Critical thresholds for the diffusion of water through low-k a-SiOCN:H dielectrics were investigated using a combination of x-ray reflectivity (XRR) mass density and positronium annihilation lifetime spectroscopy (PALS) pore size metrologies. It was observed that hermetic low-k a-SiOCN:H dielectrics were achieved only at mass densities > 2.2 g/cm3 and when the PALS pore size is less than or equivalent to the molecular diameter of water.