This paper analyzes the moisture accumulation and the increase in the effective thermal conductivity of a fibrous insulation slab due to moisture migration. The problem is modeled as a transient, multiphase flow, with variable properties, in a porous slab with impermeable, adiabatic horizontal boundaries and permeable vertical boundaries, A detailed investigation of the interaction between condensation rate, temperature, liquid content, and vapor density fields is presented. The influences of different humidity levels at the boundaries of the insulation slab are investigated in detail and the importance of the Peclet and Lewis numbers on the condensation process is demonstrated. The analysis shows that condensation, and the resulting augmentation of heat transfer, is a serious problem at large Peclet numbers (Pe > 1). Furthermore, the increase in the Nusselt number for fibrous insulation due to condensation is found to be minimized by designing an insulation slab which has a low Lewis number.