## 2 Building Energy Retrofitting
### Building Retro-Commissioning Process  
Commissioning is a crucial process typically conducted towards the completion of building construction. At this stage, the building was fully constructed, and all the necessary subsystems were installed. The commissioning process involves a series of performance tests designed to ensure that the building and its systems meet the requirements specified during the design phase. These tests encompass various aspects, such as verifying the functionality of the subsystems and assessing the overall construction quality of the building. Given the increasing significance of energy efficiency and environmental considerations, building commissioning and performance testing became even more essential and urgent. In many developed countries, building regulations mandate manual performance testing and initial commissioning prior to the building handover. The benefits of conducting commissioning at the end of the construction stage are significant, including a smoother building start-up and improved comfort for occupants. Both building owners and managers stand to gain numerous advantages from the commissioning process [32], including:  
Figure 2: DanRETRO building energy retrofitting tool with inputs, outputs, energy retrofit measures, and evaluation results [31].  
The building is ready for immediate use.  
The indoor environment meets the desired standards.  
Operating costs are optimized, predictable, and manageable within the budget.  
The building is adaptable and resilient.  
The operations staff can efficiently operate the building.  
The documentation aligns with the tools utilized by the operations team.  
Achieving an energy-efficient building with satisfied occupants.  
Well-documented services and consumption data.  
Maximizing the potential for increasing the value of the buildings.  
Furthermore, commissioning provides contractors and consultants with the following benefits: (1) ensuring the delivery of the correct building on the first attempt; (2) meeting project deadlines; (3) documenting services and maintaining quality standards; (4) minimizing defects; and (5) reducing the number of warranty claims.  
In general, the initial commissioning of newly constructed buildings in Denmark is now considered a standard quality assurance procedure in construction projects. The Danish standard DS 3090 [33] establishes the minimum requirements for ensuring quality and proper work processes during the commissioning phase. This includes conducting performance tests on key building systems such as ventilation, cooling, heating, lighting, elevators, and other services.  
However, there is less emphasis on commissioning existing buildings after retrofitting. During retrofit projects, significant modifications are made to the building, particularly in terms of the physical envelope, energy services, energy generation and supply systems, and the building automation system. Due to the involvement of multiple teams and the multidisciplinary nature of retrofit activities, combined with inaccurate calculations and suboptimal decision-making, many retrofitted buildings often contain undetected faults and malfunctioning systems and components that become apparent only after the building is put back into operation.  
As a result, there is a lack of systematic retro-commissioning for retrofitted buildings, with insufficient post-retrofit performance monitoring and evaluation. This leads to errors, faults, and malfunctions at various levels, resulting in a performance gap between the predicted energy performance promised to customers and the actual measured energy data on-site. Recent reports on Danish buildings indicate an average gap of around 30% between estimated and actual energy consumption data [34].