© Published under licence by IOP Publishing Ltd. One of the key parameters that "meter" the energy performance of the whole structure of buildings is the thermal transmittance. This parameter can be evaluated with a theoretical approach, regulated by standard ISO 6946, once the stratigraphy of the envelope and the properties of the constituent materials are known, or by using a heat flow meter (HFM), following the recommendations provided in standard ISO 9869. Recently, the use of quantitative IR Thermography (IRT) has been proposed by several researchers; this method allows to determine the overall transmittance of an envelope in a short time (especially in comparison with HFM method). However, the theoretical or experimental transmittance, measured on real buildings having walls composed by different materials, can be rather distant from those calculated or measured with different procedures. For this reason, for a correct certification of the thermal performance of a building envelope, it is necessary the availability of experimental procedures for a direct and reliable evaluation of the thermal transmittance, suitable for different walls. Research has found that, especially in historical constructions, faults in the building envelope and the age of the materials can greatly affect the HFM measurements. The aim of this work is to analyze in situ the thermal performance of three different walls which have been selected according to: different materials, different age, and different construction. On each envelope, a comparison between U-values, measured by HFM and IRT, and computed according the standard procedure, has been effected.