This post may not be espoused by adobe traditionalists, but I decided to use a cement-based 'stucco' over wire mesh for the exterior rendering of the cob wall. However, I believe I have a good case for doing so.
The traditional material for exterior abobe-wall finishing is Lime plaster. The rationale for using lime plaster on adobe is that it is a porous material and thus will allow the wall to breathe - removing moisture from the wall. In cement-based stucco, it doesn't breathe, thus the concern is moisture will build-up between the cement and the cob wall resulting in a loss of adhesion of the cement stucco from the trapped moisture.
I previously used lime plaster for the exterior after completing my adobe block out-building on this same property, but found-out over time that the lime plaster on the South and West facing walls were falling-off every two years. This would be, in part, attributed to the 40-50 deg.F., temperature fluctuation here in Arivaca, Arizona, which results in thermal expansion-contraction of the lime plaster relative to the more stable adobe wall behind it. So essentially the lime plaster worked itself free of its adhesion to the wall due to thermal expansion and contraction until it brakes-off, or the plaster collapsing. This would necessitate a maintenance project to re-apply the lime-rendering every few years which took time away from working on this cob studio
I don't want to repeat this lime-plaster failure problem on the studio, so I changed my approach and decided to go with cement stucco. Here is my rationale for why i think the cement-based stucco will 'perform' over time on my studio:
1) Per a previous blog post, I documented how I wrapped the entire cob building in wire mesh in order that the stucco has something to grab onto and hold it in place (not done on the abode out-building).
2) And in regard to the 'moisture' theory, I believe this may not be an issue in the dry climate of Arizona and compared to more temperate thus humid climates of the northern and coastal regions of North America.
Also moisture from the interior comes from people living in these adobe structures. Whether it's showering, cooking, or plants. However, the function of this building will only be as a multi-media studio, not a residence so I'm counting on minimum moisture entering the cob walls from normal household functions.
Finally. I just want robust exterior protection, not just from the weather, but insects, especially termites that are everywhere in the ground here. Also, I'm getting older (63 y/o) and do not want to spend later years back on scaffolding repairing failed exterior plastering.
Time will provide the final judgment.