Scenario-based robustness assessment for low-emission building retrofit

The building sector is responsible for over one-third of global GHG emissions and, therefore, highly contributes to the state of climate emergency. Environmentally assessing building retrofit strategies based on greenhouse gas emissions is hence crucial to minimize global warming. However, the long lifetime of buildings, combined with the ongoing transformation, introduces a high amount of uncertainty in the current analysis practice. In particular, future developments such as global warming and energy decarbonization are highly uncertain since they depend on political decisions and societal developments. A suitable method to deal with decision-making under uncertainties is robustness assessment. Supporting building retrofit decision-making with robustness analysis enables the identification of retrofit designs whose environmental performance is only minorly adversely affected by changing future conditions. Previous studies investigated probabilistic and non-probabilistic robustness approaches on energy demand, occupant comfort, peak power, and operational emissions. Other studies focused on the life cycle assessment, including operational and embodied emissions without robustness considerations. However, few studies have investigated the robustness of building retrofit decisions based on total emissions, including the operational and embodied fraction. In my thesis, I developed a multidisciplinary simulation model connecting scenario-based decision making, building energy simulation, and life cycle assessment to assess the robustness of building retrofit decisions. Various future scenarios are compiled for the scenario-based robustness assessment, and multiple robustness metrics such as minimax regret, Laplace’s principle of insufficient reasoning, Starr’s domain criterion, and others have been applied. The model granularity of the building energy simulation is chosen to capture the relevant processes while allowing a fast simulation of many design options under many future scenarios. The GHG emission calculation combines the energy simulation results and the materials used for the construction or retrofit measure. Given the climate emergency and the nature of robust decisions, multiple modeling assumptions for the GHG calculation have been compared and reassessed. Ultimately, the robustness workflow is applied to case study buildings in Switzerland and typical buildings in different European contexts to demonstrate its value. The results show that the calculation of total GHG emissions is highly affected by future scenarios like global warming and decarbonization pathways, highlighting the importance of robustness assessments. Similarly, modeling assumptions such as the considered analysis period, the environmental allocation of locally produced electricity, and the allocation of biogenic carbon can affect the ranking of retrofit strategies. Comparing the robustness of typical retrofit strategies for typical buildings in different European contexts, I found that the main context-specific parameter affecting the ranking of retrofit measures is the current grid electricity mix. At the same time, the heating system choice is the most important decision when aiming for an environmentally robust retrofit strategy. For most contexts, heat pumps or wood combustion perform robustly in terms of GHG emissions. The results further show that increasing decarbonization and potentially shorter analysis periods lead to a higher prioritization of embodied emissions, shifting the construction materials into the focus. While this is relevant in the Swiss context, for most of the investigated European contexts, the main issue for retrofit decisions will continue to be the operational emissions. This is mainly due to the still very high GHG emission electricity grid intensity of most European countries and stresses the importance of fast and strong decarbonization of the electricity grid as an enabler of the building sector decarbonization besides the reduction of energy demand. Based on my findings, I advise that in the current time of transition, methods assessing the environmental performance of buildings and building retrofits should be adapted to include expected future developments and their uncertainties. The presented methodology and application examples demonstrate a possible way to do so, and I hope they serve well as inspiration and basis for future building assessment methods.