A vapor barrier is any material or fabric that stops water vapor from passing through. As opposed to fabric that breathes i.e., allows water molecules in the air to pass through. This is the reason sporting attire and rain gear is made from material that breathes. The body keeps cool by the evaporation of sweat; heat is removed as the sweat evaporates and is carried away in the air. In other situations, such as storage bags or tents, where it is appropriate to keep moisture in or out, a non-permeable vapor barrier is used.