The estimation of wall thermal properties through an inverse problem procedure enables to increase the reliability of the model predictions for building energy eciency. Nevertheless, it requires dening an experimental campaign to obtain in situ observations for existing buildings. The quality of the estimated parameter strongly depends on the quality of the experimental data used for the parameter identication. In other words, there is a close relation between the experiment design and the precision of the retrieved parameters. The design of experiments enables to search for the optimal measurement plan. It ensures the highest precision of the parameter to be estimated. For in situ measurement in buildings, the design of experiments seeks to answer the following questions: How many sensors do we need? What is the sensor position in the wall? The optimal experiment design methodology enables us to answer those questions. The unknown parameter is the thermal conductivity of wall façade modeled considering two-dimensional heat transfer induced by time and space varying boundary conditions.