In this work the wall effect during the initial states of water vapor adsorption was studied for different piles of granules held in a cylindrical container. A new diffusion model was developed for the description of the physical phenomenon whereby water vapor penetrates along the piles of cylindrical pores. The isotherm sorption equation was incorporated as a wall boundary condition of the cylindrical pore. Finally, the differential mass balance equation was solved numerically using a finite-difference method. A second-order central differencing scheme (CDS), in a general nonuniform grid, was adopted. A mathematical expression is proposed for linear sorption isotherm (given as Equation (28) here), and a numerical procedure is presented for the case of nonlinear moisture sorption isotherm. The numerical solutions were tested through the measurement of water vapor mass uptake for different piles of silica gel and breakfast cereals, and the experimental values obtained were in good agreement.