IR thermography allows to identify the thermal anomalies due to moisture in ancient walls. Wet zones can appear warmer or colder in IR images, according to the atmospheric conditions during the scanning; furthermore, thermal monitoring, even in qualitative thermography, allows to obtain a more effective diagnosis of the defects because it records thermal behaviors of the material in different environmental conditions. Thermographic system allows an accurate analysis of transpiration effects on buildings and precise measurements of water content starting from environmental temperature, relative balance and wind speed. These variables play a major role in the causes of damages in buildings. Particularly, the evaluation of transpiration is essential to determine the evaporative rate of water content within the wall. The research has been carried out on two ancient buildings during a period of several months. The main experimental tests were on the church of 'Guardia di Sotto', Corsico, a seventeenth century building on the bank of Pavese Canal. Five thermal scanning have been disposed in different seasons from March 14, 1996 to June 16, 1997. The causes of the wet zones were identified at the basis of the walls were rising damp and rain spread in the ground. The repeated thermographies and thermo-hygrometric test allowed to distinguish the size and the location of the areas damaged by the different causes. In other cases studied - Addolorate Church, Gessate the thermal scanning and the other supporting tests confirmed the list of optimal environmental condition required to detect humidity in walls by thermography.