The significance of U values in buildings’ shell energy performance is well known in the scientific community as well as in the construction sector. In addition, conducting a validation of the energy performance of a building often requires the knowledge of real U value figures, since their theoretical peers may deviate considerably from the real ones. The theoretical U value of a shell element may be calculated using complex material and surface related information, while its practical validation is conducted usually by equipment that requires tedious installation and constant contact with the examined element. Aim of this paper is to introduce an easily deployed, plug and play, contact less technology, able to provide a reasonable approximation of the building element U value and its variation with external air speed as well as an accurate estimation of the U value change in case of a shell (wall or window) retrofit.