The Energy Performance of Buildings Directive (EPBD) recast sets out for all Member States the establishment of a comparative methodology framework to calculate cost-optimal levels of minimum energy performance requirements for buildings and building elements. In recent studies, the cost-optimal energy performance of buildings has been calculated in line with the EU's Directive, despite the calculation methodology there mentioned being only a framework that provides a general approach for national calculation methods. This paper defines the parameters needed for the EPBD phasing methodology. The results obtained from the thermal rehabilitation of the building envelope of a Portuguese residential reference building, which are conditioned by the reference building characteristics and by Lisbon's climatic conditions, make it possible to identify the best cost-efficient thermal rehabilitation measures. Conclusions on cost-efficient thermal rehabilitation are as follows: (i) the thermal rehabilitation of the roof produces the greatest variation in the primary energy building consumption (and the floor measures the smallest), (ii) the combination of thermal envelope rehabilitation measures creates synergy effects that lead to better results than single measures (regarding global costs and primary energy consumption), and (iii) it is more advantageous to proceed with a thermal rehabilitation package of measures rather than doing nothing. (C) 2015 Elsevier B.V. All rights reserved.