The Effects of Wind on EIFS Systems
In Ontario, when addressing the issue of wind effects on EFIS systems, the topic turns to stud and sheathing substrate systems. The reason for this is that on solid substrates like concrete and masonry, the substrate is so strong that it basically is impossible to pull off an adhesively attached EIFS. It is easier for the foam to come apart internally before it lets go of the substrate. However, stud and sheathing substrate systems have much less “connecting” surface area for the wind forces to act on, making it possible for serious damage to occur.
The Problem of “Wind”
First, let’s define the problem of wind in more detail. It’s a generalization to say that wind is the problem. It’s the suction force caused by a certain kind of wind that pulls a cladding off. Positive wind pressure, meaning wind that blows perpendicularly to the wall, pushes the cladding into the supporting wall, while negative wind forces, also known as suction, tend to pull the cladding off. It is interesting to note that suction forces, given the same basic wind speed, can be greater than the positive wind pressure. It is also worth noting that the wind force is higher on the back of the building and at the corners. Wind forces also increase with the height of the building.
The windiness of a building site varies with the geographic location and can vary tremendously within a region. Coastal areas, especially those prone to hurricanes and tornados, tend to be windier on average. Large urban areas, where the wind swirls and accelerates between large buildings, also experience a high degree of wind. Building codes publish tables of basic wind speeds of various areas but 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.
EIFS and Wind
Traditional EIFS is applied to a continuous surface: the sheathing. Positive wind pressure simply pushes the EIFS into the sheathing. As long as the sheathing and studs hold up, the EIFS has nowhere to go and it is not likely that any amount of observable wind will crush an EIFS.
Suction forces are a different matter. It helps to think of an EIFS-clad wall assembly as a chain consisting of the 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 attachments states that when using weak materials, like foam insulation, one needs to distribute the force over a larger number of attachment points. This maxim is also exhibited in the construction of older, all-metal airplanes, which 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 adhesive is spread over a large percentage of the foam’s surface area and any suction forces acting on the surface of an EIFS are counteracted by a large percentage of attachment points, compared to the area being acted upon by suction forces.
In contrast, consider what happens when mechanical anchors are used. The fastener must go through the foam and into a structural material. If the sheathing is structural material, such as plywood, then many fasteners 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 around 1 to 2 per square foot. The result is a much smaller percentage of attachment points compared to the overall area being acted upon by suction forces.
Furthermore, because mechanical fasteners hold onto the foam at a “point,” the wind forces are concentrated at that point. Since the foam quite brittle, it acts as the weakest link in the chain and the wind will often pull off the EIFS while the fastener stays connected to the building, also pulling the fastener head right through the foam.
A similar problem exists with the sheathing. 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.
With this understanding of the mechanism of wind failure, it follows that if an “EIFS” comes off with the sheathing still attached, then it’s not the EIFS that has failed but the sheathing. In other words, it is crucial to ensure that the sheathing is adequately attached before installing an EIFS. 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.
EIFS and Code
The 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.
Increasing the “Pull-off Capacity”
It is possible to combine the use of both adhesive and mechanical types of attachment. 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 before the “springier” fasteners can get a good grip on the foam.
Higher density foams also generally make for a wall with higher wind pull-off resistance. However, many types of EIFS do not work well with higher density foam: Such foams may better resist mechanical fastener pull-through but they bring along other disadvantages, such an increased tendency to crack.
With all these considerations in mind, here are some good suggestions for increasing the wind-load capacity of an EIFS:
- Use a stronger substrate, by selecting a thicker or inherently stronger material.
- Reduce the stud spacing if possible, to reduces the force on each sheathing fastener.
- Simply use a lot of fasteners to hold the sheathing onto the studs.
- Make sure the studs do not bend too much. Deflection of the studs worsens the pull-off forces of wind. As a general rule, light-gauge, shallow-depth studs, over long spans, is the worst.
- Use adhesives instead of mechanical anchors, wherever possible. Mechanical anchors may be strong enough for most residential applications, but simply cannot do the job alone on tall commercial buildings.
With the use of these measures in the design and installation of EIFS-clad buildings, it is possible to ensure that even the worst wind conditions will not prevail against the building’s EIFS exterior. Such designs have been tested and proven, and are frequently installed these days around the world.