The diffusive penetration of atmospheric water vapor into amorphous silica (a-SiO2) degrades the performance of electronic devices. In this letter, we calculate the range of activation energies for water diffusion in a-SiO2 such that the diffusion time through, for example, a 0.5-m protective layer is on the order of the decadal time scale, as required in typical applications. We find that for all practical purposes, silica composed of n -member rings is impenetrable to water vapor for n ≤ 5 . Thus, we conclude that the distribution of n-member rings in a-SiO2 and, specifically, the n >; 5 fraction is the critical parameter for predicting device performance.