Buildings as Systems: A Discussion of the Compatibility of Materials
Some time ago I wrote a column that was an unabashed indictment of elastomeric plasters. It provoked a great deal of response. Most fell into the category of "what do we do now?" from folks who have already used the stuff, discovered its inherent flaws and would like to get rid of it. (That's really a tough one since it is very labor intensive to remove, and you don't really want to leave it in place and install a whole new system on top.) There were also a few responses that vehemently disagreed with my assessment. Those were, for the most part, from people with a vested interest in elastomerics -- namely the manufacturers, suppliers and contractors who push the material. I believe this may be a good time to broaden the argument a little to place the role of plasters, and the other parts of a building, in context.
Buildings, regardless of the materials used in their construction, are comprised of various components, which are interlocked mechanically and interrelated functionally. The roof for example, is attached to the walls in such a manner as to allow the latter to provide support and the former to provide protection. One is non-functional without the other. It is important, then, when designing new buildings or repairing older ones that wall and roof be viewed as two elements of one system. All other components of a building including floors, windows, doors, portals, plasters, paints, etc., relate to one another as well. Key to the successful functioning of this system is the compatibility of the materials used in their construction, and by extension, their repair.
To draw an extreme analogy, a building might be compared to a human body. It is common nowadays to transplant organs and tissues from one person into another. However, it is essential that the tissue of the donor biologically "match" the tissue of the recipient. If the tissues are incompatible, the body rejects the organ and the recipient suffers a considerable setback. That does not typically occur when the tissues match.
Similarly, a building constructed of adobe will "reject" mortars or plasters made with impermeable concrete or synthetics. The rejection often occurs in disastrous ways. There are thousands of examples of this in Santa Fe alone. A common pathology seen some years after impermeable plasters are placed over soft adobe is the loss of wall due to water retention. Water finds its way into the earthen walls through cracks, leaky roofs, and broken plumbing, always resulting in deterioration. Left alone, the walls inevitably reach the point where they lose their structural qualities and collapse. The "death" of the wall is usually unforeseen because the problems are hidden behind the self-supporting hard plaster.
I have heard many times from advocates of Portland cement that it is permeable. Of course it is -- relatively: Relative to plastic, relative to latex, relative to polypropylene. Relative to the earth in an adobe wall or compared to a mud or lime plaster, it is highly impermeable. There is a very simple test that you can use to demonstrate this principle. Using bathtub caulk, attach one open end of a clear plastic tube to the surface of the material to be tested. (You can buy an official Rilem tube if you want to spend twenty-four dollars. I've used cut up turkey basters and screw-type garlic presses with equally good results.) Once attached, fill the tube with a measured amount of water and time the absorption. When you test your adobe (assuming it is non-stabilized) you won't have long to wait; while you wait for the Portland test to end, you could beneficially occupy your time critiquing or, better yet, re-writing the Uniform Building Code as it pertains to soft materials; for the elastomeric -- don't wait, just go on about your life.
So don't be deceived by the claims of permeability in Portland or the so-called Gore-tex effect in elastomerics. Both will crack, both will open at joints with wooden lintels and trim, both will eventually let water in and not let it out.
When you contemplate the materials going into your new adobe home or for your restoration project, keep in mind the concept of compatibility. Be wary of latex paints, for example, and of sprayed-on insulation on roofs. Think of the building as a system that operates smoothly when all the components can easily adjust to one another.