The coupling of a direct thermal calculation with an optimization algorithm to achieve the identification of the thermal characteristics of a building structure is presented in this paper. The resolution of the direct thermal calculation is based on an electric network representation, based on a numerical solution using the finite differences method. The optimization model minimizes a criterion such as « least squares » between the wished temperatures inside the building and the model respond (time domain) by an inverse iterative algorithm « Reflective Newton ». The proposed optimization model is then validated with an experimental case, a closed wooden structure with one heated side.