Infrared thermography has become a valuable tool for providing reliable information on the constructive components of the envelope of existing buildings and for checking if the thermal behaviour of a construction is consistent with the design parameters. However, infrared thermography has a limited capacity to offer quantitative accurate experimental results when the solar radiation incident on the facades is significant. The current study presents a methodology for theoretically calculating the external surface temperature of a multi-layer wall exposed to diurnal variations of the external environment temperature, which are caused by solar warming, hence validating a set of in situ measurements conducted using a thermographic camera over a time interval of interest. The measured external surface temperature is related to the theoretical equivalent sol-air temperature and the external and internal air temperatures by the numerical solution of the heat equation that is used to estimate the heat flux through a non homogeneous medium. In addition, the numerical results are successfully compared with those obtained using the analytical standard admittance method. Thus, the instantaneous measurements of the external surface temperature can be generalised to calculate the heat flux into the building while considering the solar radiation effects during extended periods of time. (C) 2018 Elsevier B.V. All rights reserved.