The thermal behaviour of a building is often underestimated or neglected during its construction and operation stages. In recent years, the heat flux meter (HFM) method has been commonly used to determine the U-value, a key parameter for assessing the thermal quality of the building envelope in steady-state conditions. However, this non-invasive test takes at least 72 h to execute, the accuracy is 14-28%, and it is not reliable for non-homogeneous building elements. An alternative technique is based on infrared thermography (IRT). Although it is generally used for qualitative analysis, quantitative internal IRT methods may also be adopted for in-situ measurement of the U-value. This research presents a method for determining in-situ U-values using quantitative internal IRT with a deviation of 1-2% for single-leaf walls and 3-4% for multi-leaf walls. It takes 2-3 h and can be used to provide information about the building envelope for the future refurbishment of existing buildings or to check the thermal behaviour of new building facades according to their design parameters. (C) 2017 Elsevier B.V. All rights reserved.