Multi-Objective Optimization of Low-Carbon Repair-and-Retrofit Packages for Near-Zero Energy Upgrading of Existing Affordable Housing in China’s High-Altitude Cold Regions

Background: Upgrading existing housing, particularly affordable housing in China’s high-altitude cold regions, to near-zero energy standards requires balancing three key considerations: carbon reduction, life-cycle cost, and residents’ affordability. Methods: We developed a simulation-based multi-objective optimization framework to evaluate repair-and-retrofit packages involving the building envelope, ventilation, heating electrification, on-site renewables, and control strategies, subject to social feasibility and affordability constraints. Results: The Pareto-optimal solutions revealed a clear knee region in which substantial operational carbon reductions and acceptable thermal safety could be achieved at moderate investment levels. Further decarbonization was enabled by strong system-level synergies among heat recovery ventilation, heat pumps, and photovoltaic systems. Affordability-constrained optimization shifted the feasible solution space toward options associated with lower household energy burdens and more favorable distributional outcomes. Conclusions: Policy scenario analysis indicates that grid decarbonization and targeted financial support can expand the feasible space for low-carbon pathways and improve equity, thereby enabling scalable near-zero energy upgrading strategies.