New step-by-step retrofitting model for delivering optimum timing

•Model to deliver the optimum timing of step-by-step retrofitting activities.•Comparison of interdependency of the steps, particularly due to heating systems part- and full-load operation.•Comparison of step-by-step with single-step renovation by using the cumulated energy savings as metric.•Case study considers real-life individual building renovation roadmaps. Although the Energy Performance of Buildings Directive 2018/844/EU introduced the building renovation passport and by such proposed to consider step-by-step renovation, a literature review could not identify any explicit step-by-step retrofitting optimisation model. Therefore, the present study seeks to explore the following research questions: which indications regarding the optimum timing of renovation steps can a net present value maximising model deliver; how are model’s results impacted by the interdependency of renovation steps and by homeowner’s budget restrictions. The model relies on three pillars: homeowners’ budget restrictions; building material ageing processes; and interdependency between the retrofitting steps. Implemented as a mixed-integer linear program, it maximises the net present value of households’ energy-related cash flows, and delivers the optimum timing when each step should be performed. As input data, five real-life building renovation roadmaps were used. The appropriate metric to assess building’s retrofitting energy savings is also discussed. When comparing both single-step and step-by-step approaches, the step-by-step presented 11–22% higher cumulated energy savings. Results also show that a renovation period would last between 1 and 14 years and 2 to 11 years, depending on whether interdependency of measures is considered. This has direct implications on the improvement of building stocks’ energy efficiency, and consequently, the achievement of decarbonisation targets set for 2050. In this context, the model delivers a more concrete time horizon perspective in regards to the achievement of these targets. Future work will include quantifying the economic effects of interdependency of steps and expanding the analysis for varies techno-economic building typologies.