An important property of composite material is related with its water absorption characteristic. Several articles present studies about this property and, in general, in these articles an analytical solution of the diffusion equation is used to describe the process. In this case, the effective water diffusivity is considered with a constant value. However, the water penetration modifies the internal structure of the material and this can modify the effective mass diffusivity of the product. In this article, a one‐dimensional numerical solution of the diffusion equation in Cartesian coordinates is presented, enabling to consider a constant value or a variable expression for the effective water diffusivity. A comparison between the models with constant and variable diffusivity was performed and the results indicate that the model with variable diffusivity as a decreasing function of the local moisture content describes better the process than the model with constant diffusivity.