This work intends to decipher the role of hydrophilic fillers, wheat straw fibers (WSF), on the water vapor transfer properties (sorption and diffusion) of biocomposites based on poly(3-hydroxybutyrate-co-3-hydroxyvalerate), (PHBV) as matrix. Transfer in biocomposites, measured using dynamical vapor sorption measurement, is correlated to the transfer properties of each individual constituent and to the specific structural arrangements induced by the presence of particles inside the matrix. Increasing amounts of WSF particles in the PHBV matrix lead to an increase of the water vapor sorption (WVS) of the resulting composites. This is attributed to the high sorption of hydrophilic WSF as compared to that of the neat PHBV matrix. Water vapor diffusion in composites (around 0.13 × 10−11 m2 s−1 at 20°C whatever the filler content) is always lower than in the neat matrix (0.26 × 10−11 m2 s−1) although wheat straw displays high diffusivity values (1.84 × 10−11 m2 s−1). Such unexpected behavior is related to (1) changes of structure and properties of the WSF particle once embedded in the PHBV matrix, (2) changes in the polymer matrix structure and properties in contact with fibers, and also (3) to the representativeness of the filler diffusivity, which is difficult to appraise.