In "The Coming Stucco-Pocalypse," Joe Lstiburek goes long and deep on solving problems with today's stucco exteriors. This is a must-read if you work with stucco in any capacity. Lstiburek dives not only into the physics of the forces acting on stucco, but also into the chemistry of how stucco has evolved from a porous, vapor-open material to a polymerized, more water-resistant one. But as you'd expect from Dr. Joe, he makes it easy for us to follow the salient points.

Here's the basic outline in excerpts:

Traditional lime-based stucco was three layers: scratch coat, brown coat and finish coat. Each successive layer to the exterior was more vapor open than the layer it covered. Yup. Old timers had it dialed in. Today? Not so much.
The new recipe does not always follow the old rules [each layer more vapor open than the one it covered]. Sometimes the finish coat and brown coat are less permeable than the scratch coat depending on what is added and how much. Oops.
Could we make it worse? Yes, of course. We can improve building papers. We could make them dimensionally stable. When building papers (aka “water-resistive barriers,” or wrb’s) were hygroscopic they expanded and contracted and stucco did not bond effectively to them
It became clear that there were issues with wrb’s and the first intervention was to use two layers – an outer layer that would act as a bond break and an inner layer that was the “true” wrb.

These excerpts give you the flavor but you need to read the original for the whole picture. The most important take-away is the solution: Every successful stucco job requires an assembly--a veritable sandwich--consisting of stucco, bond break, drainage mat, and wrb.

Read more. "The Coming Stucco-Pocalypse" is an easy, but important, read.

To learn more about getting stucco right, see:On traditional stucco:
Stucco That Works
On synthetic stucco:
No Leak EIFS
Synthetic Stucco Without Failures