## 3 State of the Art
### LCA and BIM  
The use of BIM can increase the capacity to assess life cycle environmental-related indicators of buildings, including carbon emissions and resource consumption. Digital models can reduce the effort of calculating the embodied environmental impacts of built assets, improving the sustainability assessment during the design, construction or operational stages, as shown by [PERSON], [PERSON] [34]. The integration of LCA in BIM is usually performed in one of the following ways:  
* Data management 1: BIM data is extracted and transferred to provide materials and product quantities for LCA calculations. However, as it is based on simplified calculation procedures using spreadsheets or custom tools, it is typically used for basic forms of LCA.
* Data management 2: Use of BIM software plug-ins that allow BIM-integrated analysis and the optimization of design alternatives. Such an approach facilitates continuous analysis that directly reflects impact calculation results.
* Data management 3: Specialized LCA tools are used to support BIM data integration and database management.  
IFC allows the inclusion of certain LCA data from EPDs. The element _LifecycleStage_ supports the modular approach used to structure the life cycle in ISO 21390 and EN 15804. The environmental indicators are stored in the element _PSet_Environmental ImpactIndicators_ and the functional unit, as defined in the LCA standards (ISO 14040 and ISO 14044), uses the property _FunctionalUnitReference_. A detailed description and analysis of the limitations of IFC in relation to LCA data are provided in Section 4.2.  
There are significant differences in the handling of BIM and LCA data. [PERSON] et al. [35] show that APIs provide satisfactory results on embodied carbon calculations. In addition, as BIM stores multi-dimensional information, it can be effectively used to assess and compare sustainability performance of buildings in relation to their thermal performance, as shown in [PERSON] et al. [36]. One of the main limitations for the use of LCA in BIM found in [PERSON] et al. [37] was the effort and resources required for the LCI creation and the data harmonization, and the manual input necessary to transfer data from EPDs (or other sources of data) into BIM tools.  
[PERSON] et al. [38] analyzed 7312 BIM objects and only 5.5% could be directly linked without any adaptation. Other publications stress the number of resources required for the transfer of LCA data into BIM models. Therefore, the use of machine-interpretable IFC formats will enhance the efficiency of environmental assessments of buildings and infrastructure based on digital models.  
[PERSON], [PERSON] & [PERSON] [39] provide a thorough explanation of five case studies to calculate the embodied emissions of buildings in early design phases using LCA data integrated into BIM models. It shows the significant time investment required for the interpretation of each element to incorporate any missing or incomplete information. The process of _model healing_ outlined in [39] is promising. However, errors in the results or missing data still endangers the reliability for incomplete models. Therefore, the primary approach should be to ensure that IFC can store reliable information, identify mandatory data and apply this _healing_ process only to tackle unsolved issues.