## 3 Research Methodology  
This research follows a structured methodology to integrate cost estimation with sustainability-focused strategies within a BIM environment, using the IFC data schema as shown in Figure 1. The aim is to develop a BIM-based framework that generates cost estimations across multiple retrofit scenarios, providing a structured approach for evaluating energy and cost trade-offs to support the selection of the optimal retrofitting solution. Figure 1: Overview of the research methodology. The methodology comprises three phases, as outlined below:  
1. **Phase 1--Development of the BIM model:** The first phase focuses on developing a detailed BIM model of the existing building to accurately capture its physical characteristics. The model was created in Autodesk Revit, reflecting the building's as-built condition, including its geometric dimensions, material properties, and construction details. This model was then exported as an IFC file (i.e., IFC4_ADD2_TC1-4.0.2.1), which served as the foundational dataset for subsequent analysis and cost calculations. 2. **Phase 2--Energy Modeling and Cost Integration:** The second phase involves two key steps: parametric energy retrofitting scenario modeling and the integration of cost factors. Together, these steps aim to comprehensively evaluate thermal performance and economic implications of various retrofit scenarios. Step 1--Parametric Energy Retrofitting Scenarios Modeling: This step focuses on the systematic development of multiple retrofit scenarios using parametric simulations in EnergyPlus. The process began with the selection of insulation materials, each characterized by distinct thermal conductivity values while maintaining a uniform material thickness across all scenarios. This standardization ensures the reliability of comparative analysis. The simulation framework enables a detailed assessment of energy performance, focusing on key metrics, such as annual heating and cooling energy demands. By utilizing parametric simulations, a broad range of retrofitting options can be explored. The modeling process leverages advanced simulation tools to predict the thermal behavior of the building under varying material configurations. This approach facilitates the identification of scenarios with the greatest energy-saving potential, forming a foundation for subsequent economic analysis. To structure and manage the building envelope components, this research applies the IfcBuildingSystem entity, which allows the grouping of building elements that perform related functions--such as insulation layers and facade components--within the IFC schema. By defining the building envelope as a structured system, individual components, such as insulation layers, facade elements, and HVAC systems, can be systematically categorized. The costs associated with each building component are structured using IfcCostItem entities and linked to their respective thermal properties through an IfcDocumentInformation relationship, enabling a more accurate evaluation of energy-saving potential. This linkage not only improves the clarity and management of retrofit options but also ensures that data are structured in an interoperable and queryable format, enabling a consistent and scalable cost estimation methodology. Step 2--Cost Factors Integration: This step integrates cost data considerations into the BIM model by associating each evaluated retrofit scenario and its geometric representation with corresponding cost items. The total cost includes material, installation, and operational energy costs for the retrofit scenarios. Maintenance costs are currently excluded due to their relatively negligible impact on the outputs of the proposed methodology; however, they could be integrated into future developments by leveraging the scalable structure of the IFC standard.