LCA calculations for climate declarations can be utilised for decision-making during design by providing insights into the environmental impact of different design options. The availability of data depends on the project phase. As described in previous chapters, the information matures as the design proceeds. Also, the recommended source of information changes for specified building parts depending on the design phase.
At an early stage in the design, the LCA expert may need to supplement more information and make assumptions based on expertise and best available knowledge than at a later stage in the project. To ensure the robustness of a life cycle assessment, any significant data gaps may be covered with conservative assumptions for building elements. However, any assumptions made must be declared in the report. The assumptions must be based on average or generic data and be justified based on the expertise of the LCA assessor. A declaration of assumptions enables iteration of the life cycle assessment when design data becomes available. Typical situations where data gaps may exist are during the early design phase; however, once the design process advances, the assumptions will be replaced with design data.
Incremental design changes occur during the design process, which may impact and require reassessing the LCA. Incremental changes may be made directly in LCA tools. Incremental changes may also be identified from BIM. Two BIM models can be compared using, for example, BIM software, and differences in, e.g., geometry and properties can be identified. The building information model description document also enables a comparison of models and the identification of changes and differences in the models. The LCA expert is responsible for documenting the used information and assumptions made in the LCA. Documentation of any changes made to the LCA is especially important for the iterations of the LCA. Well-documented steps ease the iterations of the LCA at later phases of the project.