Keeping Thermal Protection Intact
While the moisture barrier has a number of supplemental functions, its main job is to keep the thermal protective properties of the system intact by preventing external water from penetrating into the critical air spaces of the garment.
A dry system is safer, more dependable, more comfortable and a lot lighter than a wet one. All moisture barriers will shed external water, but there are significant differences in their breathability, durability, thermal integrity and long-term reliability.
Highly-breathable moisture barriers are intended to prevent water from entering into the thermal layers, while allowing body vapor from sweat to escape outward. A more breathable barrier will usually reduce the amount of moisture and body heat that can be trapped inside the gear.