This work proposed a new method for the determination of mass diffusion coefficient in hygroscopic materials. The experiment consists in submitting one side of the sample to a variation in time of the relative humidity (RH) and to collect the RH on its back side. The imposed RH is measured during the test and serves as boundary condition in a comprehensive computational code solving heat and mass transfer in porous media. This model is the physical engine embedded in the inverse procedure implemented to determine the mass diffusion coefficient. Compared to classical methods, this new method has several advantages:-it allows several samples to be measured simultaneously simply by multipexing the RH sensors,-accurate values can be obtained even when starting and ending out of equilibrium, which allows the characterization time to be drastically reduced,-the external mass transfer coefficient has negligible effect on the identified value,-non-standard Fickian behaviours can be detected by the disagreement between the measured and the simulated curves. The results show that the diffusivity obtained for spruce and for medium density fibreboard (MDF) are in good agreement with the ones found with classical methods. On the opposite, result obtained for fibreboard give unrealistic values which confirm the dual scale nature of mass transfer occurring in this kind of material for unsteady regimes.