I have been amazed at the resiliency of cob to hold-up in the rain. In Arivaca, the rainfall totals on average per year are about 15-inches. This past year (2014) we had over 20-inches. Some of the walls of this studio have been exposed to the elements for 5-years and the photo below shows a typical section of exposed wall.
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The downfall with the traditional lime-plaster here in Arivaca where temperatures fluctuate more than 40-50 degF a day, is that it eventually separates from the wall due to differing rates of thermal expansion between the plaster and cob wall. And thus every couple of years the entire building requires new application of lime plaster.
So I'm developing an experimental method, I hope will be over time will require only light maintenance.
The photo below shows a thin layer of cob (clay/straw/water) mixed with colored elastomeric, that I have been experimenting with as an alternative to lime plaster. I'm hoping the exposed straw from the eroded wall will help grab and hold this thin exterior cob (clay/straw) render. This approach is a mind-set change from the traditional lime-plaster render, that although breathable, provides a uniform 'seal' to cob/adobe buildings.
Last summer I applied a test coat of this mix on parts of the south wall to see how it would hold-up during the summer monsoon season. The photo below shows how the cob render with added elastomeric (about 1-quarts per 1/2 wheel-barrow of cob), did exhibit some erosion; but I don't think I put enough elastomeric in the summer test mix. So going forward, I'm adding 2-quarts of elastomeric to the 1/2-wheel barrow of cob.
The photo below shows most of the east side of the building rendered, with the just a quarter of the building on the right side still needing to be completed.
The photo below shows the remainder of the east side of the studio with the completed render.