Existing buildings account for 40% of total green gas emission in the atmosphere [1,2]. Historic and heritage buildings are part of this building stock, and the need for improving their energy performance, despite the derogations for officially protected ones, is supported for achieving the European 2020 Energy Strategy aims [3]. However, when dealing with these buildings, not only are the intervention measures constrained by possible architectural preservation requirements, but also the preparatory building diagnosis itself: only non-destructive and non-invasive monitoring techniques are, reasonably, allowed. Therefore, during onsite energy auditing, ad-hoc monitoring protocols should be adopted. In this study, an indirect non-invasive envelope monitoring, for evaluating brick masonry hygrothermal behavior, has been proposed and applied in a heritage building in Antwerp (Belgium). The suggested method is aimed at onsite evaluating the thermal performance of buildings traditional masonry and at quantifying the extent of its alteration due to the moisture distribution variation. Areas detected as wet during iterative passive infrared thermography and environmental monitoring, showed thermal transmittance values more than three times higher than the dry ones on the same masonry surface. (C) 2015 Elsevier B.V. All rights reserved.