WATER-RESISITIVE BARRIER MEMBRANES (WRB’s) are essential elements for most types of wall construction. While fully-adhered sheets and fluid-applied membranes are the predominate products, some manufacturers offer a mechanically-fastened membrane.
One feature of the WRB membranes is the breathability of the membrane or its water vapor permeability; that is, the ability of a material to limit the amount of vapor that can pass through. The building codes –such as the International Building Code (IBC) — separate membranes into different classes by their performance for water vapor permeability.
[refer to Chapter 2 Definitions vapor retarder class of the IBC]; that is:
Class I (0.1 perm or less)
Class II (0.1 < perm <= 1.0 perm)
Class III (1.0 < perm <= 10 perm)
Further, building codes (such as the IBC) require testing of the product to determine its permeability performance in accordance with ASTM E96 Method A, or otherwise known as the desiccant, dry-cup method [refer to Chapter 2 Definitions vapor permeable of the IBC]. The building codes are silent regarding the application of, or the use of, method B; which is the water method, or wet-cup method. The results of these two methods are different with no direct correlation between the two; that is, there is no conversion of the results from one method to the other.
The perm rating (or permeability) of a material is a performance capability, not a property of the material. This performance rating is based on the thickness of the product at the time of testing; therefore, the perm rating will differ as the thickness of the material increases or decreases. As such, the perm rating is only applicable for the tested thickness; there is no conversion formula for an alternate thickness.
The building codes define a vapor permeable membrane having a moisture vapor permeance rating of 5 perms (2.9 x 10-10 kg/Pa x s x m2) or greater [refer to chapter 2 Definitions vapor permeable of the IBC], when tested in accordance with ASTM E96 Method A.
The building codes require a Class I or Class II vapor retarder on the interior side of frame walls in zones 5, 6, 7, 8 and Marine 4 [refer to the Chapter 3 of the IBC Energy Conservation Code for zone definitions]. In addition, the building codes specifically preclude a Class I and II vapor retarder on the interior walls of frame walls in zones 1 and 2, and precludes a Class I vapor retarder on the interior side of frame walls in zones 3 and 4. [Refer to section 1405.3 of the IBC for specific vapor retarder requirements].
While ASTM E96 provides additional procedural options (C through F) for test conditions (with elevated temperatures and elevated relative humidities) using methods A and B, the Code requires compliance in accordance with method A. Perusal of the data sheets from different manufacturers provided an observation that many do not provide perm ratings based on ASTM E96, method A, but instead provide the rating in accordance with method B, or they do not indicate any method. As such, it is important to confirm the perm rating is in accordance with method A and the required thickness.
Method B of ASTM E96 will typically indicate a higher permeability than method A due to the presence of water during testing and water’s highly cohesive property.
When recommending an appropriate weather-resisitive membrane, the permeability of the membrane is not the sole determining factor. Membrane properties, such as compliance to NFPA 285, adhesion to substrate, bulk water resistance, detailing, sealing around fasteners, chemical and sealant type compatibility, and installation technique are important as well. The above is a caution to use the appropriate method (method A) for evaluating the permeability of a membrane in accordance with Code.