Within the environmental context, designing energy efficient buildings is crucial. Standard performance indicators evaluate the quantity of energy going through the wall. Such indicator considers the energy balance of the wall, i.e the first thermodynamic law. However, the main drawback of such approach is that it does not qualify the energy quality, which can be done by the second thermodynamic law. This paper proposed a performance indicator that both quantifies and qualifies the energy efficiency of a wall. It is based on the evaluation of the exergy destruction rate. The performance indicator has been developed for transient conditions induced by climatic variations of temperature and relative humidity and considering coupled heat and mass transfer in the wall. Calculations were carried out with experimental data obtained from a wall demonstrator under climatic conditions and comparisons with standard performance indicators were also performed. The corresponding results highlighted that the exergy loss allows a more accurate assessment of the energy performance and the influence of mass transfer on it. Indeed, the mass transfer can account for 30% in the exergy destruction.