Field Notes Spring/Summer 2022June 28th, 2022 | Category: Architects' Guide to Glass and Metal
When Less (as in Smaller Joints) Can be More Trouble
By Chuck Knickerbocker
You might know that Ludwig Mies van der Rohe was one of a handful of modernist architects coming to the fore in the years after World War I. He is famous for coining the expression “less is more,” which he actually picked up from a mentor. Within the glass and glazing industry, we should pay him more respect than we do since many of his designs during the 1950s used some of the first glass curtainwalls.
While we all have our interpretation of what “less is more” means (for me, it’s more about simplicity in design and execution), it only tells half the tale. Things can go south quickly when a project doesn’t find that sweet spot Mies was aiming for. This is especially so for rough openings and the windows or curtainwalls they hold.
Why? The manufacturers detail the rough opening on their shop drawings; those are reviewed; the framing is fabricated and eventually assembled, either in a factory or on a jobsite; and when it fits in the hole, everyone is happy. If only life was that simple.
Yes, the rough opening is called out on the drawings. And it’s likely-beyond likely-that the sub creating the rough opening and the sub doing the glazing are not the same. Do you think there’s any discussion beforehand about controlling those rough openings? Two of my employers being window fabricators, and we don’t see a lot of pushback on this.
Why is this Critical?
If the window fits in the opening, can’t you just caulk it in and call it good? No, no and no! There are rules of nature and product performance that impact joint size and require attention. I love the line from Death of a Salesman: “Respect must be paid.” It will be if the joints aren’t sized correctly.
Metal windows expand and contract, so the size of the jamb sealant joints must be able to stretch or shrink with that. That same joint must also flex with the deflection of the frame member due to the design windload. Head joints may be sized to accommodate movement of the structural frame, live load deflection or seismic events. A building may never see the extremes of the design parameters, but the safe bet here is not to gamble and instead size the joints to those design parameters.
The sealants also have a limited ability to deal with movement from any source. Most silicone sealants have a movement capacity of +/-50%. There are those with -50% /+100%, but the -50% will control.
The shop drawings may account for all of those factors. Good practice says they should.
On the other side of the equation is the size of the rough opening, or RO, as it’s often called on shop drawings. It’s best to build the RO with a construction tolerance of -0 inches (never smaller) and always with a “+” tolerance (let them be bigger!). From designers down to the window manufacturer and installer, all parties must agree on this. It involves the general contractor and the other trades building the openings and takes a dedicated coordination meeting before constructing the openings.
If that path isn’t followed and the windows are installed with smaller joints, it’s likely the sealant will fail in service, and that’s bad. Window installers should train their people never to install a window in a smaller-than-detailed RO. I know that’s not going to be popular, and getting the RO size corrected will delay the schedule. So what’s the path of least resistance? Put the window in and let it leak or have the pre-construction coordination and attempt to never deal with smaller openings?
When I was introduced to 3D BIM modeling, I asked a question that created a dilemma for the instructor. He had shown us how to model the RO, and then put the window in it without a sealant joint. When I raised my hand and said that when we submitted our 3D windows, and they were pasted into the model, they were going to be smaller than the windows shown. The instructor didn’t have an answer. It’s been a few years since that occurred, and I don’t
use that software enough to know if they ever resolved that type of conflict.
Allow larger sealant joints than the nominal, smallest-size-they-should-be joints shown in the shops. Bigger joints will never fail due to heat, movement or sealant capacity. Smaller joints will, and less water in the building is more than we can hope for.
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 with numerous architects, building owners and subcontractors from development of schematic design through installation. He can be contacted via email at firstname.lastname@example.org.
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