CarboSplit

Buildings are a major contributor to greenhouse gas emissions. In Switzerland, they account for approximately 40% of total energy consumption and nearly one-third of CO₂ emissions. Operational emissions need to be reduced by 90% by 2050.

A central measure is the widespread adoption of heat pumps (HP), driven by the decarbonisation of the Swiss electricity mix. However, this introduces challenges, particularly regarding the capacity of the Swiss electricity grid to absorb increased demand from building electrification while reducing its carbon footprint. Electrification of heating and mobility is generally associated with long-term greenhouse gas mitigation. For instance, electricity demand is projected to rise from 62 TWh today to 80–90 TWh by 2050 due to fossil fuel replacement in heating and transport. This allows emissions reductions from 35 MtCO₂-eq today to less than 3.3 MtCO₂-eq by 2050.

Buildings influence national GHG emissions not only through their own energy use but also by shaping the carbon intensity of the electricity grid, since demand fluctuations affect the grid’s balance between low- and high-carbon energy sources. Yet, renovation strategies tend to prioritize system upgrades (e.g., HP installation) over reducing demand via building envelope improvements. Comprehensive façade retrofits remain economically unattractive due to low energy prices and long payback periods.

Since 2010, Net Zero-Energy Buildings (NZEB) concepts have emphasized energy efficiency and renewable integration. As buildings become more efficient, embodied emissions along the construction supply chain increasingly dominate their Global Warming Potential (GWP). This underlines the need for comprehensive environmental assessments that consider both operational and embodied impacts.

Life Cycle Assessment (LCA) optimizes building performance and evaluates trade-offs. Insulation material and thickness optimization is a recurrent topic. Most LCAs assume a static electricity mix, but dynamic approaches better reflect temporal variations in carbon intensity and grid composition. Nonetheless, these studies often overlook how electricity demand affects long-term energy mix evolution. This paper explores how building LCA methods can incorporate future energy scenarios.

The study evaluates carbon impact of residential electricity consumption. The split carbon factor (SCF) method, originally proposed by LETI, is adapted to Switzerland. While LETI’s framework is UK-specific, it is adjusted for Swiss energy system, regulations, and decarbonisation targets.

The approach defines an Electricity Use Intensity target (EUItarget):

• Electricity below EUItarget → decarbonised carbon factor (CFdeca)

• Electricity above EUItarget → non-decarbonised carbon factor (CFnon-deca)

Analysis is restricted to the residential sector and considers only space heating and domestic hot water (DHW).

Case Study

A 1950s Swiss residential building with insulated brick walls, concrete roof/floor, and existing renovations. Simulated U-values: 1.2–0.1 W/(m²·K). Insulation: glass wool (low carbon) vs XPS (high carbon).

The SCF method links building insulation to electricity grid impact, improving GWP mitigation up to twice compared to static methods. It supports targeted insulation, considers embodied carbon, and is sensitive to EUItarget and CFnon-deca calibration. Limitations: assumes future grid decarbonisation, static average carbon intensity, no temporal/spatial variation. Future research: time-resolved intensity, regional scenarios, demand-side flexibility.