As you know, most EIFS is adhesively attached. This method is preferred for many reasons, not the least of which is the lack of fastener show-through, the greater ease in making a flat wall and its higher strength. Yes, adhesives are stronger. The reason is simple: They spread out the wind force over a greater area, instead of concentrating the force at single attachment points, as fasteners do. The problem is the foam insulation, which is not particularly strong itself and there's only so many fasteners one can put in a 2-feet-by-4-feet piece of insulation. It's the same reason that lots and lots of rivets are used on the thin metal skin of airplanes-it reduces the concentration of stresses on the weaker material.
When EIFS producers go to the building code groups to get their product approved, they are required to demonstrate the strength of attachment of the EIFS to various substrates, such as concrete, block, brick and various sheathing boards. Solid substrates are rarely an issue, but lightweight studs and sheathing substrates are a different story.
Test models
Determining the attachment strength of an EIFS wall assembly can be done using physical testing or mathematical analysis, or both. Florida, in particular the Dade County (Miami) area, has special wind testing requirements, due, not surprisingly, to the presence of hurricanes. Other parts of the country also have wind testing requirements, and some nearby areas follow Miami's lead. To satisfy the code people, many EIFS producers opt for testing their various EIFS wall assemblies using large panel mockups, usually 4 feet by 8 feet. These panels form one side of a large box-like test chamber and are subjected to vacuum pressure that tries to pull the EIFS off the sheathing and framing. Eventually, something "gives" and the assembly is considered to have failed at that pressure. The pressure at the point of failure can be converted into a rough equivalent wind speed, which gives an idea of how strong the wall is. A safety factor is applied to this failure force, and that reduced strength value is used to engineer EIFS walls on actual buildings.The problem with these laboratory tests is that they are just that: lab tests. They do not reflect in-service conditions, and hence do not reflect issues, such as the specific design of a building, or the effects of poor workmanship, or the deleterious effects of the entry of water. To answer the question posed by my hurricane-ravaged clients, something different is needed in terms of testing. Building a vacuum test chamber onto the side of a building, while possible, is a huge undertaking, and is rarely done. Recently, a mechanical method of subjecting a section of an EIFS to a pull-off force has been under development, and has been used with success on existing buildings. This field "pull" test method works like this:
A 2-feet-by-2-feet plywood sheet is glued to the face of the EIFS. The EIFS is cut-through at the edge of the plate, all through the substrate sheathing. A lightweight metal frame is attached to the wall and a hand winch is connected, via a cable, to the plywood plate. The winch is used to apply a force to the plywood, which in turn tries to pull apart the EIFS and substrate assembly. A meter, located in-line with the winch cable, is used to measure the pulling force in the cable as the force is applied. This force can then be used to judge how well attached the wall assembly is to the supporting building structure. This method is destructive by its nature, and obviously the test area will need to be rebuilt after testing is complete.
This method normally is used on a number of areas on a given building and thus can be used as part of an overall investigation plan to give an idea of the condition of the walls in general. Obviously, this method needs to be used with care so that the results are obtained correctly and properly interpreted.
The test method described above is under development by the well-known ASTM organization and has been used for over a decade in an informal way by a number of wall consultants. The method has thus been validated through actual use; it is not an untested test method. This method works for flat areas in the field of the EIFS and not for foam shapes, joints and corners. However, it is a whole lot better than nothing. This method also has the advantage that the force can be applied up to a predetermined limit. If the wall still stays together, then it can be repaired without removing and replacing the whole EIFS and supporting substrate system.
(Not) gone with the wind
One of the special characteristics of EIFS, in terms of it's capacity to resist wind forces, is that it is difficult to evaluate the condition of an existing EIFS and its supporting substrate without taking the wall apart and thus destroying it. Unlike curtain wall and other systems that are made of discrete elements that can be inspected and engineered, EIFS is a handmade, laminated system that defies easy investigation as to its condition. Hence, some sort of "system" test, like that described above, is needed to evaluate existing EIFS installations.The most common modes of failure in this type of test are two-fold. If the sheathing is damaged, for instance, due to water infiltration, then often the sheathing fasteners will remain attached to the studs and the EIFS, with the sheathing still attached, comes off in one piece. A similar failure mode is when the facing of the sheathing is damaged and the EIFS attachment adhesive lets go from the sheathing (i.e., the sheathing stays in place). It's rare, although possible, for the EIFS to delaminate internally. This can happen, for instance, if there is something drastically wrong with the adhesion between the various layers of which the EIFS is composed. An example of this could be a basecoat adhesive that froze after it was applied but before it had cured or if somehow the cement/adhesive ratio of the attachment adhesive was way off.
Another variation on the above in-place test procedure is to remove a small section of the EIFS and supporting wall and take it to a lab to run "pull tests" on it. This can be done on large sections of the wall, and also on small, hand-size "core" samples. The problem with small samples is that they may-or may not have-as a result of the discontinuous nature of EIFS adhesive patterns, a representative amount of EIFS adhesive holding them together, or may not have any mechanical fastener at all in the sampled area. Thus, the results tend to naturally be misleading unless a lot of samples are carefully tested. The larger ASTM field pull test, using the winch system described above, gets around this problem somewhat by used a large test sample area.
Work is under way to develop other ways of determining the pull-off strength of EIFS and their support wall structure on completed buildings. Advanced techniques, using infrared sensors and acoustical signals, have been tried but so far the results have been disappointing. One of the desirable attributes of a "better" way of assessing the attachment would be to do so nondestructively. In other words, not to damage the wall just as a result of the testing. This makes a lot of sense if the wall is OK-don't break something that's already broken. The ASTM method under development above is destructive, so one has to be willing to damage his building just to prove that it is OK. This is expensive, time consuming and leaves scars. Perhaps in the future it may be possible to simply point a special high-tech "camera" at an EIFS wall and be able to tell how well bonded it is. But for now, the method of choice is applying a simple outward force using a winch and observing what happens as the wall is taken to its limit.
