Success With EIFS, continuedFiberglass mesh. There are many types and grades of fiberglass mesh. It's all made with glass fiber and an alkaline-resistant polymer coating, but the weight of the glass and the thickness of the protective coating vary. So does the way the fabric is made: Some mesh is woven (Figure 3), and some is non-woven.

Figure 3. Woven reinforcing mesh (top left) serves to stop the cement and acrylic base coat from cracking. A coating protects the mesh against the alkaline cement, but mesh may still be weakened by moisture and chemical attack. A worker trowels the mesh into the wet base coat (bottom). For good coverage, the authors recommend applying the base coat in two layers: one layer before troweling in the mesh, and a second layer immediately afterward. Exposed mesh (top right) tends to lose strength over time.

Heavier mesh is stronger and stiffer than lighter mesh, and mesh with a thicker coating generally holds up better against the alkaline chemistry of cement compounds in the EIFS base coat. We prefer non-woven mesh because its coating is more continuous: The woven mesh can have gaps in the coating where the fibers contact each other in the weave. Unfortunately, it is difficult to find mesh manufacturers willing to fabricate non-woven mesh. Manufacturers generally offer a "standard" 4-ounce mesh (weighing 4 ounces a square yard), which is actually their lightweight mesh. They also offer higher-weight mesh types that they describe as medium-impact, high-impact, and so on. There is also a very light 2.5- to 4-ounce "detail mesh" that is used for corners, V-joints, and the like. It's common for installers to use the 4-ounce standard mesh in the field and use detail mesh wherever they need to bend around corners (ironically, the corners and joint areas that are most likely to crack often receive the lightest-weight mesh). We usually recommend a minimum 6-ounce mesh. For the first floor, or at least the first 4 feet of the wall — really, in any high-traffic area where there are likely to be lawn mowers, weed trimmers, snow shoveling, and other activities that can damage the wall — we call for high-impact mesh. You can't overlap the high-impact mesh, so you butt the pieces together, then cover the whole thing with an additional layer of the

6-ounce mesh. With that reinforcement, EIFS is almost as strong as some stucco systems, particularly a one-coat stucco. For corners, some manufacturers provide specially formed pieces made of the heavier mesh. If you have to wrap a corner with lighter mesh, you can double it up. The mesh strength is important, because it is the only thing keeping the surface lamina from cracking and allowing water penetration. Contractors may try to convince the owner that the 4-ounce standard mesh is sufficient, but our experience is that the 6-ounce or heavier mesh achieves better results. Base coat. Base coats are usually formulated with an acrylic resin combined with Portland cement. They may also contain silica sand and lime or lime byproducts. Most manufacturers offer a choice of several base coat types. The applicator can either add water to a dry mix, or add just the dry cement to a premixed wet formula that already contains water, the acrylic resin, and other ingredients. In our experience, the wet mixes are far superior to the dry mixes. We never specify a dry mix. The proportions of cement and acrylic in the base coat formulas have been an issue in recent years. European manufacturers traditionally use no more than 25% to 33% cement by weight in a base coat formula, but U.S. manufacturers typically use 50% cement. High cement content makes the base coat harder but also more brittle and more prone to cracking. In addition, the highly alkaline chemistry of cement attacks and weakens the fiberglass mesh, which also promotes cracking. Some U.S. manufacturers have developed lower-cement-content base coat formulas recently, but so far, testing has not shown these modified mixes to perform any better than the original mix. So the best advice we can give at present is to use any manufacturer's standard wet mix, but not the dry mixes.

Sealants. Joint sealing is an issue in all construction, but especially with EIFS. EIFS has been promoted as a maintenance-free cladding, and it's true that the EIFS itself often doesn't need much maintenance. But sealant joints do need maintenance, and they usually don't get any. Building owners often don't notice when the sealants fail, and sometimes they just don't care enough to maintain them well. But with EIFS, sealant joints are difficult to maintain even when someone does want to. To repair a caulk joint, you have to scrape it out or grind it back, and that can damage the EIFS surface. So the bottom line is, you need to use a sealant that will last without a lot of maintenance. You can choose

from sealants made with acrylic, polyurethane, polysulfide, or silicone. For several reasons, silicone is the best choice. First of all, silicone has a service life many times longer than other sealants can expect. It's a stable mix that normally doesn't break down from ultraviolet rays in sunlight, and it will tend to retain its properties for decades to come. Silicone has been on the market for many years, and our company has rarely if ever seen it break down. Second, silicone has better properties to begin with. In particular, it's very flexible, and it stays flexible at cold temperatures. In very cold weather, the EIFS panels shrink, the joint has to enlarge, and the sealant has to flex; but urethane or acrylic sealants typically get hard in cold weather, while silicones remain flexible. Silicones, unfortunately, tend to attract dust and dirt, and most silicone sealants can't be painted. On the other hand, you can get silicone sealants in colors to match just about any EIFS color coat or paint. Use 100% silicone sealants, not the "siliconized acrylic" varieties.

Installation Concerns

Materials are important, but most EIFS problems are caused by incorrect installation. The major manufacturers all have installation specifications, which we are generally comfortable with. They also offer installation details that over the years have gotten quite specific and comprehensive. When you do EIFS work, it's important to study those details carefully and follow directions conscientiously. One common flaw is a base coat that is too thin and does not fully cover the mesh. In one broad survey of 80 buildings in 34 states (conducted in the 1980s), our company found that 92% of the current projects had base coats that were too thin. On projects up to three years old, only about 50% had thin base coats; and on projects between three and eight years old, about 40% had thin base coats. Basically, the more recent projects more commonly had thin base coats. Apparently, as the market became more competitive, people had stopped trying to apply the material according to the instructions. Contractors reduced the amount of material they applied in order to cut costs. A base coat that is too thin will not properly resist water penetration or protect the reinforcing mesh. And if mesh is exposed to sun and rain, it will lose strength. Specs typically call for a base coat thickness of 1/16 inch (ours call for 3/32 inch). It's important to enforce the thickness requirement. However, a base coat that is too thick (5/16 inch or more) has a tendency to crack because the lamina becomes too stiff.

We recommend applying the base coat in two layers (Figure 3, above): First apply a thin layer, and then partially embed the mesh in the wet coat; later apply a second layer over the partially embedded mesh and trowel it smooth. The second layer fills in the pin holes in the first layer and covers the mesh well. For an installer who is trying to do a good job — a nice smooth layer of adequate thickness — this two-step application is just about as fast as trying to get good coverage with a single layer, and you get a much better product. It's especially important at transitions and joints, because exposed mesh and poor coverage at those points will reduce the performance of sealant joints.

Joints in the EIFS System

Whether they're part of a drainage system or a barrier system, EIFS walls seldom leak in the middle. Most leaks occur at joints and transitions. In our extensive studies, more than 97% of all water damage was found under windows; around decks; at wall-roof intersections; around penetrations for plumbing, electric, and the like; or at failed movement joints. Window leaks alone accounted for more than half the damage. Correctly installing and sealing all those joints will greatly reduce the amount of water intrusion. EPS board joints. EIFS walls have a large number of naturally occurring joints where EPS board pieces butt together. Those board joints are covered with continuous base coat and reinforcing mesh, but they are a possible cracking point for reinforced laminas. Stresses are high at window corners, so don't break the EPS board at the edge of a window — instead, cut the window corner out of a continuous piece of board. Also, don't break the EPS board over a joint in the sheathing, and it's best not to break the sheathing itself at windows or other openings. Movement joints. EIFS itself doesn't require many movement joints. Unlike traditional stucco, which needs a joint every 10 to 15 feet to allow for thermal expansion and contraction, EIFS can have a jointless run of 50 or 75 feet, so you seldom need expansion joints on an EIFS house. However, EIFS does need joints between floors in wood-frame construction. The EIFS itself doesn't move, but the wall plates and band joists of wood-frame houses develop significant cross-grain shrinkage. So an EIFS house should have a joint at every floor, as shown in Figure 4, to allow the EIFS to move instead of buckling or cracking.

Figure 4. The EPS board in an EIFS system can buckle when floor framing shrinks, causing cracks in the surface lamina, unless a movement joint is built into the system. There should be a 3/4-inch gap between the sheathing and EPS board, located at or above the midline of the band joist. A strip of peel-and-stick membrane should lap over the drainage medium on the lower floor, and under the drainage medium on the upper floor. The expansion joint between EPS boards should be sealed with backer rod and silicone sealant.

The other thing we've found is that when two large EIFS panels connect at a narrow point, the resulting stress concentration can cause cracking at the narrow section (Figure 5). At such locations, we like to install an expansion joint to allow wall areas to move independently.

Figure 5. Window corners and narrow intersections between large wall sections are points of stress concentration where cracking may occur. If possible, the design should allow for isolation joints to relieve stress at such locations. In any case, the secondary weather barrier must provide protection for wall materials beneath the EIFS.