This article reports on a methodology to determine the level of thermal insulation in old buildings through spot measurements of thermal resistance and planar infrared thermography. An old building due for renovation is used as a test case and the two methods are combined in order to obtain a total view of the building envelope. The existence of the thermal leaks on the building shell are documented and are quantified through the synchronous use of the results from spot measurements and infrared thermography analysis. The methodology proposed proves to be reliable and can also be used for non-destructive tests of the thermal characteristics of new buildings, or old buildings after renovation.