The glass curtain wall
A skyscraper can be made of glass because its outer wall holds nothing up. The frame carries the floors; the skin carries itself and the wind.
What a curtain wall is
A curtain wall is a non-structural outer skin hung off the edge of the floor slabs. It carries its own weight and resists wind pressure, and that is the whole of its job. No floor load passes through it, which is why it can be glass. The name describes the behavior: it hangs like a curtain from the structure behind it.
That answers whether a curtain wall is a window. A window is an opening cut into a wall doing structural work; a curtain wall replaces the wall, so there is no opening around it. Its parts are aluminum mullions, transoms, glass or spandrel panels, and gaskets.
Why the frame made it possible
A masonry building carries its loads in its walls, so the walls thicken toward the bottom and the windows shrink. Chicago's 16-story Monadnock Block of 1891 has brick walls six feet thick at street level.
The skeletal frame moved the load off the wall. When William LeBaron Jenney finished the 55 m Home Insurance Building in 1885, the columns took the weight and the brick outside became cladding. From that point the exterior only has to keep weather out. The curtain wall is the consequence of the frame rather than a separate invention, and architects took three decades to act on it, asthe argument over who invented the skyscraper shows.
From the Hallidie Building to Seagram
The first true glass curtain wall is usually credited to the Hallidie Building at 130 Sutter Street in San Francisco, by Willis Polk, 1918. Polk pushed the glass about three feet clear of the columns, so the facade is a sheet of glazing in a thin steel grid with nothing structural in it.
Nothing followed for a generation: the Depression and the war stopped office construction, and a sealed glass box needed air conditioning and float glass. Lever House on Park Avenue, 1952, made the glass tower a corporate building type, with heat-absorbing blue-green glass in stainless steel mullions by Gordon Bunshaft and Natalie de Blois at Skidmore, Owings & Merrill. Six years later Mies van der Rohe put the Seagram Building across the street in bronze and amber glass. By 1958 the question was how to glaze an office tower, not whether, and every record holder since, including the 108-floorWillis Tower of 1973, has worn one.
Unitized panels, or stick-built
Stick-built assembly delivers mullions, transoms and glass separately and assembles them on site, which is slow and weather-dependent on a tall building. Unitized construction builds finished panels in a factory, one story tall and one bay wide, then cranes them onto anchors cast into the slab edge, a floor at a time.
Almost every tall building is unitized. A crew can hang a floor of panels in a day, the joints are factory work instead of work done on a swing stage in the wind, and the seals go in dry. It also lets the facade climb whilefit-out proceeds below.
The glass itself
A modern panel is an insulated glass unit: two or three panes separated by a sealed argon cavity, typically 24 to 50 mm thick. A low-emissivity coating, a metal oxide layer a few atoms deep on an inner surface, reflects infrared radiation while passing visible light, which drops the U-value far below a single pane. The outer pane is often laminated, so a break leaves the fragments stuck to the interlayer rather than falling forty floors. Units around 1.5 by 3.5 m are routine and jumbo sheets run past 6 m, with size set by the factory and the crane. Where the sun is a problem, a ceramic frit is screen-printed on in dots or lines, as shading or as a pattern birds can see.
Solar gain, birds and the slab edge
Solar gain is the first problem: glass admits heat the building then pays to remove, and a west-facing glass wall drives the cooling plant all afternoon. Bird collisions are the second. Reflective glazing shows birds a picture of sky or trees, and estimates for the United States run from several hundred million to a billion deaths a year, a range that says how hard the counting is. Fritted glass with marks about 50 mm apart works, because a bird will not fly at a gap that size, and New York, Toronto and San Francisco now require it. Third is thermal bridging at the slab edge, where anchors and exposed concrete conduct heat through the insulated zone, so a wall measures worse than its glass does.
The clearest case of a facade designed without checking what its shape does to light is 20 Fenchurch Street in London, the 37-floor tower known as the Walkie-Talkie. Its south-facing wall is concave, which makes it a mirror with a focal point. In early September 2013 the reflected spot reached about 45 °C at street level and warped the panels, badge and wing mirror of a Jaguar parked across the road for an hour. The developers fitted a brise soleil, and the tower still stands in a city of nicknamed towers.
Back toward a smaller window
Energy codes have started pushing the other way. A 90 percent glazed facade struggles to meet current targets in most climates, and several recent office towers have cut the window-to-wall ratio to between 40 and 60 percent, putting insulated opaque panels back between the glass. How much of a curtain wall is glass is being argued over again for the first time since 1952.