COLUMN BUILDING SCIENCES been countless failures where the PHOTO 1 Lumpy Stucco Disaster. Stucco layer with adhered stone installed over a synthetic building paper over oriented strand board sheathing (OSB). Water held in nooks and crannies leads to a greater water load than workmanship was exceptional and traditional stucco. No drainage or capillary break or hygric redistribution. Photo 2 Disaster Made Worse. Interior supervised continuously by third plastic vapor barrier preventing inward drying coupled with a 2 x 6 insulated frame wall reducing energy flow parties.‡ The failures are now so prothereby further reducing drying. nounced we have social media stirring up the pot.§ So how is the stuff installed? The stucco layer is installed over a sheathing and water control layer... then the veneer is adhered with "thin set" mortar to the stucco. The thin set mortar is applied to the back of the veneer and then the veneer is "smooshed" into the stucco layer. Now it gets interesting. The patterns of failure are distinctive. The patterns of failure are rainwater exposure related. The failures are predominantly concentrated where there is greater rain exposure. It gets better. The failures are almost always FIGURE 1 Drainage Mat. The resulting gap provides drainage thereby addressing hydrostatic pressure concerns, provides a capillary break along with providing at the veneer-thin set interface (Photos 5 and 6). And, horizontal and vertical hygric redistribution-"aka" drying of the thin set mortar it does not matter whether the veneer is real stone or and stucco layer. manufactured. Isn't that amazing? Workmanship failure only at the Interior Lining-Gypsum Board with Latex Paint veneer-thin set interface and only where there is greater rain exposure? No workmanship failure at the thin set mortar to stucco interface-only at the veneer-thin set Cavity Insulation interface. And it is worse where the sheathing or base the stucco is installed over is less vapor permeable or Sheathing less water absorptive. More bond failures occur where the stucco is installed Water Resistive Barrier (WRB) over OSB than plywood or gypsum sheathing. More bond failures occur where the stucco is installed over Drainage Mat with Filter Fabric (Or Building Paper Over Drainage Mat) rigid insulations such as extruded polystyrene (XPS) and polyisocyanurate. Apparently workmanship is bad only Stucco Layer where we have more rain, only at the veneer-thin set interface, and predominantly over OSB or foam sheath"Thin Set" Mortar ing. This is big news. Apparently, use of OSB and foam sheathing causes bad workmanship. Thin Stone/Manufactured Stone Veneer ‡One project had three different contractors using the same materials and apparently between the three of them they were unable to find anyone to install the stuff. Really? And money was not the issue. Very high-end project. Supervision was supervised by other supervision. So how long does it take? Anywhere from six months to two years. Sometimes right away. Not right away a §https://www.facebook.com/Manufactured-Thin-Stone-MasonryVeneer-Failures-698582356878745/ Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Westford, Mass. Visit www.buildingscience.com. O C T O B E R 2 0 19 ashrae.org ASHRAE JOURNAL 71https://www.facebook.com/Manufactured-Thin-Stone-Masonry-veneer-failures-698582356878745/ https://www.facebook.com/Manufactured-Thin-Stone-Masonry-veneer-failures-698582356878745/ http://www.buildingscience.com https://www.ashrae.org