Quantitative thermography is now mostly accepted as a reliable method to measure energy performance of existing buildings, in particular the thermal transmittance U-values of opaque elements. Some researches have been conducted in this field, each presenting a different procedure verified by the application on simple case studies. Anyway, a comprehensive approach, based on a parametric analysis of walls with different typologies and exposure, but same boundary conditions, is still missing. This study proposes a systematic approach to the problem, based on a three years research activity carried on an experimental building where timber (light) and brick (heavy) structures were tested simultaneously with Infrared Thermovision Technique (ITT), also equipped with heat flow meter (HFM) sensors and a nearby meteo station. Standard deviation of U-values measured with ITT is given as well as absolute deviation against values calculated following international standards and measured with HFM method. Parameters having high significance for the achievement of good results compared to the expected U-values are assessed through a sensitivity analysis. Influence of weather conditions during the survey are also considered and a repeatable procedure is finally set up. The findings presented in the study show that the method gives good results for heavy constructions, while further studies are still needed for light and super-insulated walls. (C) 2014 Elsevier Ltd. All rights reserved.