The objective of this study is to present a validation method for simulation codes describing thermal behavior of buildings. Beginning with the classical form of comparison of theoretical and experimental results, it is proposed to improve the procedure by calculating the degree of uncertainty associated with the theoretical results. It can be shown that uncertainties associated with code input data, that is, the simulation parameters, propagate through the calculation and generate a range of uncertainty in the results. After describing the problem and objectives of the study, the computation code analysis will be presented. This concerns use of the CA-SIS (Conditionnement d'Air-Simulation de Systemes) code, and employs the TRNSYS calculation environment. Experimental validation studies for the evaluation of this code have been carried out using ETNA (Essais Thermiques Naturels ou Artificiels) cells, which have been constructed and are maintained at a site near Paris. These cells and experimental procedures are described. To determine ranges of uncertainty in the numerical results, a sensitivity analysis is first carried out by an "adjoint" method. This method and the relationship linking the uncertainty to calculated sensitivities is presented. Notably, it can be demonstrated that the adjoint method simplifies the calculation of uncertainties. The results presented focus on the cell air temperature. The experimental air temperature evolves during the climatic heating sequence within the range of uncertainty of the theoretical results. It can be deduced that the first step of validation being reached, the developer of CA-SIS code can discuss the complete validation for this configuration. The high sensitivity of the internal air temperature to the parameter of heating power shows the limitation of isothermal air volume hypothesis. From this it can be concluded that it is necessary to improve the numerical modeling of the injection heating power. (C) 2000 Elsevier Science S.A. All rights reserved.