This paper presents a study of moisture retention and isothermal transfer in a cement and lime mortar. The extended range of pore sizes and the intrinsic heterogeneity of the medium do not allow the use of a single-scale percolation network for numerically describing the topology and the physical processes related to fluid retention and transfer. Three different scales are used for the numerical description of the cement and lime mortar studied in this paper. Electron scanning pictures are used for the geometrical modelling of the material in the three different scales. The heterogeneity is associated with the fissures network, represented in the first scale, where the medium was conceived as a series of cubic blocks separated by straight channels. The second and third scale are used to represent the pores in the cement and lime paste. A 3D simple cubic percolation site network is used in the third scale to model imbibition and drainage and to predict the hydraulic conductivity in terms of the moisture content. In the second scale, pores are represented as embedded bodies in a continuous medium and the hydraulic conductivity is calculated using Maxwell-De Vries theory for composite media. At the first scale level, the isothermal mass diffusivity D-o is modelled by studying an invasion process of liquid water into the fissures network, simulating an actual imbibition experiment, from one extremity of a sample column. Finally, the results of the simulation are compared with experimentally obtained values of D-o.