We seek to determine how far, and over what timescale, water vapour penetrates the layered polymer/thin-film metal structures used in capacitance type humidity sensors. Although these sensors are sold in a mature market, to optimise performance and durability, there is much interest in gaining in-depth understanding of factors governing mass transport routes within the structures. As with many thin film systems, two possible, contrasting, routes are homogeneous diffusion and percolation. The ability of NR to elucidate the location of water inside thin films makes the technique uniquely suited to probing which of these prevail below buried interfaces in this system. Excitingly, recent developments in 'event mode' NR data acquisition will allow temporal profiling of changes in polymer microstructure with water uptake at ~1s intervals, suitable to probe typical device response times (~15s).