Impact Resistance of EIFS
In the early days of the EIFS industry in North America, the damage resistance of EIFS from impact was a big issue. Architects and building owners want to know how EIFS fared when walloped, and competitors, looking to regain market share lost to the growing use of EIFS, fueled the fire by trying to make an issue out of impact resistance. EIFS manufacturers looked for solutions to the impact question and found them. This month's column describes the basics of what happens to EIFS when you bash it, and will no doubt give you some ammo for explaining the impact issue to those who still have questions.
Contrary to popular belief, the mesh in an EIFS basecoat is not used there primarily to provide impact resistance. In fact, a functional but not very impact-durable EIFS-type cladding can be built with mesh only at the insulation joints. This requires some snazzy basecoat formulating, such as integral fiber reinforcing, but it is possible. The primary function of the mesh is to keep the edge of the insulation pieces together as they expand and contract. Without the mesh at the joints the coatings would crack at that point. The problem with not using mesh in the field of the insulation boards is that even a mild hit there would crack the EIFS coatings. It's simply not practical to put mesh only at the insulation joints, so don't even think about being cheap and trying it.
Glass mesh is used as the EIFS basecoat reinforcement for a number of technical reasons, among them being strength, stiffness and resistance to fire. A stretchy type of mesh, such as one made of plastic, doesn't reinforce the basecoat quite so well. However, certain plastics are immune to the alkali effects of the Portland cement in many basecoats. Because of its alkali sensitivity, glass mesh is coated with protective plastic. The plastic is what gives various meshes their various colors. Also, a hint: Don't assume that because the mesh is blue that it comes from a certain EIFS manufacturer, or that because it is white that it has no coating. Some coatings are clear, thereby showing the "white" of the glass, while various colors of mesh can be had by one and all.
The amount of glass, usually expressed in ounces per square yard, that is used in EIFS manufacturers' "standard-weight mesh," is enough for most building locations. In other words, how much impact resistance does an EIFS need at the top of a multi-story building where no one is present except window washers ... However, there are a multitude of areas on buildings, including near grade, at balconies and at roof parapets, where additional impact resistance is needed. Hence the universal recommendation to use heavier grades of mesh at those areas.
When the heaviest grades of mesh are used, you can really clobber the EIFS before it gets damaged. Even when it does get damaged, in splits and crushes, rather than cracks. In other words, the damage does not tend to propagate past the immediate area of contact. This is due to the flexible, synthetic nature of EIFS. Compare this behavior to numerous other common materials that are brittle. But some people still want more impact resistance. People have asked me many times about how to really beef up the impact resistance of an EIFS wall. Here are some interesting examples of some solutions that have actually been used.
Alternate solutionsThe sensible thing to do near grade areas that clearly will be subject to continuous abuse is not to use EIFS there at all. I realize that this may sound like blasphemy, but you can do the rest of the building with normal EIFS, and just use EIFS finish over concrete near the sidewalk bus stop. The EIFS finish area will look like the rest of the "full EIFS" walls, and will be essentially bulletproof.
One surefire way is to replace the glass with Kevlar. Kevlar is a man-made material and is a type of plastic. Kevlar is the stuff used to make bullet-proof vests. It's incredibly strong and also incredibly expensive. If your wallet is thick and you are truly paranoid about someone beating on your EIFS wall, it is possible to get Kevlar in an "open weave" form that can be used in EIFS.
Another way is to add yet another layer of heavy-duty EIFS mesh on top of a layer that already has heavy mesh. And then cover it all with standard mesh. This is the simplest solution, but begs a further question: What are you really trying to do? For example, the real objective may be to keep someone for going through the whole wall, and not just from beating up the EIFS. Thus the weak link in penetrating the wall may not be the EIFS, but rather the sheathing and substrate wall. A devious perpetrator with a Skil Saw and a masonry blade can cut through EIFS with ease. If the substrate and supporting wall are also lightweight, then getting all the way into the building is a no-brainer. Hence it's not smart to use EIFS for bank buildings, unless the substrate is concrete or block.
But there is a middle path. If you want to deter penetrating through the EIFS and into the stud cavity, try screwing lath onto the substrate, and then bonding the EIFS to it. Not only does this make an incredibly strong wall from a wind resistance standpoint, it is also hard to get past the lath by digging into the wall. Anyone who has demolished a stucco wall knows how ornery a cement material can be that is reinforced with steel.
Another way to avoid impact damage is via the architectural design route, namely, protecting the EIFS for impact contact in the first place. Examples abound and include embedding concrete-filled pipes into the ground as barriers near garage doors, using heavy-metal flashing at sills, parapet caps and balcony railings, and planting shrubbery to keep people and machines away from the EIFS.
You should be aware that other factors, besides the weight of the mesh, determine the impact resistance of an EIFS. The thickness and resin level in the basecoat also affects the result. Thick basecoats and resin-rich basecoats work best. The thickness of the insulation and its type also make a difference. Thick, springy insulations absorb--and give back--more energy and thus perform better. Even the springiness of the studs and sheathing is a factor. Flexible studs and sheathing work better than hard, stiff walls.
A separate issue comes up often which is related to impact, namely, how do I know if the heavy mesh was installed, and how do I determine its actual resistance in numerical terms? The standard answer to the former question is to cut into the wall and see. Obviously this damages the new wall and must be repaired. A sensitive ear can detect the difference in sound between a heavy and regular mesh area that is produced by tapping the EIFS. The heavy mesh area also feels stiffer, if you push on it.
The French connectionThe French have a real solution, though, for determining if the heavy mesh is really there. More specifically, it really determines how much force the wall can take. It's done by using a portable gizmo called a Perfotest. It's a solid rod within a tube. The rod is powered by a spring, and slides inside the tube. The tool is held against the wall, and the spring is used to make the rod smack the EIFS with a prescribed amount of force. If the heavy mesh is indeed present, the rod will bounce off the EIFS without hurting the lamina. If it's not, then it goes through. Various tips can be placed on the end of the rod.
The small diameter tips exert more force on a given area, while the large ones spread out the impact. Thus it's possible to use various tips on a Perfotest and gradually work up to a point where the tip penetrates the lamina. The Perfotest is a handy device and can be used on vertical walls in the field; very handy for resolving disputes about what mesh is actually in the lamina.
In contrast, the standard way of doing an impact resistance test in North America is to drop a rod with a round tip within a tube from various heights and to see how high you can go before the lamina is penetrated. This is fine for lab tests, where the sample can be placed horizontally, but doesn't work well in-place on existing vertical walls. Quite a bit of testing has been done in North America using this type of apparatus, which is similar to ASTM's Gardner impact test that is used on paint coatings. Designers usually specify impact resistance either in terms of the impact resistance required of the EIFS (example: so many foot-pounds of impact force), or the weight of mesh to be used (example: minimum 18 ounces/square yards).
Finally, when doing impact tests on existing walls, you need to do a lot of them on a variety of wall areas to get meaningful results. The thickness of the basecoat will naturally vary somewhat from place to place. Even the exact spot where the impacting device hits the lamina affects the results. If you hit the lamina directly over the intersection of two mesh strands, you'll get a higher reading than if you hit the mesh in the "hole" between the strands. It's thus easy to see why one would need to do a lot of tests so that the results are statistically meaningful.
There's an interesting flip side of the EIFS impact issue. Let's assume that the EIFS does get damaged. How hard is it to fix? Not hard at all, actually. You don't have to take the wall apart, or order some one-off oddball cladding to replace the damaged one. Try replacing a custom metal extrusion in a metal curtainwall system with a new one and you'll see what I mean; sit down first when you get the bill.
The impact resistance of EIFS is an issue that, for practical purposes, was resolved back in the '80s with the widespread availability of heavy grades of EIFS mesh. The impact resistance of a standard EIFS is certainly adequate, especially when used with heavy EIFS mesh in damage-prone facade areas, to allow the use of EIFS on almost all buildings.