Knowledge of an historic monument can be substantially improved by locating hidden structures, openings and the wall bonding beneath the plaster. When restoring buildings, the physical connection between the walls must be known in order to predict the risk areas for structural weakness. IR Thermography produces remarkable results, especially by means of the quantitative approach. The temperature pattern detected by thermography and analyzed in space and time maps the hidden structure of the wall. Thick walls exposed to the weather represent a challenge in detecting hidden structures by means of thermography. Frequently output is very poor because testing conditions are not optimized. Hence, appropriate testing requires careful analysis of the wall system before and after taking the thermograms. Otherwise, false alarms render the images useless. This paper describes a general procedure applied to see the hidden wall structure. It works in three steps: a) a mathematical simulation of the real test by a dedicated software, implementing the 3D thermal problem; b) a transient thermographic test, delivering a suitable heating flux on the surface for the proper time; c) processing test data, including a thermogram sequence and air temperature analysis. Here, are reported tests achieved on a XV-XVIIth century Palace at Cremona (Italy) and in the Westcott House in Springfield (OH).