The classic models describing the hygric mass transfers inside porous materials seem unsuitable in the case of bio-based materials. They are based on the assumption of instantaneous local equilibrium between relative humidity and water content (Kunzel in Simultaneous heat and moisture transport in building componentsone- and two-dimensional calculation using simple parameters, Fraunhofer IRB Verlag, Stuttgart, 1995). These two parameters evolve according to the diffusive fluxes following the sorption isotherms. This study shows that it leads to predicting much shorter times of stabilization than those experimentally obtained. A new approach is presented here; it is free from the local instantaneous equilibrium introducing a local kinetics to describe the transformation of water from vapor state to liquid state and vice versa. The local kinetics of sorption is coupled with the well-known hysteresis phenomenon. It is adjusted from bibliographic data (Collet et al. in Energy Build 62:294-303, 2013) giving mass evolution of three hemp concretes under adsorption/desorption conditions.1D cylindrical simulations allow an excellent fitting on the experiments. Finally, a semiempirical model is proposed, allowing to determine the kinetics parameters more easily.