It is very difficult to calculate and analyze with precision the thermal behavior of the walls of building envelope when coexist various modes of thermal transfer and because of the particularly random climatic phenomena. These problems are very complex in the case of special components such as passive solar wall used in "bio-climatic" architecture. In this paper, the thermal performances of passive solar systems, a classical Trombe wall and a composite Trombe-Michel wall, are studied. The models were developed by our cares with the finite differences method (FDM) [L. Zalewski, S. Lassue, B. Duthoit, M. Butez, Study of solar walls-validating a numerical simulation model, International Review on Building and Environment, 37 (1) (2002) 109-121], and with TRNSYS software [Solar Laboratory of energy (USA), Manuals of TRNSYS, University of Wisconsin-Madison, USA, 1994]. The model for a composite wall developed with FDM was validated by experimentation [1]. The comparisons between the results of simulation with TRNSYS [2] and with FDM, and between the results of simulation a classical Trombe wall and the results of simulation and a composite Trombe wall have been made. They show that the models developed by ourselves are very precise, and the composite wall has better energetic performances than the classical wall in cold and/or cloudy weather. (c) 2006 Elsevier B.V. All rights reserved.