A BIM-based multi-objective optimization decision-making method for green retrofit of existing public buildings

Introduction To address conflicting objectives, unclear life-cycle boundaries, and the difficulty of translating optimization outputs into implementation priorities in the retrofit of existing public buildings, this study proposes a BIM-driven multi-objective decision process. The contribution does not arise from the individual use of BIM, NSGA-II, or TOPSIS, which are established tools; rather, it lies in linking BIM-derived parameters, multi-domain performance analyses, life-cycle energy-economic-carbon objectives, Pareto screening, TOPSIS ranking, and engineering-oriented retrofit prioritization within a traceable building-level decision chain. Methods Annual comprehensive energy consumption, life-cycle cost, and life-cycle carbon emissions within the defined retrofit boundary are minimized, while code compliance, daylighting, and ventilation performance are treated as feasibility checks. The framework is demonstrated using an existing scientific research laboratory building. Results Under the original modeling assumptions, the compromise solution yields a calculated 26.7% reduction in annual comprehensive energy consumption, annual operating-cost savings of approximately CNY 0.8 million, a simple payback period of 6.25 years, and a net annual carbon reduction of approximately 850 t CO 2 , while satisfying the GB 50189–2015 benchmark adopted in the original case evaluation. A three-dimensional priority matrix of energy-saving contribution, investment intensity, and construction disturbance is further used to translate the mathematical solution into engineering decision information. Discussion The reported outcomes support comparative decision-making under consistent assumptions and should not be interpreted as a substitute for long-term measured post-retrofit performance.