The estimation of wall thermal properties through an inverse problem procedure enables to increase the reliability of the model predictions for building energy efficiency. Nevertheless, it requires to define an experimental campaign to obtain on-site observations for existing buildings. The design of experiments enables to search for the optimal measurement plan that ensure the highest precision of the parameter estimation. For on-site measurement in buildings, it seeks an answer to several questions such as number and position of sensors, period and duration of the experimental campaign. In this article, the Optimal Experiment Design (OED) methodology is applied for a multi-layer building wall to resolve the aforementioned issues.