Over the years I have heard several people ask about the American Clay line of plasters so I felt it was time to contact them to ask them some of these questions. Here is the Q&A session:
1. How do you handle the fact that clay is more breathable (allows more vapor to pass through it) than typical exterior coatings such as lime and, in some cases, cement based stucco? How do you keep the bales from becoming saturated by moisture that can get into the walls through the clay, but cannot get back out through the exterior coating?
Even though the clay is more permeable, it is also hygroscopic, and holds the vapor in its pore structure. It also has an affinity for water that is greater than that of the straw, thus helping the straw to dry. The clay allows the water to dissipate through a larger area and then move out of the wall system to that of lower vapor as the humidity levels drop to the interior of a building space.
For the moisture that moves through the wall and collects on the exterior, the importance of continued permeability remains high. A lime plaster, even though it is lower than a clay, is still highly permeable to vapor, and also has high dissipation of water, allowing it to move to the outside as the humidity drops to the outside, thus allowing the system to balance and not remain in the wall system.
The importance of having a climate that dries out enough to allow for the dissipation of the moisture is high. Without it, the balance will not be reached and therefore, climate must be taken into consideration when choosing your plaster both exterior and interior.
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In regards to plastering by hand or by spray application, there are several schools of thought. Some people believe that plastering by hand is better because the force of the application is higher when arm power is used to press the plaster into the bales, giving it a high level of “key” or adhesion. Others believe that the force applied by the spraying is actually better because it shoots the plaster deep into the webbing of the straw. I have discussed this question at length with several master plasterers over the years and have learned a lot about both processes. The fact of the matter is that both systems are absolutely acceptable as long as certain details are maintained.
