Towards circular life cycle assessment for the built environment: A comparison of allocation approaches

Introduction. The building sector consumes 40% of resources globally, produces 40% of global waste and 33% of greenhouse gas emissions. The transition towards a Circular Economy (CE) in the built environment is vital to achieve the Sustainable Development Goals (SDGs) of climate action and responsible consumption and production. Metrics are needed to support this transition; previous work identified Life Cycle Assessment (LCA) as the best method to analyse the environmental performance of the CE. However, current LCA methods focus on the individual system, considering a single lifecycle. Circular assessment requires a systems-perspective: buildings, components, parts and materials have multiple lifecycles. Thus, questions arise about how benefits and burdens should be allocated between systems.

Method. In this study, we compare the potential influence of applying different allocation approaches in LCA method. We calculate the environmental impacts of two 'circular building components': (1) a concrete column designed for direct reuse and (2) a recyclable roof felt. We applied four allocation approaches: (a) the cut-off approach stated in EN 15804/15978, (b) the Circular Footprint Formula (CFF) from the Product Environmental Footprint (PEF), (c) the 50:50 approach, and (d) the linearly degressive (LD) approach.

Results. The allocation approaches resulted in notable differences in impact distributions thereby incentivising different CE principles (narrowing, slowing and closing). Due to the long lifespan of building components, concerns regarding uncertainty and 'green-washing' resulting from allocation of impacts between cycles arise. However, the LD approach was, for closed-loop systems (such as the ones assessed here), found to be promising: it is simple to use; it creates incentives for narrowing, slowing and closing loops and to design for these in the future; it deals with the uncertainty, material quality and number of use cycles.

Conclusion. The comparison of allocation approaches and first recommendation on an allocation approach provides an important step towards circular life cycle assessment and, subsequently, helps promote the CE concept within the building industry.