Which life cycle assessment? Managing the risk of inconsistent building assessments across regions

System boundary: The system boundary determines the processes included in assessing a building. In the context of building LCAs, the system boundary constitutes the building life cycle stages, as indicated in Figure 1.

Figure 2 shows the different system boundary requirements of each life cycle assessment in this study.

Building element groups: Each LCA method defines which building elements are required as a minimum. The variation in minimum building element groups to be included in an assessment significantly contributes to result discrepancies.

Figure 3 shows the required building elements for each LCA methodology organised into element groups based on RICS elements as used in the RICS LCA methodology.

Floor area definitions and metrics: LCA results are typically provided per unit floor area, even when not required nor defined in EN 15978:2011. It is, however, a familiar industry approach. LCA methods use a variation of gross floor area (GFA), with each country having its own GFA definition. As GFA is used to calculate a building’s carbon footprint to allow comparison with other buildings and benchmarks, its definition impacts the calculated figures. Figure 4 maps respective GFAs and their corresponding building components. We group GFA definitions into two categories: (1) those which include the external wall thickness, and (2) those that do not.

Reference study periods: As indicated in Figure 2, a reference study period (RSP) represents the temporal boundary of an LCA. RSPs determine the impact during asset use (B module), including anticipated replacement cycles. Generally, cycles vary from 50-60 years but can include 75-100 years depending on building type. RSPs are particularly important where it is standard practice to provide metrics on a per year basis.

Impact categories: EN15978 defines indicators describing environmental impact, resource use, waste categories, and output flows leaving the system. These environmental indicators are selected by established LCA calculation methods. Figure 5 shows the proposed EN 15978:2011 impact categories compared to those in the LCA methods. Two impact categories not proposed by the EN 15978 standards were added: (1) "total use of energy," and (2) "waste processing." These categories originate from the Voluntary Sustainability Class and the BREEAM standards.

Different impacts for the same type of building

Floor area definitions are similar in Denmark and Sweden, meaning the variation in results are due to differences in system boundary and building elements. The greater number of building elements and lifecycle stages included in the Danish regulations produce a higher total GWP, even when operational energy is excluded. When emissions from operational energy are included, the GWP of the Danish building is nearly three times the Swedish building.

Figure 6 compares these buildings, where the authors descope the comparison to common life cycle stages (A1-A3) and building element groups. Even with descoping, there are still outcome differences, as the Danish calculations include biogenic carbon in A1-A3. A fairer comparison would remove biogenic carbon from scope., however, it is currently impossible with the Danish environmental product declarations (EPD) data. Consequently, it is only possible to compare A1-A3 of substructures not containing any biogenic carbon elements. Despite this, significant differences persist due to differing data sets and applying the penalty value to the generic data used in the Swedish method.