As reported in the scientific literature and mentioned in building energy performance legislation, the thermal inertia of the opaque building envelope can have a significant positive impact on the reduction of summer indoor overheating, of space cooling peak load and of electricity consumption. At the early stages of the envelope design, there is a need to use simplified as well as effective parameters to correctly characterize the thermal performance of the opaque envelope under dynamic conditions. In this work, a new methodology to describe the dynamic thermal behavior of opaque components is proposed. The approach is based on the harmonic parameters specified in ISO 13786 but it also takes into account realistic boundary conditions for different orientations and solar absorptance values. New equivalent thermal parameters are determined by means of dynamic simulations. The use of these parameters to assess the conduction heat flow rate by means of a simplified model is validated through a comparison with the results of a dynamic simulation. The method presented in the article represents an improvement with respect to the parameters usually applied at the early design stage to thermally characterize the opaque envelope of the building under dynamic conditions. In fact, it allows simplified but reliable performance parameters referring to actual boundary conditions to be obtained. The application potential of the proposed methodology lies in the drawing -up of catalogues and technical standards that supply the values of the new equivalent thermal parameters for different technical solutions. (C) 2015 Elsevier Ltd. All rights reserved.