Intervention Trajectories for Deep Energy Retrofit: a Comparative Assessment Across Climatic Contexts

Deep renovation of existing buildings is commonly planned according to the Fabric-First principle, with envelope improvements preceding heating-system upgrades. This study assesses whether a different ordering of the same retrofit measures can provide better intermediate energy and economic outcomes. Two cumulative trajectories are compared: a conventional Fabric-First sequence and a Low- Invasiveness-First sequence, in which system upgrades precede extensive insulation works. The analysis uses a coupled 0D–1D dynamic building–plant model, combining lumped energy balances for indoor zones and heating emitters with one-dimensional transient heat conduction through envelope components, driven by hourly Typical Meteorological Year data. The method is applied to a representative 1970s Italian apartment building in Salerno and Tolmezzo, representing mild Mediterranean and cold Alpine climates. Seasonal heating demand, operating expenditure, capital cost, and marginal cost-effectiveness are evaluated for each cumulative package. Both trajectories reach the same final configuration, but their intermediate performance differs. The Low-Invasiveness-First sequence generally yields lower operating costs at low and medium investment levels, especially in Salerno. Once the complete package is implemented, operating costs decrease by more than 70% in both locations. The results showthat retrofit sequencing can materially affect the timing and cost-effectiveness of renovation benefits.