Field Notes Fall 2020

Not as Simple as It Appears

By Chuck Knickerbocker

As appealing as recessing perimeter window/curtainwall details may appear on paper, I’d like to appeal to the architects out there to reconsider detailing of this nature for a number of, what I hope you’ll find to be, very legitimate reasons. This is especially true when the glazing system’s details are hidden under or beneath exterior or interior finishes, giving the appearance that the glass is just magically coming out of the head/jamb/sill (as the case may be).

For the sake of this argument, we’re not talking about any one particular system. Captured or structural silicone glazed (SSG), unitized or stick-built, curtainwall, windows or storefront—when the perimeter conditions are buried in the surrounding construction, in addition to creating opportunities, there can be some challenges. One person’s opinion: it may not be worth the risk.

First, in the example mentioned above, recessing the glazing system under or behind the exterior or interior finishes may dictate the sequence of installation. The general contractors will take exception to that and, indeed, some have. Regardless of the type that’s being detailed, the window system must be installed and shown to be weather-tight (via field testing) before the construction on either side of the glazing system can be completed. Without the adjacent materials there, the system can be accessed to affect any repairs. The same would be true if the detail was found to be leaking later. Removal of the finishes to access the system is problematic.

Second, the same is true for installing the glass the first time, and also later, should a lite of glass break. Re-glazing most likely will require removal of some of the exterior finishes, if the system is glazed from the exterior or the interior. I got a dollar that says the owner is not figuring the cost of removing and repairing those materials in the operational budget of a completed project. The owner is not recognizing it’s a cost associated with a glazing replacement.

Third, at the sill, when the glazing system’s detail is buried under the finished exterior materials, the ability of the sill to drain water out of the system might be blocked. Since many systems rely on water to drain at or near sill conditions, either at vertical mullions or at midpoints along the length of horizontal framing members, blocking the exit paths of the water isn’t a good idea. When the sill was above the finished materials, it was likely to be draining onto them. Now, below the surface, the glazing system still requires the same means/methodology to allow the water to drain. Putting a drain mat against the sill where the water exits the system might help. But over the years, if that mat or the detail in general collects dirt, and that drainage methodology ever gets blocked, it’s going to back water up into the system. Again, unclogging it requires removal of the exterior materials.

The sequencing and replacement issues are just as applicable at head and jamb conditions as they are at that sill. Water drainage issues aren’t a concern at most jamb conditions, as the water can be directed to the sill. There’s a different problem at jambs. If the frame deflects for windload, positive or negative, how might that impact the adjacent finishes? Between the glazing system and the surrounding conditions, a sealant joint is an obvious “filler,” positioned flush with the adjacent materials. But, if the specs call a maximum deflection of 3/4 inches or more for spans greater than 13 feet, 6 inches, that sealant joint width should be sized for those expected deflections, right? Taking into account the 50% movement capacity of the most forgiving sealants in the marketplace, that joint just turned into 1.5 inches wide. And I’ll bet that’s not going to be aesthetically acceptable.

At head details, there’s another factor in play. The deadload for the adjacent materials typically are supported by the structure above, while the curtainwall/window/glazing system is gravity-loaded to the floor below. That means the system anchor at the top of the glazing is slotted in some form or fashion to allow for differential movement due to either thermal expansion of the glazing system, live deflection of the structure above, or expected seismic movement. Under these conditions, while the detail often shows perfect alignment of  the glazing framing and the adjacent construction, that’s not likely to be reality once constructed. Nor is it likely over the longterm once it’s in service.

Speaking for myself and perhaps other frame suppliers and glazing subcontractors, we want to do these distinctive details— really, we do. Please consider keeping the frame outside of the adjacent construction. But again, if the owner’s willing to pay for it, they can have it as it certainly is an attractive detail. But, let’s have a conversation about them before including them in the contract documents at the start of a project, not after the “issued for construction” documents are on the street.

Chuck Knickerbocker is the curtainwall manager for Technical Glass Products (TGP), a supplier of fire-rated glass and framing systems, along with specialty architectural glazing products. With more than 35 years of curtainwall experience, he has worked successfully with numerous architects, building owners and subcontractors from development of schematic design through installation. He can be contacted via email at

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