Recent studies have shown that considering the values of superficial mass (M-s) and periodic thermal transmittance (Y-mn) of external walls is not sufficient to achieve energy savings, particularly in summer. For this reason experimental reference values of the internal areal heat capacity (k(1)) were introduced. This study aims to understand the interdependency between some thermal parameters (U, M-s, phi, F-a, Y-mn, k(1)) of massive and lightweight walls with respect to their energy performance in use in office buildings in Southern Europe. The study has analyzed eight walls with Italian standard U and Y-mn values. These walls have been then modified in order to reach k(1) values corresponding to the reference ones. The energy demand to ensure a defined level of indoor thermal comfort has been verified with thermodynamic simulations on a virtual test-room localized in two Italian cities, characterized by different climate conditions. The research results are: to develop design change strategies for external walls to achieve the k1 reference values; to quantify the thermal annual energy demand of a virtual test-room equipped with the sample walls and then equipped with the improved walls; to compare the energetic and economic impact for the improved walls against the sample ones. (C) 2012 Elsevier B.V. All rights reserved.