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A curtain wall is an outer covering of a building in which the outer walls are non-structural, utilized only to keep the weather out and the occupants in. Since the curtain wall is non-structural, it can be made of lightweight materials, such as
glass Glass is a non- crystalline, often transparency and translucency, transparent amorphous solid, that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by ...
, thereby potentially reducing construction costs. An additional advantage of glass is that natural light can penetrate deeper within the building. The curtain wall
façade A façade or facade () is generally the front part or exterior of a building. It is a Loanword, loan word from the French language, French (), which means 'frontage' or 'face'. In architecture, the façade of a building is often the most impor ...

façade
does not carry any
structural load A structural load or structural action is a force, Deformation (engineering), deformation, or acceleration applied to Structural engineering#Structural elements, structural elements. A load causes stress (physics), stress, deformation (engineering) ...
from the building other than its own dead load weight. The wall transfers lateral wind loads that are incident upon it to the main building structure through connections at floors or columns of the building. A curtain wall is designed to resist air and water infiltration, absorb sway induced by wind and
seismic Seismology (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following perio ...
forces acting on the building, withstand wind loads, and support its own weight. Curtain walls may be designed as "systems" integrating frame, wall panel, and weatherproofing materials. Steel frames have largely given way to aluminum extrusions, typically infilled with glass, which provides an architecturally pleasing look and benefits such as
daylighting Daylighting is the practice of placing windows, skylight A skylight (sometimes called a rooflight) is a light-transmitting structure that forms all or part of the roof space of a building for daylighting purposes. History Open skylights we ...
. However, the effects of light on visual comfort as well as solar heat gain in a building are more difficult to control when using large amounts of glass infill. Other common infills include stone veneer, metal panels,
louvres Louvres is a Communes of France, commune in the Val-d'Oise Departments of France, department in Île-de-France (region), Île-de-France in northern France. Education Schools in Louvres include: * Four preschools (''écoles maternelles''): Georges ...
, and operable windows or vents. Curtain wall systems differ from storefront systems in being designed to span multiple floors, taking into consideration
building sway
building sway
and movement in addition to design requirements such as thermal expansion and contraction; seismic requirements, water diversion; and thermal efficiency for cost-effective heating, cooling, and interior lighting.


History

Historically, buildings were constructed of timber, masonry, or a combination of both. Their exterior walls were load bearing, supporting much or all of the load of the entire structure. The nature of the materials resulted in inherent limits to a building's height. The development and widespread use of
structural steel Structural steel is a category of steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, ...
and later
reinforced concrete Reinforced concrete (RC), also called reinforced cement concrete (RCC), is a composite material in which concrete's relatively low ultimate tensile strength, tensile strength and ductility are compensated for by the inclusion of reinforcement h ...
allowed relatively small
columns A column or pillar in architecture upright=1.45, alt=Plan d'exécution du second étage de l'hôtel de Brionne (dessin) De Cotte 2503c – Gallica 2011 (adjusted), Plan of the second floor (attic storey) of the Hôtel de Brionne in Par ...

columns
to support large loads. The exterior walls could be non-load bearing, and thus much lighter and more open than load-bearing walls of the past. This gave way to increased use of glass as an exterior façade, and the modern-day curtain wall was born. Post and beam and
balloon frame
balloon frame
d timber structures effectively had an early version of curtain walls, as their frames supported loads that allowed the walls themselves to serve other functions, such as keeping weather out and allowing light in. When iron began to be used extensively in buildings in late 18th-century Britain, such as at
Ditherington Flax Mill Ditherington Flax Mill (promoted as the Shrewsbury Flaxmill Maltings), a flax mill located in Ditherington, a suburb of Shrewsbury, England, is the first iron-framed building in the world, and described as "the grandfather of skyscrapers",
, and later when buildings of
wrought iron Wrought iron is an iron Iron () is a with Fe (from la, ) and 26. It is a that belongs to the and of the . It is, on , right in front of (32.1% and 30.1%, respectively), forming much of Earth's and . It is the fourth most common . ...
and glass such as
The Crystal Palace The Crystal Palace was a cast iron Cast iron is a group of iron-carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its colour when fracture ...
were built, the building blocks of structural understanding were laid for the development of curtain walls.
Oriel Chambers Oriel may refer to: Places United Kingdom *Oriel Street File:Oriel College 1919 Photogravure.jpg, 180px, 1919 photogravure of Oriel College, Oxford, Oriel College from the north, with the Rhodes Building in the foreground and Oriel Street to th ...

Oriel Chambers
(1864) and
16 Cook Street 16 Cook Street, Liverpool Liverpool is a and in , England. With a population of in 2019, it is the , and its is the fifth largest in the with a population of 2.24 million. Situated on the eastern side of the , Liverpool hist ...
(1866), both built in
Liverpool Liverpool is a City status in the United Kingdom, city and metropolitan borough in Merseyside, England. With a population of in 2019, it is the List of English districts by population, tenth largest English district by population, and its ...

Liverpool
,
England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest of England and the Celtic Sea to the southwest. E ...

England
, by local architect and
civil engineer A civil engineer is a person who practices civil engineering Civil engineering is a Regulation and licensure in engineering, professional engineering discipline that deals with the design, construction, and maintenance of the physical and natu ...
Peter Ellis, are characterised by their extensive use of glass in their facades. Towards the courtyards they even boasted metal-framed glass curtain walls, which makes them two of the world's first buildings to include this architectural feature. The extensive glass walls allowed light to penetrate further into the building, utilizing more floor space and reducing lighting costs. Oriel Chambers comprises set over five floors without an
elevator U-Bahn Rapid transit in Germany consists of four U-Bahn systems and fourteen S-Bahn systems. The U-Bahn or Untergrundbahn (''underground railway'') are conventional rapid transit systems that run mostly underground, while the S-Bahn or ...

elevator
, which had only recently been invented and was not yet widespread. The
Statue of Liberty The Statue of Liberty (''Liberty Enlightening the World''; French: ''La Liberté éclairant le monde'') is a colossal neoclassical sculpture on Liberty Island in New York Harbor in New York City, in the United States. The copper statue, a ...

Statue of Liberty
(1886) features a thin, non-load-bearing copper skin. An early example of an all-steel curtain wall used in the classical style is the ' department store on ',
Berlin Berlin (; ) is the Capital city, capital and List of cities in Germany by population, largest city of Germany by both area and population. Its 3,769,495 inhabitants, as of 31 December 2019 makes it the List of cities in the European Union by ...

Berlin
, built in 1901 (since demolished). Some of the first curtain walls were made with steel
mullions A mullion is a vertical element that forms a division between units of a window or screen, or is used decoratively. When dividing adjacent window A window is an opening in a wall, door, roof, or vehicle that allows the passage of light and may ...
, and the polished plate glass was attached to the mullions with asbestos- or fiberglass-modified glazing compound. Eventually
silicone A silicone or polysiloxane is a polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its sub ...

silicone
sealant Sealant is a substance used to block the passage of s through the surface or joints or openings in materials, a type of . In building construction ''sealant'' is sometimes synonymous with ' and also serve the purposes of blocking dust, sound and ...

sealant
s or glazing tape were substituted for the glazing compound. Some designs included an outer cap to hold the glass in place and to protect the integrity of the seals. The first curtain wall installed in
New York City New York, often called New York City to distinguish it from New York State New York is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of ...

New York City
, in the
United Nations Secretariat Building The United Nations Secretariat Building is a tall skyscraper A skyscraper is a large continuously habitable building having multiple floors. Modern sources currently define skyscrapers as being at least 100 metres or 150 metres in height, th ...
(Skidmore, Owings, and Merrill, 1952), was this type of construction. Earlier
modernist Modernism is both a philosophical movement A philosophical movement refers to the phenomenon defined by a group of philosophers A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and ...
examples are the
Bauhaus The Staatliches Bauhaus (), commonly known as the Bauhaus (German: "building house"), was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German an ...

Bauhaus
in
Dessau Dessau is a town and former municipality in Germany at the confluence of the rivers Mulde The Mulde () is a river in and , . It is a left tributary of the and is long. The river is formed by the confluence, near , of the (running thro ...
(1926) and the
Hallidie Building
Hallidie Building
in
San Francisco San Francisco (; Spanish Spanish may refer to: * Items from or related to Spain: **Spaniards, a nation and ethnic group indigenous to Spain **Spanish language **Spanish cuisine Other places * Spanish, Ontario, Canada * Spanish River (dis ...

San Francisco
(1918).
Ludwig Mies van der Rohe Ludwig Mies van der Rohe ( ; ; born Maria Ludwig Michael Mies; March 27, 1886August 17, 1969) was a German-American architect An architect is a person who plans, designs and oversees the construction of buildings. To practice architecture ...
's curtain wall is one of the most important aspect of his architectural design. Mies first began prototyping the curtain wall in his high-rise residential building designs along Chicago's lakeshore, achieving the look of a curtain wall at famed
860-880 Lake Shore Drive Apartments 86 may refer to: * 86 (number), a natural number * 86 (term), a slang term for getting rid of something Dates * 86 BC, a year of the pre-Julian Roman calendar * AD 86, a common year of the Julian calendar * 1986, a common year of the Gregori ...
. He finally perfected the curtain wall at 900 910 Lake Shore Drive, where the curtain is an autonomous aluminum and glass skin. After 900 910, Mies' curtain wall appeared on all of his subsequent high-rise building designs including the famed
Seagram building The Seagram Building is a skyscraper at 375 Park Avenue, between 52nd Street (Manhattan), 52nd and 53rd Street (Manhattan), 53rd Streets, in Midtown Manhattan, New York. The building, standing tall with 38 stories and a large plaza, is recogniz ...

Seagram building
in New York. During the 1970s, the widespread use of aluminium
extrusion Extrusion is a process used to create objects of a fixed cross section (geometry), cross-sectional profile by pushing material through a Die (manufacturing), die of the desired cross-section. Its two main advantages over other manufacturing pro ...

extrusion
s for mullions began.
Aluminum alloy Aluminium alloys (or aluminum alloys; see American and British English spelling differences, spelling differences) are alloys in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silico ...
s offer the unique advantage of being able to be easily extruded into nearly any shape required for design and aesthetic purposes. Today, the design complexity and shapes available are nearly limitless. Custom shapes can be designed and manufactured with relative ease. The Omni San Diego Hotel curtain wall in California, designed by architectural firm Hornberger and Worstel and developed by JMI Realty, is an example of a unitized curtain-wall system with integrated sunshades.


Systems and principles


Stick systems

The vast majority of ground-floor curtain walls are installed as long pieces (referred to as ''sticks'') between floors vertically and between vertical members horizontally. Framing members may be fabricated in a shop, but installation and glazing is typically performed at the jobsite.


Ladder systems

Very similar to a stick system, a ladder system has mullions which can be split and then either snapped or screwed together consisting of a half box and plate. This allows sections of curtain wall to be fabricated in a shop, effectively reducing the time spent installing the system on site. The drawbacks of using such a system is reduced structural performance and visible joint lines down the length of each mullion.


Unitized systems

Unitized curtain walls entail factory fabrication and assembly of panels and may include factory glazing. These completed units are installed on the building structure to form the building enclosure. Unitized curtain wall has the advantages of: speed; lower field installation costs; and
quality control Quality control (QC) is a process by which entities review the quality of all factors involved in production. ISO 9000 The ISO 9000 family of quality management systems A quality management system (QMS) is a collection of business process ...

quality control
within an interior climate-controlled environment. The economic benefits are typically realized on large projects or in areas of high field labor rates.


Rainscreen principle

A common feature in curtain wall technology, the
rainscreen A rainscreen is an exterior detail where the (wall cladding) stands off from the - surface of an air/water barrier applied to the sheathing to create a capillary break and to allow drainage and evaporation. The ''rainscreen'' is the cladding or ...

rainscreen
principle theorizes that equilibrium of air pressure between the outside and inside of the "rainscreen" prevents water penetration into the building. For example, the glass is captured between an inner and an outer gasket in a space called the glazing rebate. The glazing rebate is ventilated to the exterior so that the pressure on the inner and outer sides of the outer gasket is the same. When the pressure is equal across this gasket, water cannot be drawn through joints or defects in the gasket.


Design concerns

A curtain wall system must be designed to handle all loads imposed on it as well as keep air and water from penetrating the building envelope.


Loads

The loads imposed on the curtain wall are transferred to the building structure through the anchors which attach the mullions to the building. ;Dead load ''Dead load'' is defined as the weight of structural elements and the permanent features on the structure. In the case of curtain walls, this load is made up of the weight of the mullions, anchors and other structural components of the curtain wall, as well as the weight of the infill material. Additional dead loads imposed on the curtain wall may include sunshades or signage attached to the curtain wall. ;Wind load ''Wind load'' is a normal force acting on the building as the result of
wind Wind is the natural movement of air or other gases relative to a planet's surface. Wind occurs on a range of scales, from thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by th ...

wind
blowing on the building. Wind pressure is resisted by the curtain wall system since it envelops and protects the building. Wind loads vary greatly throughout the world, with the largest wind loads being near the coast in
hurricane A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation Atmospheric circulation is the large-scale movement of Atmosphere of Earth, air and together with oc ...

hurricane
-prone regions. For each project location,
building code A building code (also building control or building regulations) is a set of rules that specify the standards for constructed objects such as buildings and nonbuilding structures. Buildings must conform to the code to obtain planning permission, u ...
s specify the required design wind loads. Often, a
wind tunnel Wind tunnels are large tubes with air blowing through them which are used to replicate the interaction between air and an object flying through the air or moving along the ground. Researchers use wind tunnels to learn more about how an aircraft ...

wind tunnel
study is performed on large or unusually-shaped buildings. A scale model of the building and the surrounding vicinity is built and placed in a wind tunnel to determine the wind pressures acting on the structure in question. These studies take into account
vortex shedding Image:Vortex-street-animation.gif, frame, Vortex shedding behind a circular cylinder. In this animation, the flow on the two sides of the cylinder are shown in different colors, to show that the vortices from the two sides alternate. Courtesy, Cesar ...
around corners and the effects of surrounding topography and buildings. ;Seismic load ''Seismic loads'' in curtain wall system are limited to the interstory drift induced on the building during an earthquake. In most situations, the curtain wall is able to naturally withstand
seismic Seismology (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following perio ...
and wind induced building sway because of the space provided between the glazing infill and the mullion. In tests, standard curtain wall systems are typically able to withstand up to three inches (75 mm) of relative floor movement without glass breakage or water leakage. ;Snow load ''Snow loads'' and live loads are not typically an issue in curtain walls, since curtain walls are designed to be vertical or slightly inclined. If the slope of a wall exceeds 20 degrees or so, these loads may need to be considered. ;Thermal load ''Thermal loads'' are induced in a curtain wall system because aluminum has a relatively high
coefficient of thermal expansion Thermal expansion is the tendency of matter to change its shape A shape or figure is the form of an object or its external boundary, outline, or external surface File:Water droplet lying on a damask.jpg, Water droplet lying on a damask ...
. This means that over the span of a couple of floors, the curtain wall will expand and contract some distance, relative to its length and the temperature differential. This expansion and contraction is accounted for by cutting horizontal mullions slightly short and allowing a space between the horizontal and vertical mullions. In unitized curtain wall, a gap is left between units, which is sealed from air and water penetration by gaskets. Vertically, anchors carrying wind load only (not dead load) are slotted to account for movement. Incidentally, this slot also accounts for live load deflection and creep in the floor slabs of the building structure. ;Blast load Accidental explosions and terrorist threats have brought on increased concern for the fragility of a curtain wall system in relation to blast loads. The bombing of the
Alfred P. Murrah Federal Building The Alfred P. Murrah Federal Building was a United States federal government The federal government of the United States (U.S. federal government) is the national government of the United States The United States of America (USA), ...
in
Oklahoma City Oklahoma City (), officially the City of Oklahoma City, and often shortened to OKC, is the capital Capital most commonly refers to: * Capital letter Letter case (or just case) is the distinction between the letters that are in larger ...

Oklahoma City
,
Oklahoma Oklahoma () is a U.S. state, state in the South Central United States, South Central region of the United States, bordered by the state of Texas on the south and west, Kansas on the north, Missouri on the northeast, Arkansas on the east, New ...
, has spawned much of the current research and mandates in regards to building response to blast loads. Currently, all new federal buildings in the U.S. and all U.S. embassies built on foreign soil must have some provision for resistance to bomb blasts. Since the curtain wall is at the exterior of the building, it becomes the first line of defense in a bomb attack. As such, blast resistant curtain walls are designed to withstand such forces without compromising the interior of the building to protect its occupants. Since blast loads are very high loads with short durations, the curtain wall response should be analyzed in a
dynamic Dynamics (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million ...
load analysis, with full-scale
mock-up Image:Space Shuttle America.jpg, The shuttle sitting outside the Space Shuttle America ride is an example of a mockup In manufacturing and design, a mockup, or mock-up, is a scale model, scale or physical model, full-size model of a design or dev ...

mock-up
testing performed prior to design completion and installation. Blast resistant glazing consists of
laminated glass Automobile windshield with "spider web" cracking typical of laminated safety glass. Laminated glass is a type of safety glass Safety glass is glass Glass is a non- crystalline, often transparency and translucency, transparent amorpho ...
, which is meant to break but not separate from the mullions. Similar technology is used in
hurricane A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation Atmospheric circulation is the large-scale movement of Atmosphere of Earth, air and together with oc ...

hurricane
-prone areas for impact protection from wind-borne debris.


Air Infiltration

''Air infiltration'' is the air which passes through the curtain wall from the exterior to the interior of the building. The air is infiltrated through the gaskets, through imperfect joinery between the horizontal and vertical
mullions A mullion is a vertical element that forms a division between units of a window or screen, or is used decoratively. When dividing adjacent window A window is an opening in a wall, door, roof, or vehicle that allows the passage of light and may ...
, through
weep hole A weep, a weep hole, or a weep-brick is a small opening that allows water to drain from within an assembly. Weeps are located at the bottom of the object to allow for drainage; the weep hole must be sized adequately to overcome surface tension ...

weep hole
s, and through imperfect sealing. The American Architectural Manufacturers Association (AAMA) is an industry trade group in the U.S. that has developed voluntary specifications regarding acceptable levels of air infiltration through a curtain wall.


Water penetration

''Water penetration'' is defined as water passing from the exterior of the building to the interior of the curtain wall system. Sometimes, depending on the building
specifications A specification often refers to a set of documented requirements to be satisfied by a material, design, product, or service. A specification is often a type of technical standard. There are different types of technical or engineering specification ...
, a small amount of controlled water on the interior is deemed acceptable. Controlled water penetration is defined as water that penetrates beyond the inner most vertical plane of the test specimen, but has a designed means of drainage back to the exterior. AAMA Voluntary Specifications allow for controlled water penetration while the underlying ASTM E1105 test method would define such water penetration as a failure. To test the ability of a curtain wall to withstand water penetration in the field, an ASTM E1105 water spray rack system is placed on the exterior side of the test specimen, and a positive air pressure difference is applied to the system. This set up simulates a wind driven rain event on the curtain wall to check for field performance of the product and of the installation. Field quality control and assurance checks for water penetration has become the norm as builders and installers apply such quality programs to help reduce the number of water damage litigation suits against their work.


Deflection

One of the disadvantages of using aluminum for mullions is that its
modulus of elasticity An elastic modulus (also known as modulus of elasticity) is a quantity that measures an object or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is defi ...
is about one-third that of steel. This translates to three times more
deflection Deflection or deflexion may refer to: * Deflection (ballistics), a technique of shooting ahead of a moving target so that the target and projectile will collide * Deflection (chess), a tactic that forces an opposing chess piece to leave a square ...
in an aluminum mullion compared to a similar steel section under a given load. Building specifications set deflection limits for perpendicular (wind-induced) and in-plane (dead load-induced) deflections. These deflection limits are not imposed due to strength capacities of the mullions. Rather, they are designed to limit deflection of the glass (which may break under excessive deflection), and to ensure that the glass does not come out of its pocket in the mullion. Deflection limits are also necessary to control movement at the interior of the curtain wall. Building construction may be such that there is a wall located near the mullion, and excessive deflection can cause the mullion to contact the wall and cause damage. Also, if deflection of a wall is quite noticeable, public perception may raise undue concern that the wall is not strong enough. Deflection limits are typically expressed as the distance between anchor points divided by a constant number. A deflection limit of L/175 is common in curtain wall specifications, based on experience with deflection limits that are unlikely to cause damage to the glass held by the mullion. Say a given curtain wall is anchored at 12 foot (144 in) floor heights. The allowable deflection would then be 144/175 = 0.823 inches, which means the wall is allowed to deflect inward or outward a maximum of 0.823 inches at the maximum wind pressure. However, some panels require stricter movement restrictions, or certainly those that prohibit a torque-like motion. Deflection in mullions is controlled by different shapes and depths of curtain wall members. The depth of a given curtain wall system is usually controlled by the
area moment of inertia The second moment of area, or second area moment, or quadratic moment of area and also known as the area moment of inertia, is a geometrical property of an area Area is the quantity that expresses the extent of a two-dimensional region, ...
required to keep deflection limits under the specification. Another way to limit deflections in a given section is to add steel reinforcement to the inside tube of the mullion. Since steel deflects at one-third the rate of aluminum, the steel will resist much of the load at a lower cost or smaller depth.


Strength

Strength (or maximum usable stress) available to a particular material is not related to its material stiffness (the material property governing deflection); it is a separate criterion in curtain wall
design A design is a plan or specification for the construction of an object or system or for the implementation of an activity or process, or the result of that plan or specification in the form of a prototype A prototype is an early sample, mode ...
and
analysis Analysis is the process of breaking a complex topic or substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical composit ...
. This often affects the selection of materials and sizes for design of the system. The allowable bending strength for certain aluminum alloys, such as those typically used in curtain wall framing, approaches the allowable bending strength of steel alloys used in building construction.


Thermal criteria

Relative to other building components, aluminum has a high heat transfer coefficient, meaning that aluminum is a very good
conductor Conductor or conduction may refer to: Music * Conductor (music), a person who leads a musical ensemble like, for example, an orchestra. * Conductor (album), ''Conductor'' (album), an album by indie rock band The Comas * Conduction, a type of ...

conductor
of heat. This translates into high heat loss through aluminum curtain wall mullions. There are several ways to compensate for this heat loss, the most common way being the addition of thermal breaks. '' Thermal breaks'' are barriers between exterior metal and interior metal, usually made of
polyvinyl chloride Polyvinyl chloride (colloquial Colloquialism or colloquial language is the style (sociolinguistics), linguistic style used for casual (informal) communication. It is the most common functional style of speech, the idiom normally employed in ...
(PVC). These breaks provide a significant decrease in the
thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

thermal conductivity
of the curtain wall. However, since the thermal break interrupts the aluminum mullion, the overall moment of inertia of the mullion is reduced and must be accounted for in the structural analysis and deflection analysis of the system. Thermal conductivity of the curtain wall system is important because of heat loss through the wall, which affects the heating and cooling costs of the building. On a poorly performing curtain wall,
condensation Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

condensation
may form on the interior of the mullions. This could cause damage to adjacent interior trim and walls. Rigid
insulation
insulation
is provided in
spandrel A spandrel is a roughly triangular space, usually found in pairs, between the top of an arch and a rectangular frame; between the tops of two adjacent arches or one of the four spaces between a circle within a square. They are frequently fill ...

spandrel
areas to provide a higher R-value at these locations.


Infills

''Infill'' refers to the large panels that are inserted into the curtain wall between mullions. Infills are typically glass but may be made up of nearly any exterior building element. Some common infills include metal panels, louvers, and
photovoltaic Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercia ...
panels.


Glass

Float glass Float glass is a sheet of glass Glass is a non- crystalline, often transparency and translucency, transparent amorphous solid, that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optic ...
is by far the most common curtain wall glazing type. It can be manufactured in an almost infinite combination of color, thickness, and opacity. For commercial construction, the two most common thicknesses are 1/4 inch (6 mm) monolithic and 1 inch (25 mm) insulating glass. 1/4 inch glass is typically used only in
spandrel A spandrel is a roughly triangular space, usually found in pairs, between the top of an arch and a rectangular frame; between the tops of two adjacent arches or one of the four spaces between a circle within a square. They are frequently fill ...

spandrel
areas, while insulating glass is used for the rest of the building (sometimes spandrel glass is specified as insulating glass as well). The 1 inch insulation glass is typically made up of two 1/4-inch lites of glass with a 1/2 inch (12 mm) airspace. The air inside is usually atmospheric air, but some
inert gas An ''inert gas'' is a gas that does not undergo chemical reactions under a set of given conditions. The noble gases The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical element I ...
es, such as
argon Argon is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

argon
or
krypton Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the ...

krypton
may be used to offer better
thermal transmittance Thermal transmittance is the rate of transfer of heat through matter. The thermal transmittance of a material (such as insulation or concrete) or an assembly (such as a wall or window) is expressed as a R-value (insulation)#U-factor.2FU-value, U-v ...
values. In Europe, triple-pane insulating glass infill is now common. In Scandinavia first curtain walls with quadruple-pane have been built. Larger thicknesses are typically employed for buildings or areas with higher thermal,
relative humidity Humidity is the concentration of water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vap ...

relative humidity
, or sound transmission requirements, such as laboratory areas or
recording studio A recording studio is a specialized facility for Sound recording and reproduction, sound recording, Audio mixing (recorded music), mixing, and audio production of instrumental or vocal musical performances, spoken words, and other sounds. T ...

recording studio
s. In residential construction, thicknesses commonly used are 1/8 inch (3 mm) monolithic and 5/8 inch (16 mm) insulating glass. Glass may be used which is
transparent Transparency, transparence or transparent most often refer to transparency and translucency, the physical property of allowing the transmission of light through a material. They may also refer to: Literal uses * Transparency (photography), a sti ...
,
translucent In the field of optics Optics is the branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order ...
, or opaque, or in varying degrees thereof. ''Transparent'' glass usually refers to ''vision'' glass in a curtain wall. Spandrel or vision glass may also contain translucent glass, which could be for security or aesthetic purposes. ''Opaque'' glass is used in areas to hide a column or spandrel beam or shear wall behind the curtain wall. Another method of hiding spandrel areas is through ''shadow box'' construction (providing a dark enclosed space behind the transparent or translucent glass). Shadow box construction creates a perception of depth behind the glass that is sometimes desired.


Fabric veneer

Fabric is another type of material which is common for curtain walls. Fabric is often much less expensive and serves as a less permanent solution. Unlike glass or stone, fabric is much faster to install, less expensive, and often much easier to modify after it is installed. Because of low density of fabrics total weight of structure is very low then strength consideration of structure is not too important.


Stone veneer

Thin blocks (3 to 4 inches (75–100 mm)) of stone can be inset within a curtain wall system. The type of stone used is limited only by the strength of the stone and the ability to manufacture it in the proper shape and size. Common stone types used are:
calcium silicate Calcium silicate is the chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held togeth ...
,
granite Granite () is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cool ...

granite
,
marble Marble is a metamorphic rock , a type of metamorphic rock Metamorphic rocks arise from the transformation of existing rock (geology), rock to new types of rock, in a process called metamorphism upright=1.35, Schematic representation of ...

marble
,
travertine Travertine ( ) is a form of terrestrial limestone Limestone is a common type of carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the l ...

travertine
,
limestone Limestone is a common type of carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its na ...

limestone
, and
engineered stoneEngineered stone is a composite material made of crushed stone bound together by an adhesive, (most commonly polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material c ...
. To reduce weight and improve strength, the natural stone may be attached to an aluminum honeycomb backing.


Panels

Metal panels can take various forms including stainless steel, aluminum plate; aluminum composite panels consisting of two thin aluminum sheets sandwiching a thin plastic interlayer; copper wall cladding, and panels consisting of metal sheets bonded to rigid insulation, with or without an inner metal sheet to create a sandwich panel. Other opaque panel materials include
fiber-reinforced plastic Fibre-reinforced plastic (FRP; also called fiber-reinforced polymer, or fiber-reinforced plastic) is a composite material A composite material (also called a composition material or shortened to composite, which is the common name) is a material ...
(FRP) and
terracotta Terracotta, terra cotta, or terra-cotta (; Italian Italian may refer to: * Anything of, from, or related to the country and nation of Italy ** Italians, an ethnic group or simply a citizen of the Italian Republic ** Italian language, a Romance ...

terracotta
. Terracotta curtain wall panels were first used in Europe, but only a few manufacturers produce high quality modern terracotta curtain wall panels.


Louvers

A
louver A louver (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United States. ...
is provided in an area where mechanical equipment located inside the building requires ventilation or fresh air to operate. They can also serve as a means of allowing outside air to filter into the building to take advantage of favorable climatic conditions and minimize the usage of energy-consuming
HVAC Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal rad ...

HVAC
systems. Curtain wall systems can be adapted to accept most types of louver systems to maintain the same architectural sightlines and style while providing desired functionality.


Windows and vents

Most curtain wall glazing is fixed, meaning there is no access to the exterior of the building except through doors. However, windows or vents can be glazed into the curtain wall system as well, to provide required ventilation or operable windows. Nearly any window type can be made to fit into a curtain wall system.


Fire safety

Firestop A firestop or fire-stopping is a form of passive fire protection Passive fire protection (PFP) is components or systems of a building or structure that slows or impedes the spread the effects of fire or smoke without system activation, and usually ...
ping at the '' perimeter slab edge'', which is a gap between the floor and the curtain wall, is essential to slow the passage of fire and combustion gases between floors. Spandrel areas must have non-combustible insulation at the interior face of the curtain wall. Some building codes require the mullion to be wrapped in heat-retarding insulation near the ceiling to prevent the mullions from melting and spreading the fire to the floor above. The firestop at the perimeter slab edge is considered a continuation of the
fire-resistance rating A fire-resistance rating typically means the duration for which a passive fire protection Passive fire protection (PFP) is components or systems of a building or structure that slows or impedes the spread the effects of fire or smoke without sys ...
of the floor slab. The curtain wall itself, however, is not ordinarily required to have a rating. This causes a quandary as
compartmentalization (fire protection) Compartmentalization in structures, such as land-based building A building, or edifice, is a structure with a roof and walls standing more or less permanently in one place, such as a house or factory. Buildings come in a variety of sizes, shap ...
is typically based upon ''closed'' compartments to avoid fire and smoke migrations beyond each engaged compartment. A curtain wall by its very nature prevents the completion of the compartment (or envelope). The use of fire sprinklers has been shown to mitigate this matter. As such, unless the building is sprinklered, fire may still travel up the curtain wall, if the glass on the exposed floor is shattered from heat, causing flames to lick up the outside of the building. Falling glass can endanger pedestrians, firefighters and firehoses below. An example of this is the 1988 First Interstate Tower fire in Los Angeles, California. The fire leapfrogged up the tower by shattering the glass and then consuming the aluminum framing holding the glass. Aluminum's melting temperature is 660 °C, whereas building fires can reach 1,100 °C. The melting point of aluminum is typically reached within minutes of the start of a fire. Fireman knock-out glazing panels are often required for venting and emergency access from the exterior. Knock-out panels are generally fully Architectural Glass, tempered glass to allow full fracturing of the panel into small pieces and relatively safe removal from the opening.


Maintenance and repair

Curtain walls and perimeter sealants require maintenance to maximize service life. Perimeter sealants, properly designed and installed, have a typical service life of 10 to 15 years. Removal and replacement of perimeter sealants require meticulous surface preparation and proper detailing. Aluminum frames are generally painted or anodising, anodized. Care must be taken when cleaning areas around anodized material as some cleaning agents will destroy the finish. Factory applied fluoropolymer thermoset coatings have good resistance to environmental degradation and require only periodic cleaning. Recoating with an air-dry fluoropolymer coating is possible but requires special surface preparation and is not as durable as the baked-on original coating. Anodized aluminum frames cannot be "re-anodized" in place but can be cleaned and protected by proprietary clear coatings to improve appearance and durability. Stainless steel curtain walls require no coatings, and embossed, as opposed to abrasively finished, surfaces maintain their original appearance indefinitely without cleaning or other maintenance. Some specially textured matte stainless steel surface finishes are hydrophobic and resist airborne and rain-borne pollutants.McGuire, Michael F., "Stainless Steel for Design Engineers", ASM International, 2008. This has been valuable in the American Southwest and in the Mideast for avoiding dust, as well as avoiding soot and smoke staining in polluted urban areas.


See also

* Mullion wall * Insulated glazing * Quadruple glazing * Copper in architecture


References


External links


European Commission's portal for efficient Curtain WallingEN 13830: Curtain Walling - Product StandardEN 13119: Curtain Walling - TerminologyUnderstanding Curtain Wall & Window Wall differences
{{Use dmy dates, date=April 2017 Types of wall Building engineering Construction Architectural elements