Of the many topics and products covered by Kawneer’s Commercial Training Department, one that has resonated the most in recent years has been a segment called “Preventable Curtain Wall Failures”.
We all know the cost of call-backs and remedial work can often make the difference between profit and loss. And all too frequently it feels like glaziers are required to accept responsibility for things they cannot control. Contrary to this feeling, when fabricating and installing a curtain wall, the most common causes of system failure are entirely within the glazier’s ability to address and prevent.
I’d like to share with you the most common sources of system failure as reported by our installation managers and curtain wall engineers. These do not lie with the building structure itself or with other trades, but can be addressed and prevented by the glazier.
The first source of potential failures — Seals around and within the curtain wall system. The perimeter caulk joint serves two main purposes: preventing air/moisture from penetrating the façade of the building and separating dissimilar materials. Since curtain wall systems are required to be able to move with the building, wind load drift, live load deflection and creep and shrinkage can all strain and potentially tear inadequate perimeter seals.
Shop drawings and installation instructions will indicate the sizes and locations of the perimeter seals in relation to the framing members.
- If the project’s shop drawings indicate a ½-inch perimeter seal to allow for the aforementioned field conditions, installing a mere ¼-inch joint can lead to leaks, condensation and even glass breakage.
- There should be no gaps in the perimeter seal, nor should it interfere with the ability of the system to weep (or drain) out any infiltrated water.
- The perimeter seal should also isolate the exterior and interior framing members from each other so cold exterior air is not in contact with the interior facing jambs, heads and sills — thereby reducing the potential for condensation.
Other sealant related failures can result from inadequately cleaning the contact surfaces and from using sealants that are not compatible with all the materials that they come in contact with. Omitting any of a curtain wall system’s critical internal seals that contribute to the overall air and water tightness of the system can also result in failures.
<-- Be sure to apply all of the manufacturer's critical seals as shown on their installation instructions. Each seal contributes to the air and water tightness of the system.
At right, an example of the seals at the horizontal joint in one of our curtain walls. Note: the sealant over the fastener head and around the joint plug. The omission of either can provide a seam to allow air and water to be drawn into the system.
A second source of potential failures concerns the assembly and installation of the components themselves.
Shims: Simple decisions like supplying shims of the proper composition and installing them at the correct locations can prevent unnecessary stress on the frame and the glass. Non-load-bearing shims can compress. Wood shims can deteriorate. Shims located under the anchors and not between the anchors and the mullions transfer excessive loads to the anchors themselves and to the perimeter fasteners.
<-- The shims at the mullions should be located between the mullion and the anchor.
The function of the system anchors is to attach the curtain wall to the building and to transfer its load to the structure itself. If the shims are located under the anchor, the weight of the system will bow the anchor, and put excessive stress on the perimeter fasteners themselves.
Setting Blocks: Glass failure is too often the result of installing setting blocks not designed for the system or from locating them improperly. Kawneer’s setting blocks are designed and tested specifically for the individual system. Their size and hardness is critical. Setting blocks without the correct hardness can strain the lites. Setting blocks of the incorrect size can create improper pressure points on the glass lites or the spacers.
Gaskets: Most gaskets are designed to be cut into individual pieces not to be run continuous around corners. Stretched gaskets of insufficient length will shrink-back. Gaskets run around corners will pull away. Gaps can allow air to infiltrate and condensation can result.
<- Unless using a system with molded gasket assemblies, one continuous gasket will pull away at the corners and allow air and water infiltration.
A third source of potential failures concerns water management.
Because of the manner most curtain wall systems control water, it is critical to properly locate weep holes in pressure plates and covers. It is also imperative to establish and maintain the required air seals in the system. If the pressure plate fasteners are not those designed for the system, are not located as specified in the installation instructions, or the proper level of torque is not applied to them, air can infiltrate and draw moisture with it.
<- The weep holes in the pressure plate and cover are also offset in a stair-step pattern to prevent any water from being blown back into the system by wind currents.
Some applications, such as impact framing, require closer spacing of the pressure plate fasteners. In many cases the 9” on-center is reduced to 6” or even 3”. If the proper torque is not applied, a fastener in every hole will still not prevent air and water infiltration.
In Kawneer’s 100+ years, we have visited our share of jobsites. Too often expensive remedial work is the result of something as simple as the lack of attention to what is thought of as a minor detail. What are some areas of concern for you?
I’d love to talk to you more about how we can work together to prevent failures and ensure the success of a job.