Vapor barrier mastics are defined as have a perm rating of less than 1. The acceptable test method in our industry is ASTM E96. ASTM E96 tests for the amount of water that penetrates the mastics (grains water/sq in). Less than 1 is a vapor barrier, between 1 and 10 is semi-permeable vapor barrier and greater than 10 is breather mastic.
Vapor barriers mastics are important in chilled water applications, especially where there is high humidity. Humid air allowed to come into contact with chilled piping creates condensation. The condensation causes the insulation to become wet, thus insulating less and leading to a system failure. Vapor barriers also seal the insulation system for energy savings and protect the exposed fibers from damage.
For outdoor use, a weather barrier/breather mastic should be used; such as DP 5050 and DP 5060.