difficult to pin down with a precise definition. Because it can be examined from multiple
perspectives—in terms of functional relationships, as an aesthetic object, or as a massproduced
system available within the construction marketplace—some ambiguity is both
inevitable and provocative.
In the first case, the curtain wall is defined in terms of its functional relationship to the
building’s structure. It then refers to the cladding, or enclosure, of a building as
something both separate from and attached to the building’s skeletal framework. Where
load-bearing walls provide both structure and enclosure, there can be no curtain wall.
However, difficulties emerge within this first definition when the question of “in-fill” is
considered: are conventional windows (or other in-fill material), when fixed inside the
boundaries of a structural frame, considered to represent curtain-wall construction? Such
construction is certainly “attached” to the structural frame but not exactly “hanging” from
it. When is a window just a window within a frame, and when does it transform into a
curtain wall? The answer might have more to do with one’s aesthetic bias than with the
actual functional relationship between cladding and structure.
From a functional perspective, curtain walls necessarily appeared precisely at the same
time as skeletal frameworks—toward the end of the 19th century. Yet the first such walls
were often strikingly similar to the thick masonry walls that they might have been
expected to supersede. Although no longer load-bearing structures, relatively thick
masonry curtain walls continued to be used in steel-and concrete-framed buildings for
Entries A–F 639
other reasons. First, thinner masonry walls—before the development of internal cavities
to block the migration of moisture through the wall—tended to have problems with water
penetration. Second, lighter facades consisting of metal or glass panels were often
considered aesthetically unsuitable for serious works of architecture because of a legacy
and tradition that linked monumental architecture to masonry construction. Third, the use
of more modern cladding alternatives required breakthroughs in environmental control
technologies—air conditioning and insulation being the most important—before they
could be deployed over large surface areas enclosing habitable spaces. Finally, building
code officials, increasingly sensitive to the real danger of urban conflagrations, prevented
the use of new, lightweight materials in exterior walls—even after other technical and
environmental issues had been addressed—if they were unable to match the proven fire
resistance of masonry.
Although defined initially in terms of its functional relationship to structure, toward
the middle of the 20th century the curtain wall began to be alternatively defined by its
function as an environmental filter—as a membrane mediating between desired interior
conditions and variable exterior circumstances. Sunscreens (bris es -soleil), double glazing, and
pressure-equalized rain screens were among the functional responses to this concern,
culminating in the late 20th century’s technologically sophisticated “bioclimatic” designs.
In these “green” buildings, an array of computer- and user-controlled devices may be
embedded within the curtain wall to encourage the use of fresh air and natural daylighting
while at the same time aiming to improve user comfort, reduce energy consumption, and
promote a “sustainable” lifestyle.
Curtain walls can also be defined as the embodiment of an aesthetic intention—the
second of the three perspectives mentioned previously. Numerous such curtain-wall
themes can be identified in 20th-century architecture. They coalesce, in general, around
the revolutionary “new” materials of metal and glass: metal (as industrialized, massproduced,
streamlined panel), glass (as transparent or reflective surface, crystalline solid,
or harbinger of an enlightened culture), or metal and glass com- bined (as woven “fabric”
or abstract grid). Still, other more traditional materials and systems, including stucco,
concrete, brick, and stone veneer, have also played a role in validating the curtain wall
within various aesthetic domains and not merely as the by-product of functional
considerations. The ideal of an all-glass skin perhaps was the most persistent curtain-wall
theme of the 20th century. Starting with metal window systems containing relatively
small glass panes and moving toward larger glass sizes with smaller mullion profiles, the
most technically advanced glass walls of the late 20th century managed to eliminate
mullions entirely, whether by using the glass itself as a structural material, relying on
structural sealant joints, or by pinning the glass to elegantly detailed lightweight steel
substructures.
Ironically, the initial aesthetic formulation of the modern metal-and-glass curtain wall
preceded the invention of multistory skeletal frameworks. Greenhouses were being built
in Europe, even in the mid-17th century, with large areas of glass divided by wooden, and
later iron, mullions. By the mid-19th century, skins of metal and glass were commonly
used for the roofs of markets, gallerias, and train stations. London’s Crystal Palace of
1851 was extremely influential not only in validating the architectural use of iron and
glass but also in foreshadowing its rationalization as an industrialized system.
It is as a system—the third perspective mentioned previously—that the curtain wall
became widely available within the building construction marketplace. Early 20thcentury
curtain walls tended to be unique and custom made, fabricated individually from
the cast iron, rolled steel, and plate glass that were just beginning to appear as
industrialized commodities. However, by the mid-1930s the emerging sheet-metal
technologies (and aesthetics) associated with the mass production of airplanes and
automobiles began to be seriously adapted to building construction, especially the
development of metal curtain-wall panels. Starting at the end of World War II, the 20th
century’s ubiquitous metal-and-glass curtain-wall systems—repetitive grids of extruded
aluminum mullions and horizontal rails fastened to a building’s structural skeleton and
supporting panels of glass or metal—increasingly began to appear on commercial and
institutional buildings. The newly invented float process made large areas of glass even
more feasible beginning in the 1950s.
Other panelized curtain-wall systems also appeared as cladding options: these
included composite metal panels containing lightweight cores of honeycombed material
or foam plastic insulation sandwiched between two layers of thin sheet metal (aluminum
or steel); precast concrete panels, custom designed for each job but still manufactured
within a rationalized, systematic production setting; and thin stone veneer panels, factory
Entries A–F 641
cut to a thickness as little as one inch, then attached to the building’s structure using
proprietary metal clips and anchors. Even traditional brick and stucco became integrated
into manufactured curtain-wall systems: brick as part of layered cavity wall systems and
stucco, most commonly in the form of EIFS (exterior insulation and finish systems),
consisting of thin polymer-based plaster laminae applied with fiberglass reinforcing mesh
to a surface of rigid foam insulation. Among the numerous architects or designers
associated with the development of curtain-wall technology or its aesthetic refinement, a
partial list would include Nicholas Grimshaw, Norman Foster, Walter Gropius, Le
Corbusier, Richard Meier, Jean Nouvel, I.M.Pei, Cesar Pelli, Jean Prouvé, Peter Rice,
Kevin Roche, Ludwig Mies van der Rohe, Eero Saarinen, Ken Yeang, and the firm of
Skidmore, Owings and Merrill.
