Why does anyone actually build with this stuff?
Stucco is wonderful stuff, and has been used for thousands of years in hot, dry countries; Roman stucco is still visible in Pompeii. Stucco is also cheap; in Vancouver, British Columbia, it was used on thousands of condos built in the nineties, where it caused the Leaky Condo Crisis. A major public inquiry concluded that the crisis caused "a litany of horrific experiences, personal tragedies, and dashed dreams" endured by homeowners. It hasn't gone away either; there are still thousands of units that need repair.
Usually, people who write about such things as building science are dry and technical, but then there is Joseph Lstiburek of Building Science Corporation. He looks at the problem of using stucco on wood, which is still being done. He says we are heading for another stucco-pocalypse. But first, a bit of history:
We used to put the stucco over brick and stone. If things got wet, so what? Nothing to rot. And the walls were not insulated. Lots of energy flow. Lots of energy available for drying. Lots of drying. Life was good. Then we started to put stucco over wood. Wood rots. But it does not rot unless you get it real wet for a long time. We didn’t get the wood real wet for a long time. And more importantly it was real wood. And we didn’t insulate the walls. Lots of drying available even if the real wood got real wet... We then insulated. And we insulated. And insulated some more. This reduced the ability of the assemblies to dry when they got wet.
Traditional stucco was permeable to moisture, so if a wall got wet, it would dry out. Then they added acrylics and binders to it that turns it into a skin on the outside of the building, so the OSB board rotted and the insulation got soaked. It was never really suited to wood construction but people keep trying. The manufacturers of stucco keep coming up with systems to provide a drainage layer, but it really isn't enough. Lstiburek concludes:
Things are getting worse. But things have to get intolerably bad before we change. We didn’t learn from Vancouver. I predict that they are going to get intolerably bad sooner than later. The stucco-pocalypse is coming.
He is still doing stucco, but on top of a 3/8" air space. When I worked as a developer and an architect, I wouldn't touch the stuff, and after the Vancouver crisis I have never understood why anyone would. If you are going to build a rainscreen, why go with stucco on OSB? In Arizona, on top of block, maybe. But in Cascadia? Surely it is still asking for trouble.
But nobody knows this stuff better than Joe. Have a good read at Building Science Corporation.