Today's building professionals understand that air and water resistive barrier (WRB) membranes are essential elements in modern wall construction. These membranes typically consist of fully-adhered sheet or liquid membranes. Mechanically-attached sheet products are still used as well.
A less understood feature associated with WRB membranes is water vapor permeability or "breathability". The "cutoff" between "permeable" and "non-permeable" products is little-defined and hotly-debated. Manufacturers have added more to the confusion by publishing incomplete information about their products' vapor permeability and by presenting arbitrary vapor permeability classifications as mandatory requirements.
Carlisle Coatings & Waterproofing has prepared this chart to help architects and engineers in specifying the permeability of WRB membranes. The chart provides full disclosure of the vapor permeability properties of CCW's membrane air barrier (WRB) product line and a comparison of these properties to reference values in Building Code.
When choosing a suitable membrane, however, building scientists say permeability alone should not be the determining factor. Membrane properties which are often more important than permeability include adhesion to substrate, water resistance, detailing, seal around fasteners, chemical compatibility, and installation technique.
Ultimately, the project's wall assemblies need a WRB/air barrier membrane which will provide a durable and continuous system and will function effectively under the project-specific conditions.