Permeability, Permeability Testing and Perm Ratings

Permeability is a measure of the amount of water vapor (moisture) that can pass through a specified material  in a certain amount of time. The measure and degree of permeability is expressed in units referred to as Perms. Materials with high Perm levels will allow more moisture or water vapor to pass through than those with lower Perm values.

While no specific test procedure has been officially recognized, or sanctioned, by the construction industry to date, the ASTM E-96-00 Standard Test for Water Vapor Transmission of Materials is being used  by many companies to measure the permeability of their wallcovering products.  The E-96 has 6 different versions of the testing procedure.

• Procedure A is Desiccant Method at 73.4°F (23°C)
• Procedure B is Water Method at 73.4°F (23°C)
• Procedure BW is Inverted Water Method at 73.4°F (23°C)
• Procedure C is Desiccant Method at 90°F (32.2°C)
• Procedure D is Water Method at 90°F (32.2°C)
• Procedure E is Desiccant Method at 100°F (37.8°C)

There are two variations that are commonly used in the wallcovering industry.  One variation (Procedure A) is to use the desiccant method (dry cup) and a “sandwich” composite of the drywall, primer, adhesive and wallcovering.  The other variation (Procedure B) is to use the water cup (wet cup) to test the wallcovering film alone.  As the test is run, the samples are placed in a constant temperature and humidity  controlled area and the sample dishes are weighed to determine if the dish is gaining (desiccant method) or losing moisture (water method).  The weight changes are documented over a time period and then calculations are made to determine the water vapor transmission (WVT) and permeability of the material tested. 

It is important to keep in mind that, depending on the test method used, reported permeability values can vary substantiality. Typically “film only” permeability testing (Procedure B) will result in substantially higher permeability values than “sandwich” testing (Procedure A) which tests the wallcovering “film” after “installation” on dry wall with a primer and adhesive.   Also, testing results can also vary widely unless a strictly controlled and consistently applied test protocol is followed.

Testing of wallcovering “film” or “sandwich” permeability under controlled laboratory conditions will not reflect the many variables that can effect wall system permeability  under actual building conditions. Laboratory permeability values should not be used to make recommendations regarding the appropriateness of a wallcovering for a particular building.  Such a recommendation can only be made by an experienced building professional who is familiar with the specific building, building requirements, local conditions and the characteristics of wallcovering.