In order to investigate hygroscopic moisture transfer in fibrous material, experiments were conducted to measure the mass flux and moisture distribution through a sample of glass fiberboard (138kg/m3). The samples ranging in thickness from 0.0388 to 0.053 m were exposed isothermally to relative humidity gradients ranging from 35 to 100%. The equilibrium moisture content was used as the driving force for transfer. The mass conductivity was calculated from the flux and distribution data and found to vary from 0.199 to 0.348 g/cm-hr (5.53 × 10−6to 9.67 × 10−6kg/m-s) increasing slightly with increasing relative humidity difference. The data was also used to calculate the mass diffusivity and permeability of the fiberglass. The results from this study were combined with data from earlier work on thermal mass transfer to determine the thermal mass conductivity of fiberboard and to provide an overall description of nonisothermal mass transfer. A better understanding of overall hygroscopic moisture transfer is important in the determination of drying and condensation rates.