Reconciling embodied and operational carbon ::a split carbon factor methodology for building insulation in Switzerland's energy transition

The decarbonisation of the electricity network is a cornerstone of Switzerland’s 2050 energy strategy. In this context, buildings represent both a challenge and an opportunity, as they account for over one-third of the final energy demand. The increasing use of heat pumps, coupled with a low-carbon electricity mix, is driving down operational carbon emissions. However, this shift introduces a paradox: if electricity is considered fully decarbonised, further insulation may seem counterproductive, as its embodied carbon might outweigh operational savings. In this context, Life Cycle Assessment (LCA) would suggest that insulation is unnecessary – a conclusion that risks undermining long-term decarbonisation efforts of energy supply. This highlights a critical research gap. Without continued insulation efforts to reduce thermal energy demand, renewable electricity production may fall short of future needs. A new allocation method is thus required to reflect the true carbon impact of excessive electricity use and preserve the relevance of energy sufficiency in a carbon-neutral future. This research develops a Swiss-adapted methodology based on the LETI “split carbon factor” method. It assigns a decarbonised carbon factor (0.016 kgCO₂-eq/kWh) to electricity consumption below a defined electricity use intensity target (19 kWh/m²·year), aligned with the 2050 renewable energy budget, and a non-decarbonised carbon factor (0.175 kgCO₂-eq/kWh) above this limit. Applied to a case study of a renovated residential building, the method emphasizes the value of improving insulation up to a given target. Beyond this threshold, further operational carbon savings remain, but with diminishing returns, supporting a more targeted strategy. It prioritizes high-energy-consuming buildings while reducing pressure on already efficient new constructions. By penalizing poorly insulated buildings, the method reconciles operational and embodied carbon accounting, bridging the gap between LCA outcomes and energy transition goals. It offers a more realistic framework for assessing building performance while supporting both design and policy development within Switzerland’s low-carbon pathway.