Most of my work is in exterior remediation, both as a builder specializing in exterior work and (more and more these days) as a building consultant inspecting and advising owners and builders when moisture problems arise. I see a lot of failures, and most of the hands-on work we do is fixing other builders' mistakes. Daily I am reminded of the need for wider understanding of best practices, and that is especially true with adhered concrete masonry veneer (ACMV). We call this material "stone," but we all know that it is a non-load-bearing concrete cladding made to look like pieces of stone set in mortar that is bonded to a stucco scratch coat. We really need to be treating this material like stucco with chunks of concrete in it. When we lose sight of that, problems arise.
ACMV has been increasing in popularity as a cladding on mid-range and high-end homes for the last decade, and across the Midwest where I work, a huge number of these homes are now showing signs of water-damage because the ACMV wasn't installed correctly the first time. I am an Exterior Design Institute (EDI) third-party EIFS and building envelope inspector, and this work has recently led me to work with relocation service companies. When a person is relocated by a company to a home with an EIFS exterior, it has for a long time been an automatic trigger for an inspection, owing to all the acute moisture problems—often leading to severe mold problems—suffered by buildings with EIFS. Now, at one of the three national companies I do inspections for, ACMV on the exterior of a home has become an automatic trigger for inspection, too. I believe that the problems are worse with ACMV, and they will make the EIFS problems look like a drop in the bucket.
The best practices described in this article apply equally well to new construction, when they should always be used, as well as to remediation. But if you apply them in new construction, you won't have to apply them as a repair.
I've written a number of articles for about both ACMV and EIFS failures. In this article, I'm going to focus on doing it right the first time. But the photos come from a remediation job. It cost $33,000 to repair the problems on this 14-year-old home. Adding a drainage membrane from the outset would have cost only about a buck-and-a-half per square foot installed. That's pretty cheap insurance to keep the envelope dry and performing well, considering the alternatives.
If you get anything from this article, let it be this: ACMV must drain. This is true of any cladding system, but with ACMV it's especially important because all those chunks of concrete hold a lot of water. If that water can't drain, it's more likely to get sucked into the framed wall than it is to evaporate to the outside. Brick veneer works the same way, except that most builders know (and code demands) that there is a minimum 1-inch air space behind the veneer (although a 2-inch gap is recommended by the Brick Institute of America). While there can still be problems with this detail (namely from mortar droppings and squeeze-out that fills the space and creates numerous bridges for water to wick to the framing) for the most part the assembly works to allow moisture to drain to the outside from behind the brick. To create a drainable system, I always use a rainscreen material, such as Keene Driwall, which is shown in the photos for this job. Recently I have been using MTI Cavity Gravity, which was developed for brick walls but also works with EIFS and ACMV. It stays rigid, which makes applying the lath easier and helps to maintain an even thickness for the scratch coat.
A rainscreen material makes the whole wall very forgiving, so you can get away with holes, thin spots in the scratch coat, and other mishaps in the assembly that are bound to occur no matter how diligent you are when inspecting the work. But what you can't skimp on is providing a place for the water to drain at the bottom. In fact, if you provide drainage but don't allow it to exit, you can accelerate the water damage because a build-up of water will concentrate at the base of the walls.
To ensure good drainage at the base of walls, we always apply a weep screed. This is the essential piece that will allow drain water to exit the cladding assembly. At the exterior steps on the house, we had to install aluminum flashing first to bridge over the sheathing transition. This flashing extends down the wall into the drainable subgrade beneath the steps and sidewalk. This gave us a clean surface against which to adhere the sealant after installing backer rod in the gap between the concrete steps and the flashing.
After installing the weep screed, we cover the walls with housewrap and building paper, and finally, install the rainscreen material. Lath and the scratch coat will go over the rainscreen mat, followed by mortar and stone.
I get a lot of flak from builders about specifying both housewrap and black paper, but it doesn't cost that much more, especially if it prevents the high cost of a remediation. Without the rainscreen material, black paper would serve as an essential sacrificial layer over the housewrap, allowing water to drain between the two sheets. By itself, housewrap can form a capillary bond to mortar or when the two materials are pressed directly against each other. Either way, this can allow water to pass through the housewrap.
With a rainscreen material, this is less of an issue, but I like having black paper to slow solar vapor drive. Because ACMV holds a lot of water, it is prone to evaporation toward the inside when the sun is beating down on a wet wall. Housewraps are made to be permeable to promote drying, but that means moisture vapor can pass through them from outside to inside. The black paper will not prevent solar vapor drive, but it will slow it down. If we can slow down the wetting time, we have a better chance of avoiding problems.