The in situ determination of overall heat transfer coefficient (transmittance) of the opaque components which constitute the envelope of existing buildings, allows a more accurate evaluation of the real energetic consumptions of a building and, furthermore, this value is a useful tool for energetic certification. The value of transmittance can be calculated theoretically using the European Standard EN ISO 6946 or can be obtained from the analysis of experimental recorded temperature and heat flux values. Several articles are present in the literature that propose experimental and measurement data analysis methods on test components under real conditions and various evaluations have already been developed using numerical simulations for different theoretical aspects. The aim of this paper is the evaluation of the deflection of the heat flux of external walls of a building provoked by heat flux sensors (HFS) owing to their different thermal conductivity compared to the conductance of the wall on which they are positioned. In order to do this evaluation, a finite differences calculation algorithm was developed and implemented on PC, which allowed the comparison between the experimental and calculated values of temperature and heat flux for the walls containing a sensor for the in situ evaluation of transmittance. (C) 2017 Elsevier B.V. All rights reserved.