Wind and EIFS
First, the issue of wind behavior mostly applies to stud and sheathing substrate systems. On solid substrates like concrete and masonry, the substrate is so strong that it basically is impossible to pull off an adhesively attached EIFS-the foam will come apart internally before the foam lets go from the substrate.
Second, "wind" itself is not the problem per se. It's the lack of wind, or suction force, that pulls a cladding off. Positive wind pressure pushes the cladding into the supporting wall, while negative wind forces, also known as suction, tend to pull the cladding off. The odd thing is that suction forces, given the same basic wind speed, can be greater than the positive wind pressure. Also, the wind force is higher on the back of the building, and also at the corners. Wind forces also increase with the height of the building.
Blown awayThe windiness of a building site varies with the geographic location and can vary tremendously in a small area. Coastal areas, especially those with hurricanes and tornado areas, tend to be the worse. Large urban areas, where the wind swirls and accelerates between large buildings, also require special consideration. The building codes publish tables of basic wind speeds of various areas in the country but keep in mind, this information is regional in nature. The wind maps do not show what goes on at a specific site. For that information, local airport data is often used, or better yet, site-specific studies are done using instruments or wind tunnel tests on building mockups.
In terms of wind action on EIFS, traditional EIFS is applied to a continuous surface: the sheathing. Since wind pressure in relation to small areas of EIFS is fairly uniform, the pressure simply pushes the EIFS into the sheathing. As long as the sheathing and studs hold up, the EIFS has nowhere to go. I have yet to see an EIFS that has been flattened by being pushed into a wall by wind.
Suction forces are a different matter and the behavior of the EIFS, and the rest of the wall is another matter entirely. In essence, think of an EIFS-clad wall assembly as a chain: finish, basecoat, foam, attachment system (adhesives and/or mechanical anchors), substrate, substrate attachment, studs, and the rest of the building's structure. If one of these links in the chain breaks, then the EIFS may come loose. Clearly, it's hard to suck the finish off the basecoat. But some of the other links in the chain are more vulnerable to breakage.
One of the maxims in designing attachment is this: When using weak materials, like foam insulation, one needs to spread out the force over a large area. This is why older all-metal airplanes use zillions of rivets to hold the thin sheet metal together. This is also why adhesive attachment of EIFS is stronger than the mechanical anchors. The reasons are several. Obviously, by using a notched trowel or other widely dispersed adhesive pattern, the wind forces coming through the foam is spread over a large area. This is good from two standpoints. First, the bond of the adhesive to the foam is spread out over a large area, and, second, the bond of the adhesive to the sheathing is spread out too.
In contrast, consider what happens when mechanical anchors are used. The fastener must go into a structural material. If the sheathing is structural, such as plywood, then there are many fasteners that can be used in a given sheet of foam. However, with substrates that cannot accept a fastener easily, such as gypsum-based products, the only available fastening points are usually the studs. This severely limits the number of fasteners that can be used. The maximum is 1 to 2 per square foot. This is a lot of fasteners, meaning a dozen or more per 4-feet-by-8-feet piece of foam. This is more than is normally used but it can be done. It's also expensive.
Because mechanical fasteners hold onto the foam at a "point," the wind force is concentrated at that point. Since the foam is not too strong, if the wind does pull off the EIFS, the fastener often stays connected to the building and the EIFS, with foam still attached, comes loose; the fastener head pulls-through the foam. Notice that I said, "Pull off the EIFS." A similar problem exists with the sheathing, as follows:
Like foam, gypsum-based sheathings are not heavy-duty structural materials. The small, drywall-type screws and nails used to attach gypsum products to studs concentrate the pull-off forces at the screws heads. In high wind conditions, it takes a lot of fasteners to keep such sheathings attached to the studs. The number of required fasteners is several times that are required to hold drywall onto studs in interior applications.
Code EIFSThe code people are aware of this sheathing fastening issue, and the technical reports they issue for specific products often contain tables that give the maximum wind load capacity of various sheathing types and thicknesses, as well as for various screw types and spacings, and for various stud spacings. As you can see, there are a lot of factors in play that determine the final strength of an EIFS wall. If you want to see some examples of the kinds of wind load capacities that current commercial EIFS products can withstand, go to www.icc-es.org. This is the Web site for the International Code Council, a building code agency. Look up an EIFS producer's technical report (also known as Evaluation Report). In these reports one will find a table or other description of various EIFS walls, along with the wind capacity of the wall.
The moral here is that if the "EIFS" comes off with the sheathing still attached to the EIFS, then it's not the EIFS that has failed but the sheathing. Thus, it's prudent to make sure that the sheathing is adequately attached. As the installation of the sheathing is not always done by the EIFS installer, some coordination is needed to make sure that the whole substrate system is strong enough.
You're probably thinking, what about adhesives and mechanical fasteners? Yes, that helps. You won't get the combined pull off capacity of both but the value is usually higher than adhesives alone. The reason for this is that adhesives create a rigid bond to the substrate and absorb almost all the force, until the "springier" fasteners can get a good grip on the foam.
Higher density foams also make for a wall with higher wind pull off resistance. The rub is that many EIFS do not work well with higher density foam: The foam may better resist mechanical fastener pull through but higher density foams create other problems, such an increased tendency to crack.
So what do you do if you need very high wind load capacity? There are many options. Here are some:
• Use a stronger substrate. Take it thicker or use a material that is inherently stronger.
• Reduce the stud spacing. This reduces the forces per sheathing fastener.
• Use a lot of fasteners to hold the sheathing onto the studs.
• Make sure the studs do not bend too much. This action, called "deflection," exacerbates the sheathing coming off the studs. Light-gauge, shallow-depth studs, over long spans, are the worst.
• Use adhesives instead of mechanical anchors. Mechanical anchors are plenty stronger enough for most residential applications, but simply cannot do the job alone on tall commercial buildings.
The strongest EIFS wall I've ever designed and actually tested was intended for a tall building in the cyclone-prone area of the western Pacific Rim. Heavy-gauge, brake-formed steel studs were used. Cement board sheathing was glued and screwed to the studs. Then, 2.5 flat galvanized lath was screwed into the studs. The EIFS was then attached using an adhesive. The basecoat used Kevlar instead of glass mesh. This made the whole panel very stiff and almost immune to flying debris. When tested as a full size wall panel using a powerful vacuum machine, we could not break the wall. It withstood more than 300 pounds per square foot. How strong is that? Well, 20 pounds per square foot comes out to about 85 miles per hour wind speed. The relationship between wind speed and wind pressure is not a straight line (40 psf does not equal 170 mph, etc.) but suffice to say that this EIFS wall was not going anywhere. But it wasn't any $10 a square foot wall either.