a b s t r a c tThis paper describes the implementation of an integrated cost optimality and environmental assessment involving alternative energy efficiency retrofit packages for a building that dates from the beginning of the 20th century. A building typical of the building stock in the centre of Coimbra (located in the central region of Portugal and recently classified as a UNESCO World Heritage Site) was used to illustrate the methodology presented. The results were also analysed for the same building in two other locations. A life-cycle (LC) model was implemented to assess different energy efficiency measures for an apartment. The economic assessment complied with European Directive 2010/31/EU. The results show that the lowest life-cycle environmental impacts were obtained for insulation thicknesses between 50 and 120 mm, which are also cost-optimal. It is also shown that insulation thicknesses of more than 80 mm do not improve energy efficiency or global cost reduction. This paper shows that, even though historic buildings in Portugal do not have to comply with building energy codes, significant energy savings can be achieved for them without changing their historic character. It was also concluded that economic and environmental costs can both be minimised by choosing the most suitable energy efficiency retrofit measures.

a b s t r a c tEuropean Union (EU) regulations aim to ensure that the energy performance of buildings meets the cost-optimality criteria for energy efficiency measures. The methodological framework proposed in EU Delegated Regulation 244 is addressed to national authorities (not investors); the optimal cost level is calculated to develop regulations applicable at domestic level. Despite the complexity and the large number of possible combinations of economically viable efficiency measures, the real options for improving energy performance available to decision makers in building retrofit can be established. Our study considers a multi-objective optimization approach to identify the minimum global cost and primary energy needs of 154,000 combinations of energy efficiency measures. The proposed model is solved by the NSGA-II multi-objective evolutionary algorithm. As a result, the cost-optimal levels and a return on investment approach are compared for a set of suitable solutions for a reference building. Eighteen combinations of retrofit measures are selected and an analysis of the influence of real options on investments is proposed. We show that a sound methodological approach to determining the advantages of this type of investment should be offered so that Member States can provide valuable information and ensure that the minimum requirements are profitable to most investors.

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The European Union has proposed a common methodology to identify energy-efficient measures with the minimum global cost throughout the lifecycle. It is known that the energy impact of a specific measure is influenced by other measures in the same package, which affect its profitability. For this reason, the cost-optimal package cannot be a simple combination of individual cost-optimal retrofitting measures. Thus, to define a cost optimal solution, the energy performance and the global cost of a large number of packages need to be calculated and analysed. To overcome the expensive computational effort of this type of study, this work proposes an expeditious procedure for selecting the optimal energy source for heating and domestic hot water (DHW) service generation as a function of the following variables: energy needs and costs, initial investment, primary energy conversion factors and efficiency of energy systems. The method was applied to a reference dwelling in the Portuguese building stock. With the expeditious cost optimality approach, once an energy system configuration has been determined, an estimation of the cost-optimal package can be established as the sum of the most profitable options for each element.