Vapor Barriers (Second -Level Inner Layer)
A vapor barrier is a clothing item that is waterproof, serving as a barrier to the transportation of water vapor. When worn near the skin, it keeps vapor near the skin. Eventually the humidity level rises to the point where the body senses a high humidity level and shuts off perspiration. This prevents evaporative heat loss and slows dehydration. It is best not to use vapor barriers directly against the skin because any evaporation of moisture directly at the skin surface leads to greater heat loss. Wearing polypropylene or some other hydrophobic layer between the skin and the vapor barrier allows the moisture to be transported away from the skin. There is no doubt that a vapor barrier system is effective for some people in some conditions. Before using a vapor barrier, consider your activity level, the amount you naturally sweat, and your "moisture comfort".
Taken from "The Backpacker’s Field Manual" by Rick Curtis, Director of Princeton University Outdoor Action. Published by Three Rivers Press, New York.