The impact of lifespan assumptions in LCA: Comparing the replacement of building parts versus building layers—A housing case study

The circular economy transition of the built environment is of high priority in the EU, a challenge even more pressing in the housing sector. Conceptualising buildings as ensembles of standardised and prefabricated products, which can be separated into both defined building parts or layers is an accepted circular design approach facilitating future replacement and reuse. Life Cycle Assessment (LCA) is a tool for achieving circularity by informing design choices based on predefined lifespans. However, there is conflicting top-down guidance about whether to assume individual lifespans for constituent components or to group these into building layers when carrying out whole building LCAs. This study reviews the latest guidance on building layers and parts according to the European Level(s) framework, ISO 20887 standard for Design for Disassembly and Adaptability, and the Shearing Layers concept. An energy efficient housing case study was used to compare organisation of the Life Cycle Inventory into separate lifespans for components and layers aligned to Shearing Layers, with lifespans defined by Level(s) Indicator 2.1. The study focussed on Module B4 replacements over a 100-year period. The findings reveal that assuming the replacement of building components as opposed to layers results in greater carbon emissions. In both cases, emissions were approximately double the amount of upfront carbon to produce the initial building. These findings demonstrate the importance of lifespan assumptions in LCA, which should be further developed. The study provides an LCA template for practitioners to organise the building inventory and apply lifespan assumptions, improving rationale behind design decisions.