In this study, diffusion of water into ethylene ionomers neutralized with sodium or zinc cations (Na‐ionomer and Zn‐ionomer, respectively) was studied by in situ Fourier transform near infrared (NIR) spectroscopy. Water ingress was measured by NIR from the edge of the laminated glass toward the center and the measured water concentration profiles were analyzed by Fick's second law or the modified Jacobs–Jones model, which describes diffusion into a two‐phase system of different density and polarity. It was found that the modified Jacobs–Jones model agrees well with the measured water concentration profiles of ionomers, while the Fickian model fits well to the poly(vinyl‐butyral) (PVB) results. The modified Jacobs–Jones model was also confirmed to apply to water vapor transmission rate (WVTR) data of another sodium ionomer (Na‐ionomer‐HTM) with a higher melting temperature than Na‐ionomer. The water diffusion coefficient for Na‐ionomer, Zn‐ionomer, and PVB are compared, and the diffusion parameters are discussed considering the higher‐ordered structures of ionomers studied by small‐angle X‐ray scattering (SAXS). Simulating conditions that PV modules encounter in field applications, model projections show that Zn‐ionomer and Na‐ionomer‐HTM give significantly lower water permeation than PVB, and due to its low diffusion coefficient, Na‐ionomer will have significantly less water ingress than PVB. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48929.