The ash deposition is one of the major concerns when evaluating the performance and reliability of coal-fired steam generators. In order to take into account the insulation effect of ash deposits, up to now, several models have been reported, but none has taken into account the porous structure of a typical ash deposit under the boiler operational conditions. This work proposes a correlation for the effective thermal conductivity based on a porous structure that represents the growing behavior of a typical ash deposit. The correlation is developed based on a finite volume analysis, which takes into account the rarefaction effects due to the close relation between the geometric scale and the mean free path of the molecules. This effect is considered through the temperature jump, which modifies the boundary condition under which the Fourier equation can still be applied. The found results from the proposed correlation are in good agreement with the measured effective thermal conductivity and gives a good estimative of the energy loss due ash deposition on superheaters of coal-fired steam generators. (C) 2017 Elsevier B.V. All rights reserved.