The characterization of building thermal behaviour is crucial to achieve the low-carbon objectives of the European Union by 2050. In this way, the knowledge of the thermal transmittance is being developed as a significant factor of the thermophysical properties of the envelope. In the existing building, the theoretical calculation has several limitations with non-destructive techniques typical of the deterioration of the elements. Many experimental methods obtain therefore more representative results. The experimental method developed in ISO 9869-1 is the most standardized, although it presents limitations in the heat flux measurement. However, the thermometric method obtains the thermal transmittance with the surface temperature. This research is focused on the evaluation of the thermal transmittance based on ISO 9869-1 (average method and average method with correction for storage effects), but using variables measured with the thermometric method. For this purpose, multilayer perceptrons were used as post-processing techniques. The models were trained by using a dataset of 22,820 simulated tests of representative walls of the building stock in Spain. The determination coefficient was greater than 98% in both analysis approaches. Individual models were also generated for each building period because they significantly influenced the input variables. The results showed that thermal transmittance values can be obtained without measuring the heat flux, and the error associated with the use of tabulated values for the total internal heat transfer can be removed. This research would guarantee a high assessment tax of buildings establishing adequate energy conservation measures to improve their energy performance. (C) 2019 Elsevier B.V. All rights reserved.