This article presents the results of a research aimed at obtaining detailed knowledge on the thermal characteristics of the building for the summer performance, particularly focusing on thermal insulation level. A new methodology for the analysis of the parameters having the most effect on space cooling energy performance of a building is presented. The proposed methodology involves analysing the different contributions to the internal air convective heat balance and their interrelations with different boundary conditions. It is applied, by means of a multi-step dynamic numerical simulation, to a parametric analysis of two case studies, a residential and an office building. The contribution of the energy transmission through the building envelope to the building energy performance for cooling is compared with the contribution of the energy not transmitted through the envelope. The influence of the single parameters related to the envelope, and in particular the effect of the thermal insulation level, are investigated through the application of a sensitivity analysis. The methodology could be used in different applications: in the energy design of new buildings or in an energy audit of existing buildings, because of its potentiality to identify critical situations and to quantify the effects of improving solutions. (C) 2012 Elsevier B.V. All rights reserved.