Permeability, Perm Ratings and Testing Procedures
Without a single test procedure, Perm ratings can vary significantly
Permeability describes the capacity of a substance to allow the passage of water vapor (moisture). A substance that freely allows water vapor to pass through itself is said to be permeable. ‘Perm’ is the unit of measure for permeability – the higher the Perm number of a product, the greater its permeability.
Vinyl wallcovering – as a result of its construction – is considered non-permeable because it allows little or no transfer of water between its surfaces. The permeability of vinyl wallcovering can be increased, however, through perforation – the process of punching tiny holes in the surface of the wallcovering to allow the passage of water.
The strategy of using perforated vinyl wallcovering in certain building installations is to minimize the incidence of mold on the wall surface behind the wallcovering. Perforating the wallcovering allows it to "breathe," allowing water vapor to pass through its surface, so that water is not trapped behind the surface where it can contribute to mold growth. (See ‘Cautions’ below).
Unfortunately at this time, the construction industry has not prescribed a single, specific test procedure to measure Perms or the permeability of interior materials. As a result, the reported permeability values of various products can differ significantly, which may result in some possible confusion when specifying interior products, like vinyl wallcovering. Which is the best choice?
Until a single testing method is defined, you should know that LANARK Wallcovering that can be perforated at our manufacturing facility as a custom feature, is tested using the American Society for Testing and Materials (ASTM) E-96-00 Standard Test for Water Vapor Transmission of Materials. There are six different versions of the E-96 testing procedure. The two most commonly used in the wallcovering industry are:
- Procedure A is Desiccant Method at 73.4°F (23°C)/’Dry cup’
- Procedure B is Water Method at 73.4°F (23°C)/’Wet cup’
When comparing the Perm ratings of perforated wallcoverings, you should inquire as to the test method used to determine the Perm ratings of the competitive products being compared – you may not be comparing "apples to apples.”
Contact your LANARK Wallcovering distributor for more information about perforation, pricing and minimum quantity requirements to have this feature added as a custom option to your LANARK Wallcovering order.
Testing of wallcovering permeability under controlled laboratory conditions cannot account for all the many possible variables that can impact a wall system's overall permeability in "real world" conditions. As such, Perm ratings generated in the laboratory should not be the only factor considered when choosing wallcovering for a specific building installation. Instead, an experienced building professional with knowledge of the conditions and characteristics of the specific building and its locale should always be consulted to determine whether vinyl wallcovering – even if perforated – would be the appropriate choice.
Wallcovering – including perforated wallcovering – is not a solution for and should not be used in any buildings which have or may have wall or wall cavity moisture accumulation or other moisture problems. Wallcovering – including perforated wallcovering – will not prevent mold growth or other moisture related damage if moisture accumulation is permitted to occur in a wall or wall cavity.