The building elements’ thermal transmittance index (known as U value) is the most essential parameter for the estimation of its thermal losses, and subsequently its energy performance. Laboratory measurements of the U value are practically of interest only for pre-fabricated or standardised building elements; hence, the commonly acceptable practice is to use theoretical estimations, taking into account the thermal resistance indexes of the building elements’ layers. In this context, this paper aims at the assessment of the uncertainty budget of one of the already used in-situ building elements’ thermal transmittance measurement, as long as this is applied using a thermal camera instead of contact thermometers. A specially designed and easily constructed auxiliary measurement set-up is presented and is used in comparison with simultaneous thermocouple measurements, to verify the adequacy of the auxiliary set up response time. This study concludes with the analysis of the uncertainty that affects the proposed procedure, along with an assessment of the impact in the final result. The uncertainty of these measurements is strongly related with the respective decisions on building upgrade and renovation. Therefore, managing it with a simplified procedure will be rather profitable both for those seeking to perform an in-situ U value measurement and for those who are engaged into a decision process upon renovating or not an existing building.