Over the years, many ways have been developed to improve the impact resistance of EIFS. A related issue was the whole matter of "how to test how much impact resistance a given EIFS has."

In this era of increased concern about the physical security of buildings, this month's column will focus on some of the key aspects of the subject of the impact of EIFS.

The first, and key thing, to understand about walls in general, is that most parts of most walls do not have a need for much resistance to impact damage: There simply isn't much that is likely to run into the wall except a drunk pigeon. When any potential source of impact damage is present, it is usually from people, vehicles, varmints (such a woodpeckers) or equipment. Thus, any EIFS that will shed rain, look good and insulate, will do its job, most of the time. However, there are some common wall areas where there are exceptions and these include:

• Near sidewalks and walkways

• At entranceways, corners and doors

• Wherever vehicles are moving about, including cars, trucks, lawn mowers, snow blowers and forklifts

• Where equipment is stored, such as shopping carts, bicycles and BBQs

• Where people "hang out," such as roof top gardens and balconies

• Where equipment, such as ladders and swing stages, come in contact with the EIFS. This includes numerous common types of maintenance operations, such as window washing.

Where's the beef?

The primary way to increase the impact resistance of an EIFS is to add more reinforcing mesh to the basecoat. The so-called "heavy duty" EIFS meshes weigh four to five times as much as "regular" mesh and give about that many times more resistance to perforation. Since heavy meshes are too thick to be lapped at their edges, the edges are then "butted" (not lapped), and the heavy basecoat layer is overlaid with another layer of normal weight mesh. In other words, heavy meshes are never used alone in a single layer.

However, there are limits to how much mesh can be placed in a single base layer. This is an issue worth understanding. The problem is that the mesh is mesh. This sounds dumb, but the truth is as follows: If you "up" the glass level (i.e., used an even heavier amount of "glass" per square foot), the holes in the mesh shrink. This would make it hard for the basecoat adhesive to "key" into the mesh. The practical limit for glass-per-layer is around 20 ounces per square yard, which understandably, is the heaviest mesh commercially available.

The type of basecoat, be it cementitious or noncementitious, also matters, noncementitious basecoats, being more flexible, tend to dent rather than crack. The thickness of the basecoat is also a big deal. The mesh needs to be fully embedded in the basecoat adhesive, in order for the mesh strands to dissipate the forces that they are subjected to. As I've pontificated numerous times before, a good, beefy, thick, basecoat, in the European tradition, makes for a much better EIFS wall and this includes in the area of impact resistance.

The type of foam insulation also makes a difference. Generally, the denser the better. However, almost all foams now in common use in North America are EPS, which has a low (1.0 pcf) density. Up through the '80s, the now-rarely-used Type PM EIFS, which used a higher density, extruded PS foam, and thick, cementitious coatings, was one answer to the impact problem.

This brings up the question of, "How do you know how much impact resistance is in an installed EIFS wall?" One way to see how much impact resistance (i.e., mesh) is actually "there," in a given, in-place EIFS installation, is to cut in and look. Is this practical? This mars the wall and requires ugly patching. A pre-emptive way to be sure how much mesh is there is to inspect the heavy mesh areas as they are being installed.

There's another way, a nondestructive way, for in-place walls, to determine how impact resistant a wall is.

Europe's testing

There are many off-the-wall, in-the-lab tests for impact resistance. These tests operate on a wall mockup that is oriented horizontally (unlike a real wall, which, by definition, is vertical), and thus are unsuitable for in-place testing. To digress, I suppose some building owner might allow a huge section of his new EIFS wall to removed and tested, but chances are slim. These horizontal tests are thus most for laboratory-type, product development purposes.

The most common, horizontal, lab-type EIFS impact test uses a rod that is dropped through a tube from various heights. The drop-height, at which penetration occurs, is a measure of impact resistance. This test was originally used to measure adhesion of paint to sheet metal but actually is fairly predictive of difference in the composition of EIFS wall assemblies.

In the early '90s, the U.S. Army Corps of Construction Engineering Research Laboratory did an extensive series of laboratory-type tests on the impact resistance of EIFS. (You can learn more about this report by logging onto http://www.cecer.army.mil/

td/tips/product/details.cfm?ID=136&LAB=1.) The results confirmed what was already known but at least confirms it in writing. The report does not name EIFS brands but was conducted on a large number of different products.

An interesting observation from testing EIFS for impact resistance is that the substrate makes a difference. EIFS, attached to hard, rigid substrates, fares worse, while light, springy substrates, like some stud walls, fare better. Why? Because with the rigid wall, the EIFS must absorb all the impact energy, while a springy wall bounces back, therefore absorbing less energy. It makes sense, if you think about it. It's sort of like the Zen way of dealing with force: Deflect it rather than trying to deal with it head-on.

Testing EIFS samples for impact resistance is tricky. The test sample needs to be carefully supported, and multiple impacts, "hits," are needed. The various readings are then averaged to give a representative value. For example, if the impacting device hits right onto a mesh strand, the result will be different than if the "hit" occurs in a "hole" between mesh strands. The usual pass/fail criteria for impact testing is "perforating" the EIFS lamina. In other words, you can see the foam. Simply denting the lamina, but not breaching it, is not considered a failure, since water could not get in and the foam would not be affected by the sun.

The usual way of reporting impact test results is not to use hard numbers, but to divide the results into three or four ranges. Since impact testing is full of variables, this gives a more representative indication of the true behavior of the EIFS.

Plastic mesh has been tried, in lieu of glass mesh, to reinforce EIFS basecoats. It works well in some ways, and not well in others. It does OK, from an impact testing perspective, especially in noncementitious basecoats, where the stretchiness of the plastic mesh and the stretchiness of the basecoat adhesive, create an EIFS lamina that dents, rather than punctures. The real issue, though, is that the plastic melts in a fire, making the wall sensitive to fire code requirements that the EIFS stay intact. Hence, the use of plastic mesh has not caught on, because it has, in a sense, caught on fire.

The most unusual impact "test" I've seen occurred at a high school. Some students were outside taking archery classes, and one errant arrow hit a large EIFS wall. It took about five minutes for all the students to give the wall a test shot, resulting in a wall that looked like a porcupine.

The most impact-resistant EIFS I've seen was in a cyclone-prone area of Asia, where Kevlar, the material used to make bulletproof vests, was used. The wall was incredibly impact resistant, and also incredibly expensive; it was almost literally bulletproof.

The recent arrival of Hurricane Charlie in southwest Florida shows what real impact damage can do. In fact, as a result of the even more powerful previous Hurricane Andrew, test methods have been developed that fire 2x4s from an air cannon at a large wall mockups. Plain EIFS, even with a heavy mesh basecoat, does not "pass" but beefing up the sheathing substrate with extra mesh will keep the 2x4 from roaring into your bedroom, which might prove exciting but that's subject for another Walls & Ceilings article.

Editor's note: