A sheet, spray or trowel-applied membrane or material layer that controls the passage of liquid water even after long or continuous exposure to moisture. (OK readers, gadflies and other building science crazies, I need a performance metric or series of performance metrics. Do we go down the route of the window industry? Or do we go somewhere else? Email me with your thoughts—seriously—lets get this one figured out before the next decade is out: joe@buildingscience.com.)