I recently visited a job that had a wall assembly showing exterior veneer stone installed over cement scratch coat applied over metal lathe. My interest lay in the detail that followed, the metal lathe was installed over what looked like a polypropylene spun fabric that was applied over a corrugated barrier, looked like MTI’s Perforated Control Cavity but different, that was overlying 30# felt that was applied over standard Tyvek stapled to the plywood. Is this overkill? Couldn’t the Tyvek be eliminated? What is the purposed of the polypropylene fabric? Augment the drainage plane behind the stucco with a plane before and after the corrugated panel? Structural to augment the metal lathe to avoid cracking?

To increase the R-value of the same wall, if one were to apply 2 inches of rigid panel XPS directly over the plywood, could this same order be followed and eliminate the Tyvek as it would serve as the WRB? Setting up a metal lathe/stucco or stone veneer wall 2 1/2 inches away from the structural member of the wall concerns me. Despite the “outsulation” concept I don’t see it done. Even with longer fasteners spaced accordingly, does the weight moved so far away from the wall pose a possible weakness? Would it be wiser to use a lighter EIFS system in this situation? Thanks for any input/ constructive comments.