The use of insulating glass units (IGU) is standard practice to reach a high thermal performance of the building envelope. Infrared thermography (IRT) offers a quick, non-destructive method to evaluate their thermal performance in situ. By visualizing surface temperatures, the thermal resistance of IGU's may be estimated. Due to the low thermal mass, a situation close to steady state conditions can be obtained quickly under stable weather conditions. However, the specular reflectance of glass and its relatively low emissivity (0.837) may result in unreliable infrared images. In spite of these obstacles, the use of IRT could address a practical need from the building industry as other inspection tools often render inadequate. This paper investigates the potential of IRT for thermal performance estimation of IGU's. Firstly, a sensitivity analysis on the boundary conditions was conducted using numerical simulations. Subsequently, quantitative IR-measurements were conducted on different glass types, in the lab and in-situ. Results show that specular reflection should be avoided to obtain reliable measurements. Finally, the thermal resistance of IGU's was calculated with the measured surface temperature, both in the lab and in situ. This showed that the use of inaccurate outside and inside heat transfer coefficients and non-representative inside and outside temperatures renders inadequate thermal resistances, even if IRT was executed in cloudy windless weather and with a temperature difference over 15 degrees C across the IGU. In these conditions, only single glazing, IGU's without low-e and IGU's with low-e coating are distinguishable. This undermines the practical implementation of IRT as assessment tool for IGU's. (C) 2016 Elsevier B.V. All rights reserved.