We measured the thickness and temperature dependence of moisture permeation in highly defective (10–20 at. % hydrogen) Al2O3 thin films grown by atomic layer deposition on polymer substrates. We found that when films were grown at higher temperature or were thicker, independent of growth temperature, they were better moisture barriers. We determined the threshold thickness for measurement-limited barrier performance to be 7.5 nm for growth at 100 °C compared to 9.6 nm at 50 °C. We explained the permeability of these highly defective films with a new model, which relates moisture permeability to a critical density of defects and not due to pinholes.