The lower the permeability, the higher is the resistance of the material to water vapor intrusion. Permeance is not a material property. It is a performance evaluation of the test specimen. ASTM E96 has a set of six standard conditions for the test. It is important to be aware that no two different test conditions are likely to yield the same test results on the same material.
Water vapor can penetrate all but a few materials.
Therefore, in order to make accurate comparisons the materials being compared must be tested using identical conditions. Water vapor permeability can be a critical design consideration because water vapor has the ability to penetrate materials that are unaffected by water in the liquid form. Water vapor intrusion is a particular concern to insulation systems subjected to a thermal gradient. Driving forces are created due to vapor pressure differences between ambient conditions and the colder operating conditions of the piping. These forces drive water vapor into the insulation system, where it may be retained as water vapor, condense to liquid water or condense and freeze to form ice.