Closing In the Building: Curtain Wall and Glazing

Once a building's structure is up, the glazier starts closing in its exterior with curtain wall, the grid of aluminum framing and glass that hangs off the structural frame rather than supporting any of the building's structural load itself. Getting the building closed in matters for more than just appearance: until the envelope is weather-tight, the interior trades waiting to start drywall, flooring, and finishes can't move in behind a wall that's still open to the weather. A curtain wall isn't a single wall panel either, it's dozens or hundreds of individual units, each one engineered to flex slightly with the building's wind sway and thermal movement without cracking or opening a path for water.

Curtain wall arrives on-site in one of two ways, and the choice changes the entire installation schedule. Unitized curtain wall is assembled complete, glass and framing together, back at the factory and arrives ready to hang, letting a crew install somewhere around 300 to 500 square feet of wall in a single day. Stick-built curtain wall instead arrives as separate framing members and glass that get assembled piece by piece in place on the building, a slower process that typically covers only 80 to 150 square feet a day. Unitized systems get chosen for tall buildings on tight schedules precisely because that installation speed matters, while stick-built systems stay common on smaller or more custom jobs where the factory tooling cost for unitized panels wouldn't pay for itself. Either way, every joint between panels gets filled with joint sealant engineered to stretch and compress with that same building movement without losing its seal.

A curtain wall doesn't try to stop every drop of water from getting past its outer surface, that would be nearly impossible to guarantee over the life of a building. Instead, most systems use pressure-equalized design: an air gap just behind the outer glass lets any water that gets past the first seal drain back out through small weep holes rather than pushing further into the wall, the same logic a mason relies on with flashing and weep holes in a brick wall, just applied to an aluminum and glass system instead. A thermal break, a strip of less conductive material set into the aluminum framing, keeps the cold outer frame from directly touching the warm interior frame and causing condensation inside the building. Both details work invisibly once the wall is finished, which is exactly why they're inspected closely during installation rather than left to be caught later, since neither one can be verified anymore once the building envelope is sealed up and finished.

Not every piece of glazing in that wall is the same type of glass, and code is specific about where each type goes. Tempered or laminated safety glass, which breaks into small, relatively harmless pieces instead of large sharp shards, is required near doors, stairways, and low sills where someone could plausibly fall into it, while ordinary glass is acceptable elsewhere. Before the whole order of curtain wall is released for fabrication, a mockup panel is typically built and field-tested for water penetration under ASTM E1105, spraying water at the panel under pressure for a sustained period while checking the interior for any sign of leakage. Passing that single mockup is what gives the project confidence to fabricate and install an entire building's worth of panels built to the same design.

The single most common place a finished building actually leaks is where the curtain wall meets the roof at a parapet, the short wall that caps a roof's edge. The glazier's sill flashing at the top of the wall has to tie continuously into the roofer's base flashing at that same transition, and if the two trades install their pieces without coordinating the overlap, water finds the gap between them no matter how well either trade's own work performs on its own. Getting the building envelope right, from the curtain wall's pressure-equalized design down to this one transition detail, is what lets the rest of the project move inside on schedule instead of chasing a leak after the interior finishes are already in. If closing in a building is the kind of work that interests you, the field trades interview guide covers what hiring for these roles looks for.