Evaluation of renovation strategies: cost-optimal, CO₂e optimal, or total energy optimal?

Abstract

Currently, building renovation decisions are often driven by immediate investment expenses and short-term energy cost savings, rather than long-term optimisation. The difference between the national carbon footprint (CF) methodologies and the LEVEL(s) framework methodology is significant in relation to the application of energy scenarios. Energy scenarios are created at the national level and are not harmonised. CO2e emission factor of Estonian grid electricity is among the highest in Europe and carbon footprints of Estonian buildings are typically dominated by operational energy use (B6) in Life-Cycle Assessment methodology. Therefore, carbon optimisation leads to better energy performance, than the previous cost-optimal benchmark. The application of the LEVEL(S) methodology would further emphasize the dominance of operational energy stage (B6) in Estonian results and guide towards even higher energy efficiency targets.This study shows, that CF optimal renovation solutions differ from cost-optimal solutions and that CF optimal can be considered a more relevant energy efficiency benchmark for deep renovations, instead of cost optimal. It provides an analysis of different renovation solutions for a typical apartment building renovated in 2021. These solutions were examined using three parameters: life-cycle costs, total energy demand, and CF by applying the Estonian assessment method for the CF of construction works.The results of this case study indicate that the renovation options for both CO₂e optimal end operational energy use (B6) throughout the life cycle provide similar results. The CO₂e optimal solution leads to lower building energy performance indicator level, than conditioned by cost optimal solution.The current increasing energy prices are more substantially supporting solutions with lower CF, as total energy demand is an important economic factor not only in operational energy stage (B6), but also in material production, which was also investigated in this study. The results show that, if operational energy (B6) is excluded, there are significant differences in embodied carbon between the materials used for renovation. Lower carbon footprint renovation strategy involves the usage of pre-fabricated timber frame elements, mineral wool for insulation, wooden cladding or cement fibreboard in the facade. The prevalent External Thermal Insulation Composite System (ETICS) had the highest CF value and maintenance requirements during the building’s lifetime. As the main objective of the Energy Performance of Buildings Directive is to drive greenhouse gas reductions, then the steering mechanism should be based on CO2e optimal instead of cost optimal.