Non-destructive thermal diagnosis of walls is a crucial theme for the renovation of old buildings, the examination of building structures and the accreditation of new buildings. Nevertheless, there are very few diagnostic methods for the characterization of thick walls. In this study, an innovative approach that combines terahertz radiation method and thermal probe method is proposed. Firstly, a method using THz (terahertz) radiation is investigated and primary experimental results are shown. This THz radiation method has a large potential to characterize the dense building materials (concrete, plaster) in a short time. However, this method is less sensitive to the light insulating materials which are totally transparent to such THz radiation, and it is also impossible to identify the structure and layer order in the case of multilayer walls. Then, based on the classical thermal probe method, three multilayer walls formed of plaster, concrete and insulation materials which are common components of building walls are considered. A short time thermal probe method with an assumption of semi-infinite boundary condition is innovatively proposed. This thermal method is able to identify the first layer of wall and characterize light insulating materials or insulating and slightly capacitive materials. Therefore, with the two methods, both the dense materials and light insulating materials can be characterized. Finally, a combined application of the two methods is discussed and analyzed. (C) 2017 Elsevier B.V. All rights reserved.