The Energy Performance Certification system of existing residential buildings in Belgium includes clear procedures describing how the experts can determine the necessary input data. In general, these input data are directly observed in the building. For some data, this observation can be impossible. An exhaustive list of alternative sources of information has been defined for those input data that cannot be directly observed during the building visit. This fact sheet describes this concept, focusing on transmission characteristics of the building envelope, and presents examples of decision trees that the experts have to follow to determine EPC input data in order to find a good balance between their accuracy and their reproducibility.