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Know Your Perms

Q: I’m thinking of suggesting an alternate sheathing on an exterior wall bid. Should I be concerned with vapor retarders?

A: Whenever you are suggesting changes to an exterior wall, you should make sure you are not introducing a second vapor retarder. This is especially true when trying to meet the higher levels of thermal efficiency as required by newer energy codes. These energy codes include the 2015 International Energy Conservation Code and the 2016 ASHRAE 90.1 Energy Standard for Buildings Except Low-Rise Residential Buildings. The first is a code and the second is a standard, which some states use as a guideline for code enforcement.


Regardless of which is used, there is a code mandated requirement for increasing the thermal performance of the exterior wall assembly. The concern is that depending on the material properties of the building products that are used, there is a potential for creating a wall with a double vapor retarder. The result of a double vapor retarder is the probability of trapping water in its liquid state within the wall cavity.


Every building material has some ability to allow water in its vapor state to migrate or pass through the material itself. The measure that is used to measure this ability is called a material’s “perm” rating. A perm rating is the recognized rating system that measures the amount of water vapor that can pass through a material. The higher the perm rating, the more readily vapor will pass through a material. Building product manufacturers test their products for perm rating by using the ASTM test standard E-96, Standard Test Methods for Water Vapor Transmission of Materials.


Using that as a test standard, building materials are broken down into four categories. They are as follows:

  • Materials that are greater than 10 perms are considered vapor permeable materials.
  • Materials that are 10 perms or less and greater than 1.0 perms are considered vapor semi-permeable.
  • Materials that are 1.0 perm or less and greater than 0.1 perm are considered vapor semi-impermeable.
  • Materials less than 0.1 perm are considered vapor impermeable.

The units of a perm rating is the number of grains of water vapor that pass through a square foot of material per hour when the vapor pressure is equal to one inch of mercury. Looking at various building materials, 6 mil polyethylene sheets have a perm rating of 0.03 perms. Gypsum sheathing has a perm rating greater than 30. Foil-face polyisocyanurate sheathing is less than 1.0, and extruded polystyrene is 1.1 perms.


The traditional theory of using a vapor retarder is to limit the amount of water vapor that can enter the wall cavity. The concern is the potential for condensation within the cavity. The conditions that are conducive for condensation are a vapor drive that pushes water vapor into the cavity coupled with a temperature differential within the cavity. Water vapor is driven from areas of high water vapor content to areas of lower water vapor content. Also, vapor goes from a higher temperature to an area of lower temperature. From an exterior wall condition, the common drive is for moisture-laden air to move from moist warm air to cold dry air, a common condition in norther climates. The problem is compounded if there is a significant temperature drop occurring in that cavity at the same time, which can occur with insulation in the cavity. To combat this, designers impede the flow of moisture laden air with the introduction of a vapor impermeable material on the warm moist side.


A good design is one where an assembly is allowed to dry in at least one direction. For example, on a roof deck with roofing installed on top, the deck can dry in a downward direction. Conversely a concrete slab on grade will dry in an upward direction. For a wall, there must be a direction for water vapor to move out.


Problems occur where there is no direction for this vapor movement. Water becomes trapped within the cavity, which will eventually lead to mold and degradation of the building materials. So, if a material with a low perm rating is installed on one side of the exterior wall, a much higher rating should be used on the opposite side.


The selection of water impermeable materials and their location in a wall is best left to the architect of record. A contractor should be aware of the perm ratings of the various materials used in the wall and should advise the design team of any concerns of a potential double vapor retarder. Also, any alternates that are considered should be analyzed in regard to a proposed material’s perm rating and subsequent impact on wall performance.

Robert Grupe is AWCI’s director of technical services. Send your questions to, or call him directly at (703) 538.1611.

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