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.
Classification
|
Definition
|
Permeance
|
I
|
Vapor
Impermeable
|
Greater
than or equal to 0.1 perm
|
II
|
Vapor
Semi-Impermeable
|
Greater
than 0.1 perm but less than or equal to 1.0 perm
|
III
|
Vapor
Semi-Permeable
|
Greater
than 1.0 perm but less than or equal to 10 perms
|
|
Vapor
Permeable
|
Greater
than 10 perms
|
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
Chris.Kann@CarlisleCCM.com with questions.