Energy audits and check of energy performances are more and more urgent for achieving energy saving in existing buildings. In this context, it is very important to estimate U-value of buildings and, to this aim, different approaches are available. U-value is mainly estimated from the knowledge of thermal properties of the single layers of the wall using design data or analogies with similar buildings. To get a more accurate estimation, field analyses should be made like endoscopic investigation or logs. Nowadays, also in situ measurement of U-value using heat flow meters are available. Such measurements involve quite simple operational and calculation issues but, on the other hand, the disadvantage of long duration times and not negligible measurement uncertainties. In this paper, the authors present the results of an experimental campaign aimed both to assess the metrological performance of HFMs and to evaluate the influence of the ambient conditions. Furthermore, in situ U-values have been compared with the estimated ones from design data and field analyses. The results of the test show a good behavior of HFMs when tests are conducted according to ISO 9869. Nevertheless, operative measurement conditions and characteristics of the envelope component under investigation can strongly affect in situ U-value accuracy. (C) 2015 Elsevier B.V. All rights reserved.