Impermeability—Water Vapor Resistance
All but two standards reviewed have a requirement that addresses the passage of water vapor. The standards that address water vapor resistance all refer to ASTM Test Methods for Water Vapor Transmission of Materials (ASTM E 96). ASTM E 96 describes three basic test methods: the water method, the inverted water method, and the desiccant method. ASTM E 96 also includes six sets of test conditions, referred to as “Procedures.” The test method allows for calculating results as water vapor transmission, permeance, or permeability. The waterproofing material standards reviewed here utilize most of the variations allowed in ASTM E 96, which makes comparison of materials challenging. Refer to Table 2 for a review of standards.
As seen in Table 2, there is agreement among many standards writers that water vapor resistance is a significant issue; however, none of the standards require the same combination of ASTM E 96 test procedures and methodology for expressing results. This diversity in test methods (water, inverted water, and desiccant) is surprising considering that ASTM E 96 states that the inverted water method should be used, “Where water is expected to be in contact with the barrier in service…” Waterproofing is clearly a barrier that by definition is intended to be in contact with water.
In this context, bentonite waterproofing deserves particular attention. Bentonite transforms from a dry
Standard________________________ Requirement____________________ Test Method
granular material to a dense layer capable of stopping the flow of liquid water by absorbing water (hydrating). This widely used product is designed, quite unlike other waterproofing, to hold moisture against the structure. The wide acceptance of bentonite raises a question regarding whether waterproofing materials must be vapor resistant.
Nonetheless, there is wide support for permeance requirements, perhaps in part because permeance is an indicator of a products resistance to long-term water absorption. By convention in the U. S. construction industry, a material is considered a vapor barrier if it has a permeance of less than 8.7 ng/Pa. s. m2 (1 Perm (inch-pound)). While some materials may have much lower permeance, this value may have broad support as a basic requirement for waterproofing.