Now or later? Exploring the potential of step-by-step passive retrofitting approach in buildings

: This study presents a systematic framework for evaluating the long-term economic and thermal performance of step-by-step retrofitting strategies for existing buildings. Focusing on envelope interventions, it explores a wide range of scenarios including different levels and implementation timings of insulation upgrades for walls, roofs, and floors, along with high-performance window replacements and external shading systems. Over a 25-year planning horizon, 326 unique cases and 18 421 retrofit combinations out of them are calculated using dynamic energy simulation incorporating thermal comfort metrics and productivity gains alongside energy consumption. To assess financial viability, net present value (NPV) is calculated considering investment costs, energy savings, and work productivity due to thermal comfort. The framework integrates deterioration rates for current building envelope components along with discounting to enable more reliable decisions under constraints related to timing and budget. Findings show that a staged retrofitting approach may outperform one-step implementation in terms of total NPV, especially when thermal comfort is monetized, highlighting the importance of balancing energy savings with occupant well-being in low-energy-cost contexts. The results illustrate how the framework can support building owners in identifying cost-effective and comfort-enhancing retrofit pathways that align with long-term sustainability goals.