The curtain wall

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A non-load-bearing external envelope hung on the structural frame, carrying only its own weight and the wind pressure on its face, and transferring both back to the frame at each floor. It is the direct consequence of the skeleton frame: once the wall has no structural job, its thickness is set by weather, insulation and fire rather than by the height of the building above it. Early curtain walls were masonry or terracotta panels; the fully glazed metal-and-glass wall becomes the dominant commercial idiom in the mid-20th century. The trade is real and still unresolved: a glass envelope maximises daylight and view and minimises weight, and it is thermally poor compared with a heavy wall, which is why a large fraction of the operating energy of a mid-century glass tower goes into correcting the envelope with mechanical heating and cooling.