## 2 Theoretical Background
### Interoperability Strategies in BIM2 BEM  
Current approaches to interoperability (Figure 3) fall into two basic categories:  
**Technology-based approaches**  
* **Integrated**: Tightly coupled tools (e.g., BIM-embedded BEM plugins) minimize geometry errors but limit flexibility [20].
* **Central**: Open formats like IFC/gbXML enable cross-tool compatibility but risk data loss during conversion [21; 22].
* **Distributed**: Visual programming languages (e.g., Grasshopper/Dynamo) bridge BIM and BEM engines (EnergyPlus and Radiance) but require technical expertise [20].  
Figure 3: Scheme of BIM2 BEM interoperability approaches (the blue and pink boxes indicate the approaches in which the work from the two projects is categorized).  
Since IFC cannot accommodate all necessary data exchanges, there are several methods available for updating and extending missing entities. [PERSON] et al. (2015) [23] describe potential methods for extending IFC, as detailed in Figure 4.  
**Process-Driven Frameworks**  
Standardization initiatives like ISO 19650-1 [16] and IDS (Information Delivery Specifications) address workflow fragmentation by [24]:  
* Defining **Exchange Requirements (ER)**: Specifying data needs per project phase.
* Structuring **Information Delivery Manuals (IDM)**: Aligning processes with stakeholder roles.
* Enforcing **IDS Compliance**: Validating data quality and completeness.  
All BIM-based BEM approaches have their strengths and weaknesses, but many struggle to optimize the design process during the early stages of a project.  
The following table provides an overview of issues identified in various studies [20; 25; 26; 27], along with potential solutions to these challenges.  
In the BIM2 IndiLight and BIM2 BEM-Flow projects, a combined approach was adopted, and the methods used are highlighted in Figure 3 (blue and pink). By using a combined approach, the following challenges are addressed:  
1. Repetitive Project/Model Preparation: * Success requires extensive upfront project/model preparation, but evolving design requirements and boundary conditions force teams to repeat this process for each new project.
2. BEM Tools Lack User-Friendliness: * Building energy simulation (BEM) tools are perceived as complex and inaccessible to non-experts, hindering their adoption in integrated workflows. * Their functions often misalign with architects'/planners' needs, creating friction in collaborative design processes.
3. Time-Consuming and Redundant Workflows: * BEM tools typically require manual creation of separate energy models, adding time and effort to the planning process.
4. Integration Challenges: * The disconnect between BEM tools and integrated planning workflows limits their practical utility, especially in early design stages where rapid iteration is critical.  
Figure 4: Approaches for model enrichment [23].  
The preparation and post-processing of the digital asset, as well as the conversion and adaptation of BEM formats (e.g., converting formats, adjusting IFC files, and adding missing data in specific BEM tools), require a deep understanding of building physics and the entire process from early-stage to detailed design. Standardization, which reduces individual procedures while still supporting the overall workflow, can help to resolve some of this complexity. Standards such as ISO 17412-1 [28], ISO 29481-1 [29], and ISO 19650-1 [16] aim to address two major challenges: simplifying the BIM process and providing precise definitions of exchange criteria. These standards help to ensure that BIM processes and data transfers are well-structured and clearly defined, resulting in more effective project workflows.