This ASHRAE project looked at the nature, significance and control of solar-driven vapor diffusion in wall systems. The project combined experimental and simulation work to provide an in-depth characterization of the phenomena occurring during inwards vapor diffusion in insulated wall assemblies. This paper presents the modeling of the results of a field study that was performed over a period of 2 years and where possible occurrence of solar driven diffusion was documented for different wall assemblies. The specifics of the modeling such walls under climatic conditions are presented and discussed. The verification of the modeling results against the field testing results once more demonstrated the importance of obtaining the complete description of the wall systems, along with any degradation that occurred during the test period. Excellent agreement was found for the wall case when accurate material properties and description of wall systems was available. It is important to use material property data of the specific wall system during the validation phase of the hygrothermal modeling.