A 3D tri-phasic numerical model was developed to predict water vapor permeability in composite materials made of polypropylene (PP) as matrix and cellulose particles as fillers, with existence of an interphase around permeable inclusions. About 70 tri-phasic structures composed of ellipsoidal, heterogenous-size particles were generated to represent composites with four different filler contents () with interfacial region at the filler/matrix interface (either 1 or 2 μm thick) displaying its own permeability. The relative permeability (i.e., ratio between composite and neat matrix permeability) was calculated from Finite Element Method (FEM) simulations on these structures. A good prediction of experimental relative permeability for the whole filler content range investigated was observed. The presence of a percolating interphase observed in some structures explains the high permeabilization observed for high . The proposed 3D numerical model was confronted to five state-of-the art analytical models and was the only one able to describe the observed complex structures with identification of reliable characteristics for the interphase (thickness, permeability).