A method is described to determine the liquid water diffusivity in capillary active materials and a model for the mathematical description of this material property is proposed. Heat flux measurements by means of a guarded hot plate apparatus are employed for the assessment of the total energy transfer through dry and water-saturated calcium silicate samples. The radiative and conductive contributions to the heat transfer are estimated using an empirical model and separated from the enthalpy fluxes, connected to the water (liquid and vapor) transfer. After mathematical transformation the liquid water diffusivity can be calculated. The proposed method differs significantly from other procedures generally employed to the same aim, and is advantageous since no further equipment but a guarded hot plate and a balance is needed. Considerations on the effect of the water content on the vapor diffusion resistance factor are made, taking into account the experimental results. The obtained results are compared with data by other authors and an error estimation is included. (C) 2016 Elsevier Ltd. All rights reserved.