Thermal conductivity is one of the most important parameters for construction and building materials. Indeed, extensive efforts have been put in by Europe in order to decrease energy consumption at various levels. However, with regards to historical buildings, the measures of thermal performance are not simple and they, in fact, pose a wide range of error. In this paper, an intermingled fractal model has been proposed, which is capable of reproducing the porous microstructure. Moreover, by converting this model into an electrical circuit, it is capable of predicting thermal conductivity. The results are in perfect agreement with experimental ones obtained by climatic chamber. (C) 2016 Elsevier B.V. All rights reserved.