Most curtainwalls are nonstructural. The World Trade Center was an exception.
Readers may recognize my name as the columnist for the monthly W&C column, The Finish Line, about EIFS. Prior to my immersion into the world of EIFS, I spent many years designing metal wall and window systems for a major wall system producer and installer, a company now called the Centria Co.

The term "curtainwall" comes from the fact that curtainwall-type walls do not support the building (unlike some other wall constructions, such as some forms of masonry and concrete, that do) but rather simply "hang" like a "curtain" and provide weatherproofing and insulation. Some people consider stucco and EIFS to be a form of curtainwall but usually the term means metal and glass systems.

Good knowledge of the curtainwall is helpful in making the entire wall system work as a complete assembly, since, at some point, the stucco or EIFS or other opaque wall cladding meets the curtainwall.

Curtainwall is almost always produced in a ready-to-install form.

Behind this curtainwall

Like stucco and EIFS, most curtainwalls are nonstructural; they do not help support the building. An exception was the World Trade Center, in which the walls did support, at least in part, the building's perimeter. Given that curtainwalls are not intended to support the weight of the floors above, curtainwalls are usually a lightweight form of construction. Although curtainwall can be used on buildings of any height, they are especially suitable for tall buildings, for a number of reasons:

• Curtainwall's light weight, especially on tall buildings, reduces the forces on the foundations, making the whole building lighter.

• Curtainwall is a form of prefabricated construction, which is important on tall buildings in the sense that it is easily installed, like a gigantic Erector Set, at great heights above the ground.

• Curtainwall is an engineered type of cladding, each system being drawn-up down to the last bolt and nut, for a specific building. Some curtainwalls are completely custom systems (like the WTC), but most are some sort of pre-designed system. Either way, a lot of engineering goes into the initial design of these systems and into their use of a specific building. This engineering for a specific building, called "shop drawings," is what allows the countless individual pieces to be manufactured and erected. Shop drawings are also used in prefabricated EIFS and stucco panels. This is in stark contrast, on nonprefabricated stucco and EIFS walls, which are made to fit, at the job site, from hand-applied, on-site-created materials.

• Curtainwall is almost always produced in a ready-to-install form, and thus can be installed year ‘round as discrete building units. Curtainwall also rarely employs "wet" materials, and therefore also can be erected year ‘round.

There are some fundamental differences between discrete building unit-type products, like curtainwall, and apply-in-a-large-continuous-area-type products, like stucco and EIFS. These differences are important to understand in order to produce a good wall.

Stucco and EIFS materials are fundamentally "natural" in their nature in the sense that they do not "move" much in response to temperature; they are mostly "rock" as opposed to man-made materials like metals, glass and plastics. Curtainwall, on the other hand, although also inorganic, tends to "move" a lot. The reason is that most curtainwall is made of aluminum, which has a high coefficient of thermal expansion. In a nutshell, this means that care needs to be taken at the interface between the curtainwall and the stucco-ish products, as the two materials are moving at different rates due to change in their temperature, i.e., the weather. This means that attention needs to be given to the interface between these wall elements, lest water and air get into the wall. The lack of attention to such interface details is frequently the cause of building envelope failure. It's a big deal.

Curtainwall is a precision product, with aluminum extrusion being produced down to tolerances of 1/1,000th of an inch. Stucco-ish products are not in the same ballpark, tolerance-wise. The point is that where these "systems" come together, this difference in tolerance needs to be taken into account.

For example, molded rubber gaskets are sometimes used to "mate" and weatherseal adjacent curtainwall elements. This is possible because curtainwall is a precision product. Gaskets do not work in curtainwall-to-stucco interface because the curtainwall side of the joint is precise while the stucco side of the joint is not. The point is that where these two types of claddings meet must be carefully designed and constructed so that they do not leak.

The many joints in curtainwall require vigilance to protect against the damages of moisture intrusion.

Case the joint

Curtainwall systems have lots of joints in them; stucco and EIFS do not. Every time an extrusion, gasket, piece of glass or a nut/bolt comes together, an opportunity exists for water entry. This means that a well-designed curtainwall must have provision for dealing with water leakage, and they do. This takes the form of weeps and drainage channels, and various ways of equalizing air pressure so moisture does not get in. If it does, then at least there is a built-in mechanism for dealing with it.

To deal with the above interface issue, flashings and caulking often are part of the water management strategy. Usually, relying on sealants alone is risky but can work if the building is well maintained, and the joints are properly designed and installed. Usually, it's smart to have a back-up "seal," such as good flashings.

When designing caulking joints, keep in mind that choosing the correct type of caulking needs to take into account both sides of the joint-namely the curtainwall side and stucco side. The type of caulking used needs to reflect the caulking's affinity for these different materials. In particular, different types of caulking primers may be needed on either side of a single joint.

Another area that needs attention when curtainwall and stucco/EIFS come together is deflection. The term "deflection" refers to the movement of the wall in the plane of the wall, usually as a result of wind. Curtainwall and stucco/EIFS deflect at different rates, causing differential movement where they come together. This location is often where caulking occurs, and thus is critical. Curtainwall and stucco/EIFS also are not the same in terms of their brittleness. Stucco is quite brittle; EIFS is not. Glass is brittle; aluminum is not. When one of these systems, such as a metal stud wall, supports the other, this deflection matter needs to be taken into account, lest one wall component induce cracks in the other.

Curtainwall and EIFS/stucco are very frequently used side by side on the same building. Many of the problems I run into as a consultant have to do with not handling the interface between these two system types properly. This often leads to water leaks, and hoards of lawyers. The key is to look carefully at these interfaces and not to assume that whatever is next to your product will be handled right by someone else (the "not-my-problem" syndrome). Rather, the best approach is to be proactive as a building envelope team, to address these issues before getting on-site and to get it right the first time. In my experience, one of the best uses of a contractor's time is to get together for a preconstruction meeting to make sure that everyone is on the same page. This can help avoid a lot of headaches later.