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.
2. Isn’t the quality of the American Clay finish heavily dependent on the quality of the substrate? In this case, the adhesion and overall quality of the American Clay will be based on the quality of the earthen scratch and brown coats. Does the makeup of those coats effect the finish?
It is true that the American Clay finish is dependent on the quality of the substrate, but that is true of any finish plaster or stucco. Typically the “makeup” of the coats will determine whether it will work or not as a sound base. If the substrate is both sound and solid then the American Clay will attach well. If the base is loose and not very sound, there would be issues.
3. How do you handle wet areas with American Clay such as bathrooms and kitchen walls near sinks? Do you have to treat the finished surface of the clay to waterproof it? Howe does that effect the overall performance of the product?
With the clay, the areas to seal and protect are areas that are going to be regularly exposed to items that will saturate, stain and degrade the material. Behind a kitchen sink or a bathroom sink (or even around a toilet with young boys and even men that tend to miss often). For the rest of the areas in the kitchen or bathroom, using the clay to help mitigate and disperse the moisture is an advantage. Keeping in mind that good ventilation to allow the moisture to dissipate, after a hot shower for example, is still important.
The type of sealer determines how the permeability would be degraded. A silicate would work great for water but does not block oil stains very well, yet it remains very permeable. Similarly, a limewash works for water as well, but not oils. Acrylics are another option but move away from the natural finishes. They do protect against both water and oils; however, they can reduce moisture transmission out of the system. Oil based sealers, like polymerized linseed oils are great for protection against all of the above but do not breath. Wax is a good option, but limits additional coats over the area later.
4. I’ve heard that up to 4 coats of finish are required to get the product to look as beautiful as it does in your advertising. Is that true?
No, with a sound, even base coat, one or two coats is all that is necessary. If it is taking more coats, the base was either very coarse or not very level. That said, the base may be two coats thick, so the overall plastering process from start to finish can be considered 4 coats.
5. Is there a wait time in between coats for the product application? In other words, does the subsurface need to dry first before it is re-wet for the plaster application?
The base does need to be dry prior to the applications of American Clay. The need to pre wet your base is dependent on the site conditions (i.e. the difference between Seattle during the wet season, or Tucson in July. I would not pre wet my base in Seattle, but I would in Tucson).
6. Are there adhesion problems if the base coats are not earthen? Can AC be applied over a lime scratch and brown coat? What issues might one encounter?
The base does not need to be earthen. Our plasters go over a lime brown without issues, as long as it is a true brown coat with adequate key. If it is a smooth lime finish, it would require additional prep prior to lay application of either a primer to provide tooth or a bonder to provide adhesion. Cement brown coats will work in the same manner as lime. Gypsum brown coats can be an issue, and require a sealer prior to the clay application. For the mentioned exceptions above, the clay would begin to pull away from the brown coat upon application of the second coat if the detailing noted above was missed.
7. How do you wash the walls over time?
The required washing of the walls that we are used to with paint is not generally required with any natural finish. A light brushing or vacuuming of the wall to remove cobwebs and dust attached to texture on the wall is typical. For stains or marks, a very well wrung sponge used to wipe the surface will most always do the trick. For deeper stains a 25% white vinegar solution sprayed on the surface will help. Allow this to dry, and then wipe with the sponge mentioned prior. If it is a stain that cannot be removed with washing, the plaster can be removed and patched with reserved material from the original project.
8. How do you change colors over time if you decide to change a room color? Can the walls be painted or color washed?
The are many ways to change the colors over time. Painting with any number of materials is possible. A color wash worked into the clay works well for subtle shifts, and if a complete change is desired, another coat of AC plaster can be applied.
(Note: Straw Bale Innovations is in no way affiliated with American Clay and this post is purely for informational purposes only)