The performance decay of a building component is influenced by a huge variety of parameters, interacting between them and determining different results in terms of service life and performance over time. For this reason, evaluating and modeling the performance of a component requires the comprehension of the way and the measure the single factors affect it, and turning this into an analytical model is a very complex operation. Moreover, components of existing buildings are subjected to an additional incognita: the characteristics and the durability of the materials, which is hardly a certain information, and can vary between similar buildings, affecting the accuracy of the evaluation. This study, conducted according to the guidelines of ISO 15686:7 code, investigates on a common solution for interventions on existing buildings, the ETICS, surveying their current performance degree through thermographic camera tests. The aim is to appreciate the difference in the performance decay between different façades of the same building - a homogeneity that allows to estimate this parameter alone - in order to reach an analytical expression of the variation of performance decay according to the difference in the exposure. The sample is constituted by buildings of large dimensions and well-known original projects and details of successive interventions, located in the city of Naples. The data obtained with in-field measurements were subjected to statistical analysis, to allow the quantitative determination of the impact of this parameter.