The Finish Line: Impact
When EIFS was first introduced into the North American market in the late ’60s, there was skepticism about its performance. Designers and specifiers asked, “How well would such a lightweight wall cladding perform when subjected to the usual rigors that wall systems are exposed to?”
One of the areas of concern was resistance to damage by being impacted by external forces, such as banging or hitting the EIFS with tools, people or vehicles. It was common back then for competing wall systems to demonstrate the ease with which the EIFS lamina could be penetrated by a sharp object, such as a screwdriver. This was an impediment to marketing and acceptance of EIFS as a bona fide, yet new, product. This concern led to a search for ways to improve the impact resistance of EIFS, and this month’s column tells about that search.
The reinforcing mesh in the EIFS basecoat is there for two reasons. The primary one is to keep the lamina from cracking at the insulation board joints, due to the expansion and contraction of the insulation boards with temperature changes. The other reason is to provide additional impact resistance; the EIFS lamina itself being too weak and thin to resist ordinary impacts.
Initially, the obvious way to increase impact resistance was to use multiple layers of mesh in the basecoat. This required multiple passes on the wall, and did increase the impact resistance-but not enough. A more robust EIFS was needed, but how?
FOAM DENSITY AND BASECOAT THICKNESS AND HARDNESS
Increasing the density of the foam insulation helps somewhat in terms of impact resistance but it also increases the stresses in the mesh between ends of the insulation boards, increasing the tendency for the lamina to crack. It also increases the cost: more density equals higher-priced foam.
Increasing the thickness of the basecoat and its hardness also improves impact resistance. It uses more basecoat adhesive and more cement (to increase hardness) but also makes it more brittle. Not a good solution either.
In Europe, where modern PB-type EIFS was developed after World War II, this issue had already been addressed before the use of EIFS in the U.S. had occurred. It took a few years to uncover the solution. The solution was to use special heavy-weight mesh specifically engineered for use with EIFS. Sto Corp. led the charge with its Armor Mat product, followed closely by other EIFS producers who found similar mesh sources and started promoting them, too.
This new mesh was too thick to be used in a single layer because the overlapping of adjacent mesh pieces (needed to prevent cracking of the lamina) would create a bump in the wall, and would look bad. The solution was to put a base layer of heavy mesh in its own basecoat on top of the foam-without lapping the ends of the heavy mesh pieces-and then to cover the heavy layer with another basecoat using normal mesh. It is a two-step basecoat process, and is now the standard way of doing it.
MESH WEIGHTS AND CORNERS
Ordinary “light” EIFS reinforcing mesh weighs about a quarter to a third of the heavy-weight meshes. It is fairly limp and can be bent around tight corners. The heavy mesh is thicker and much stiffer. The increased stiffness of heavy mesh makes it hard to bend around corners or to put it in aesthetic reveals or complex decorative foam shapes. To get around this bending problem, factory pre-bent L-shaped pieces of mesh are offered by the EIFS producers.
The most vulnerable part of an EIFS is an exposed outside corner. Edges of openings, such as doors and columns with EIFS cladding, are prime victims of impact damage. Multiple layers of reinforcing mesh can be wrapped around these edges but there are limits to how much the edge can be beefed up. Sometimes, the best solution is not to use EIFS at all in such locations or to cover the EIFS with something hard.
The former solution-not using EIFS at such locations-can involve using stucco, masonry or concrete with EIFS finish. This is common near sidewalks, and at decks and balconies.
The latter solution can involve metal or wood plates attached to the corner. Heavy-gauge sheet metal or extruded aluminum angles will work. The most attractive way to attach them is to use a structural adhesive, in a caulking type form. I’ve seen pop rivets and screws used but they usually work their way free, unless you can get them all the way through the EIFS and into something solid in the substrate.
Another solution is to put something sturdy in the way so impacting objects, like shopping carts and barbecues grills, can’t bang the EIFS. Pipes embedded in the ground or sidewalk will work, as will cast stone bumpers at the base and waist-height of the wall.
In terms of measuring the impact resistance of EIFS, ASTM has a method that is a modification of a test originally designed to measure the denting susceptibility of sheet metal. The test method is designated as ASTM E2486, and uses a vertical tube in which slides a pointed metal rod. The EIFS specimen is placed horizontally and the rod dropped through the tube from various heights. The impact performance is judged based on how high the rod can fall before the EIFS lamina is breached. The heights are broken down into categories as shown in the reprint (Photo 1) of the table in the ASTM document. The categories roughly correspond to the normal, medium and heavy-weight meshes commonly used in EIFS. The “J” (a “joule”) and “inch-pound” are technical measurement units for force, and are System International measurement units (aka metric) and American Technical measurements (pounds, inches, etc.), respectively.
This test is done in a lab, and unless a sample is removed from a building wall and laid flat, it has no use on existing EIFS installations. It is primarily a research tool and is also used in specifications for indicating which areas on a building need extra reinforcing mesh. The European “Perfotest” test method, described below, can be used on existing in-place walls, as well as in a lab, and is a more useful test method.
Dade County is where Miami is located. It is the leading building code jurisdiction in the region, and has special regulations for hurricane issues. One regulation has to do with the affects of flying debris on buildings. In particular, Dade County has a test whereby a 2x4 wood stud is shot at a vertical wall mock-up at high velocity by an air cannon. (An air cannon is a laboratory device that uses high-pressure air to propel the projectile rather than explosives.) It’s a fun test to watch, especially on windows. The pass/fail criteria is that the wall not be “breached” by the 2x4. For EIFS, the term “breached” is defined as the 2x4 making its way through the EIFS, past the exterior sheathing and into the stud cavity.
This Dade County test is widely used in hurricane-prone areas, and is applicable to many forms of wall construction, not just EIFS. A normal EIFS wall, with studs, sheathing and light reinforcing mesh, cannot pass this test. The EIFS needs to be substantially beefed up, which is typically done by reinforcing the lamina and substrate. This test is also widely used in other hurricane-prone areas, and thus is a precedent-setter.
In Europe, EIFS is regulated in many countries by the Union Européenne pour l’Agrément Technique dans la Construction, which is a network of independent institutes, centers or organizations that are engaged in the issue of technical approvals for innovative construction products or systems. This group is a forum for many countries to work together and to develop standards for a huge range of products, including EIFS. This international cooperation helps promote commerce, and makes product development and marketing much easier.
In Europe, the UEATC calls for using a handy, hand-held impact-testing device called the PerfoTest. The PerfoTest uses a spring-powered indentor gizmo to whack the surface of the EIFS (see Photo 2). If the EIFS surface doesn’t break as a result of the hit, then the proper, specified mesh weight is in the lamina. It’s a clever method, because if the wall is OK (i.e., is sufficiently impact resistant), the EIFS won’t fail. Thus, this test method is nondestructive if the wall is sufficient.
The photo shows the indentor rod being released. The PerfoTest tool is held tightly against the wall, the indentor rod is pulled back against a spring, then the indentor rod is let go, and it whacks the wall surface.
A highly desirable feature of this test method is that it can also be used on vertical, in-place walls. The ASTM method described earlier requires the EIFS sample to be placed horizontally-hardly a wall-like position. This makes the PerfoTest useful not only off-site in laboratory tests, but for QC on job sites. This method has not been widely accepted in North America but remains a standard method in many of the European Union countries.
The quest for a more impact resistant EIFS continues. Exotic materials, such as Kevlar and carbon fiber, have tried to get extra strength into the EIFS lamina, but their cost is off the scale and is really only suitable for specialty uses. Plastic meshes have been tried too, but they don’t fare well in fire tests.
One of the nice design features of EIFS is that you can place extra reinforcement where you need it. Many other wall system designs have a fixed amount of impact resistance. Thus this “use-it-where-you-need-it” aspect can be a real cost saver for EIFS.
The impact resistance of EIFS used to be a big issue but has faded as an anti-EIFS marketing issue. This makes sense, since most parts of walls-such as upper stories-don’t need much impact resistance anyway. W&C