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A model is proposed that accounts for the isothermal drying process of hardened cement-based materials. It constitutes a further development of previous works. The equations of isothermal drying are derived (i) from mass balance equations written for the liquid water phase, water vapour and dry air, (ii) from the Fick's law governing the relative diffusion process of water vapour and dry air to the gaseous mixture, and (iii) from the Darcy's law describing the transport of wet air and liquid water. Intrinsic liquid water (Kl) and gas (Kg) permeabilities are distinguished, since the concept of intrinsic permeability, which is independent of the fluid nature, is not relevant for a cementitious material. New laws for gas transfers are introduced according to measurements on concrete specimens. Thus, a semi-empirical law gives the effective diffusion coefficient of water vapour vs. porosity and degree of liquid water saturation. In the same way, a new function, expressing the relative permeability to gas with respect to this degree of saturation, is proposed on the basis of experimental results. In order to describe the global movement of gas, viscous and slip flows are taken into account according to the Klinkenberg's concept. A numerical study shows, on the one hand, that a gas depression (below the atmospheric gas pressure) can be observed and, on the other hand, that transfers of water in the gas phase may significantly contribute to the drying of cementitious materials in addition to liquid water transport by capillarity movements. Simplified approaches and their range of application are presented.