BIM-based building LCA is here defined as a process where BIM provides adequate information on quantities to enable the formulation of a list of materials/products. This information is then linked with the emission data. The main benefit of BIM for normative LCA is reducing manual work in gathering correct quantities. This is one of the most time-consuming tasks when performing an LCA calculation today. However, inaccuracy in quantity take-off is still an issue and may affect the LCA outcome. The information content and identification of objects and materials in BIMs are not standardised, and therefore, project-specific knowledge is required in interpreting model data and mapping correct emission data to perform LCA reliably. Interviewees considered this a major constraint for BIM-based normative LCA.
No unified system exists for the classification of BIM objects, which poses a challenge when computing quantities for the LCA and mapping emission factors to the objects. This work would be more efficient when the object classification was recorded in the models in a standardised manner. A challenge is also that few experts know BIM and LCA, making it harder to understand each other’s needs and constraints.
In addition to time savings, the risks of human errors could be mitigated by applying documented BIM to LCA pipelines. Reducing the amount of manual labour in LCA would also make it easier to compare different design alternatives concerning the building design's carbon emissions. BIM-based LCA enables following the life cycle effects of design decisions in the design process, from rough estimations to more refined designs. This may increase the understanding of LCA amongst the other parties involved in a building project, thus resulting in more climate-aware decisions.
BIM models may encompass the majority of building components required for conducting an LCA calculation, but they may also feature components not within the scope of LCA. The LCA expert needs to consider the LCA scope when performing calculations from BIM-based material take-off lists. BIM checking and coordination activities are already present in the design process, and quality control is also essential to the LCA performers.
Conversely, certain components are typically not included in the early design stages or may be absent after detailed design, for example, screed, coatings, fittings, and smaller pipes. In different projects, reinforcement may or may not be included in the model; typically, only the concrete structure is modelled. The LCA specialist must acknowledge the components not included in the BIM and make relevant assumptions.
The general calculation rules for building LCA set requirements for the system boundaries, life-cycle modules, reporting units, the result's accuracy, and the use of data sources. These demands vary between the Nordic countries and Estonia. In addition to the general rules, countries have varying guidelines for normative building LCA reporting, such as the life-cycle phase when the building LCA report is required (See section Nordic harmonisation of building LCA – constraints and enablers). All of these affect the requirements for the BIM modelling process. For example, Finland will require a climate declaration (building carbon footprint and handprint) in the building permit phase. At that phase, only the architectural model exists with a certain level of detail; typically, specific products are not selected. Thus, the building LCA calculation is to a great extent based on the general emission values using the national co2data.fi emission database. Surface materials are also not yet specified at this point; thus, assumptions about them are made for the LCA.
A BIM-based LCA necessitates the technical integration of product data – product ID or type and quantity – and emission data in the right format. However, the calculation of product-related emissions is only part of the building's entire carbon footprint. In addition, information on energy consumption during use (B6) and energy sources is needed to include the emissions caused by them. These data are not retrieved from BIM and may not be available in machine-readable format, which challenges the automation of building LCA.
When calculating product-related emissions, information about product lifespan is also needed, in addition to quantities and emission data to calculate the replacement of parts (B4) during the assumed 50-year calculation period. In addition, emissions from services and systems, such as transportation, construction operations, deconstruction, and building services systems, are at present calculated during the building permit phase based on general assumptions, such as building type and size.
Some emission databases, like Finnish CO2data.fi, contain some generic values to calculate some modules (e.g., A4-A5) or some systems (e.g., MEP) based on the type of the building and gross or heated area of the building. This enables calculation during early phases, e.g., building permitting phase, when details and systems are not yet modelled. Current practice allows for the building floor areas to be modelled as spaces in the BIM, but the modelling conventions are not harmonised, thus, human interaction is required if this data from the model is employed in the LCA.
The accuracy of BIM-based LCA depends on the level of detail of the models available, i.e., the phase of the construction project and the purposes of the available models. The normative LCA in Nordics and Estonia is done at least during the building permitting phase, handover phase or both. The models and datasets available differ quite significantly between these phases. Commercial building products are rarely specified at the permitting phase, and emission data need to be based on nationally accepted databases or generic EPDs. For structures and systems that are not included in the design models, it is up to the LCA specialist to make correct assumptions on their quantities.
The conventions on BIM usage for normative LCA in the handover phase vary. It is not typical that information on installed products is updated in as-built models even though the model may have been updated to match, e.g., the location of installations in geometry. LCA calculation in the handover phase may rely on contractors’ information collection of installed products and quantities and manual gathering of EPDs and other reference data sources. The EPD databases may not include the same product identifiers, such as GTIN code, that are used in contractor’s purchasing and logistics, and therefore, linking EPDs to collected information on installed equipment may require manual steps. Many products do not still have EPDs, and the products may be project-specific engineered-to-order (ETO) products. In these cases, generic emission data is still employed.