The large majority of the building heat losses occur through the building. Hence, the accurate evaluation of energy leakages, quantified by the thermal transmittance (U-value), is necessary, especially for energy labelling or city energy planning purposes, to foresee proper retrofit intervention and energy strategies. Among the techniques for the U-value assessment, the one that employs the quantitative infrared thermography (IRT) has spread in the last years, thanks to the possibility of easing the abovementioned processes due to reliable results, fast inspection, measurement carried out on large areas. However, a work that collects all the available techniques, explaining their weak and strength points, together with analogies and differences among the literature experiences, and which focuses on IRT, has not been carried out until now. This study starts from the common approaches for the U-value evaluation (analogies with coeval buildings, the calculation method, the in-situ measurements and the laboratory tests), with the underlying standard procedures and the most important advantages, problems, and potential sources of errors defined by the literature. Then, the IRT technique, and its development through the years, is detailed and discussed, focusing on analogies and differences among the available literature sources. Also, several recurring energy related problems, such as the detection and estimation of thermal bridging as well as the assessment of the ε-value of building materials, are shown. Finally, the qualification of IRT personnel and the perspectives in the building sector are briefly explained, to remark the need for specialized thermographers who deal with an ever evolving methodology.
