Experiments have assessed how moisture and heat influence the functional and structural integrity of butyl rubber matrix composites used as protective barriers. This paper, which is first of a series, reports the findings on the subjects of moisture diffusion and its effect on ultrasonic wave propagation of composite barriers. On exposure to high humidity at elevated temperatures, carbon black-reinforced butyl rubber composite barriers were found to absorb substantial moisture, with the hygroscopic reinforcement phase making a more significant contribution than the matrix. The moisture diffusion coefficients for both composite and unreinforced vulcanizate were close to each other at each exposure temperature, and had the same temperature dependence. The calculated activation energies of moisture diffusion indicate that the moisture absorption rate of the carbon black phase is governed by the moisture diffusion rate of the matrix. Ultrasonic wave speed appeared to be a viable means of monitoring the moisture absorption of composite barriers. On exposure to moist heat, relative ultrasonic wave speeds for both composite and unreinforced vulcanizate decreased continuously.