Low-density thermal insulation foams demonstrate an increase in thermal conductivity (or reduction in R-value) with increased water content in the foam interior. The rate and amount of moisture transport through closed-cell foamed plastics differ for liquid water and water vapor by approximately an order of magnitude. Water vapor penetrates foamed plastics, such as closed-cell thermal insulations, much more readily when it is subject to thermal and water vapor pressure gradients. We studied water vapor permeation into low-density thermal insulations upon exposure to water vapor drive conditions of varying magnitude, and we monitored the thermal properties of these materials. We confirmed that the relative change in R-value is correlated with the volume fraction of water in the material, regardless of the material’s composition. Further, our studies indicate that the density of the foamed closed-cell plastic materials influences the slope of the linear correlation, with a more pronounced effect for higher-density thermal insulations. We also share moisture absorption rates for a variety of thermal insulations and quantify how these rates depend on exterior conditions (e.g., temperature and humidity settings). To complement these data, we offer drying rates for the same materials. Finally, we relate our findings to several typical applications and compare the analytical predictions with field results.