Moisture condensation in unvented interstitial spaces is a significant issue in marine climates, particularly affecting townhomes, condominiums, and apartment complexes in the Pacific Northwest. Vapor migration in these homes typically follows an upward path, reaching the interstitial spaces above the top floor. Closed-cell foam insulation applied to the interior roof sheathing can partially shield the interstitial space from the temperature extremes on a roof. However, improper foam installation may lead to gaps, allowing attic air to infiltrate the insulation and interact with structural elements that are thermally bridged to the exterior. In cold weather, condensation occurs in these areas, causing cumulative damage to both structural lumber and insulation. Since this deterioration often remains hidden, substantial damage may occur before detection. This study investigates moisture transport to an unvented attic of a representative home in the Greater Seattle area. Hygrothermal data was collected using data loggers at multiple locations over a one-week period. The study observed the impact of daily household activities such as cooking and showering on moisture accumulation. The temperature and relative humidity (RH) profiles confirm an upward airflow pattern. Notably, elevated humidity levels in the bathroom persist for up to five hours after a late-night shower, likely due to inadequate ventilation. Moist air from the bathroom migrates into the attic space, contributing to condensation and moisture-related damage. The study provides recommendations for improving attic and roof design, enhancing ventilation strategies, and optimizing the balance between thermal efficiency and moisture management.