Misconceptions About Permeance in Wall Air Barriers

In moderate climate regions, and especially in southern states, specifiers are often tasked with selecting an air barrier that is vapor permeable. In many cases, they are advised by product manufacturers’ reps that products with a higher perm rating will deliver better performance. Various manufacturers have used this tactic to drive the sale of their products and limit competition.

To counter this misleading marketing technique, it is imperative to understand permeance, how it relates to vapor retarder classification, and what it all means in terms of building performance. 

Permeance indicates the rate of water vapor transmission through a material and is dependent on the material’s thickness, much like R-value in heat transmission. Permeance is often abbreviated to “perm”, which is the unit of measure used for vapor retarder classifications. A material’s perm rating is also what is needed when comparing the water vapor transmission of different building products. 

The table below shows vapor retarder classification as accepted by the International Building Code (IBC). It is important to note that the less permeable a material is, the greater its resistance to water vapor transmission.

As the table above illustrates, any material with a perm rating greater than 10 is classified as PERMEABLE. Selecting a product solely because it has a higher perm rating than the definition of permeable doesn’t add any meaningful benefits to the performance of the system.

The most important thing to consider when comparing perm ratings of various products is the test in which the perm rating was determined. ASTM E96 is the Standard Test Method for Water Vapor Transmission of Materials.

ASTM E96 contains two test methods to determine the perm rating of materials: Method A (the desiccant method) and Method B (the water method). Results from these two test methods vary considerably and cannot be compared in any way. Therefore, it is extremely important when comparing and choosing a vapor permeable or vapor impermeable air barrier that the results are from the same ASTM E96 test method. Method B is the most commonly used for classifying materials due to the higher results it yields, representing a worst-case situation with an excess presence of moisture.

Please contact Chris Kann at [email protected] with questions.