85% expected to still be in use by 2050 [13]. Despite ongoing renovations, the process is slow, leaving many buildings reliant on outdated technologies and fossil fuels [14]. Retrofitting these structures presents opportunities for significant energy efficiency improvements and emission reductions [15]. Aligned with the EU Green Deal, the European Commission's Renovation Wave Strategy aims to double renovation rates in the next decade. This initiative targets improving energy performance in 35 million buildings by 2030, potentially creating up to 160,000 jobs [16]. It also addresses energy poverty, with 34 million Europeans unable to afford adequate heating or cooling, while enhancing health, well-being, and energy security. The energy retrofit process consists of five phases according to [PERSON] et al. [17] (Figure 1):  
1. Project Setup and Pre-Retrofit Survey: Define the project scope, engage stakeholders, and collect baseline data on building design, HVAC systems, and energy use. 2. Energy Audit and Performance Assessment: Evaluate energy consumption using tools such as thermal imaging and blower door tests to identify inefficiencies. Identification of Retrofit Options: Analyze measures using simulations and modeling to select cost-effective strategies aligned with project objectives. * Site Implementation and Commissioning: Install measures like insulation, energy-efficient systems, and renewable technologies, ensuring proper operation through testing and adjustments. * Validation and Verification: Monitor post-retrofit energy performance and compare it to pre-retrofit levels to validate savings and identify further optimization opportunities. Energy retrofitting can be categorized into three main types:  
* Envelope Retrofits: Improve insulation, windows, doors, and roofing to reduce heat loss and gain. Studies show substantial energy savings, particularly in poorly insulated buildings as shown by [PERSON] and [PERSON] [18]. * System Upgrades: Replace HVAC systems, lighting, and hot water systems with energy-efficient technologies, including renewable energy sources like solar panels. These upgrades reduce energy consumption while enhancing occupant comfort as shown by [PERSON] et al. [19]. * Operational Improvements: Implement smart controls and optimize system operations to improve efficiency without major physical changes. Examples include intelligent energy management systems and occupant behavior modifications as shown by [PERSON] et al. [20]. Energy retrofits offer numerous benefits, including lower energy consumption [21], reduced greenhouse gas emissions, enhanced indoor comfort, increased property values, and job creation. For instance, [PERSON] [12] analyzed the impact of retrofitting on three schools, highlighting significant energy savings and operational improvements as shown in Table 1.  
Figure 1: Typical energy retrofitting process adapted from [PERSON] et al. [17]. The retrofitting measures in the three schools, as discussed by [PERSON] [12], showed varied energy savings. Schools A and B achieved significant heating reductions of 15.7% and 15.4%, respectively, while School C saw lower savings of 9.3%, likely due to differences in the initial conditions or measures implemented. For electricity consumption, School B had the highest reduction at 15.0%, followed by School C at 13.2%, and School A with 2.2%, indicating a greater focus on heating improvements in School A. These results emphasize tailoring retrofitting strategies to each building's specific needs to maximize energy efficiency.