Elastomeric Plaster: Silver Bullet or Blight?
A few years ago I was gratified to see a Dateline NBC television news magazine segment outlining the failure of elastomeric plasters and the resultant lawsuits against the manufacturers. The problem, it seems, is that the more tightly you encase your house in impermeable sheathings, the more quickly the structure will deteriorate. That's common sense when you give it a moment's thought: water trapped behind plaster will adversely affect the structure of the wall itself. And water, as we all know, will get behind plaster somehow, sometime.
We seem often to forget that plaster is intended to be protective and sacrificial. It should be designed and installed for the benefit of load-bearing parts of the building. For years now I have been commenting ("ranting" according to some) about problems caused by Portland cement renders over adobe. Now, ironically, I find myself in a position of advocating for Portland in certain situations where it is an alternative to the elastomerics.
Unlike Rastra-block, an innovation which I believe to be a generally reliable product that is stridently and misleadingly advertised, I find absolutely no redeeming characteristics in any of the elastomeric products which share a similar promotional hyperbole as Rastra. For starters they have those notorious "limited" warranties that are rendered invalid if the weather was not just so, the building was idiosyncratic, or if the applicator was just in a cranky mood that day. And when this stuff fails it is visually spectacular; buildings look as though they have been struck by the blight.
But, as with any silver bullet solution to a problem, elastomerics are sold as the fix. Here are some the problems we get called upon to fix a year or so later.
Elastomerics have an amazing propensity to peel, a condition that begins at one weak spot where the material is poorly adhered to the substrate. This usually begins under windows or canales at spots where water is concentrated and has more options for entering a pinhole or a poor connection to wood or flashing.
Sometimes, in its most visually appalling state, the acrylic material forms blisters that grow, sag and sometimes burst. I will refrain from describing the appearance of the fluid that emerges and will comment only that this is one time when the word "pathology" can be used to vividly describe an architectural problem.
Elastomerics crack. This, given the promotional material you get from the Internet or the supplier, is the most egregious failing. Being flexible, elastomerics are hyped to be crack-proof. I suspect that they are when applied over sheets of steel and kept at a constant temperature. But buildings move -- new buildings especially -- and as they do, cracks appear. If the substrate to which an elastomeric is applied cracks in anything over a hairline, it comes right on through the "crack-proof" coating. That opens the door and invites the moisture in for a long-term residency.
There are many, many more characteristics of elastomeric coatings than I have room to disparage here. When they first came into common usage about 10 years ago I hated them for their appearance: The colors are false to my eye because they are so uniform; the surface lacks depth and does not respond to the environment by temporarily streaking in a rain storm or by absorbing sunlight -- on the contrary, elastomerics tend to reflect light, and that in a rather pallid way. Finally, in my view, buildings should weather; the most attractive buildings are those that weather well and achieve a bit of a patina.
So, if you won't use mud or lime for plaster (at least on adobe), use Portland cement. But if you are considering elastomerics as an alternative to cementitious stucco, don't. In many cases, preparation of your walls for the acrylic involves the use Portland for the base coat. It will pay you to be consistent and finish the job with the same material.
More on elastomerics and buildings as systems.