Curtain Wall (architecture)
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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 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 rapid cooling (quenching) of ...
, 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 () (also written 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 t ...
does not carry any
structural load A structural load or structural action is a force, deformation, or acceleration applied to structural elements. A load causes stress, deformation, and displacement in a structure. Structural analysis, a discipline in engineering, analyzes the ef ...
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 σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...
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, skylights, other openings, and Reflective surfaces (climate engineering), reflective surfaces so that sunlight (direct or indirect) can provide effective internal lighting. Particular attention is ...
. 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 Stone veneer is a thin layer of any stone used as decorative facing material that is not meant to be load bearing. Stone cladding is a stone veneer, or simulated stone, applied to a building or other structure made of a material other than stone. ...
, metal panels, louvres, and operable windows or vents. Curtain wall systems differ from storefront systems in being designed to span multiple floors, taking into consideration 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 and the maximum size of window openings. The development and widespread use of
structural steel Structural steel is a category of steel used for making construction materials in a variety of shapes. Many structural steel shapes take the form of an elongated beam having a profile of a specific cross section. Structural steel shapes, sizes, ...
and later
reinforced concrete Reinforced concrete (RC), also called reinforced cement concrete (RCC) and ferroconcrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having hig ...
allowed relatively small
columns A column or pillar in architecture and structural engineering is a structural element that transmits, through compression, the weight of the structure above to other structural elements below. In other words, a column is a compression membe ...
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 A balloon is a flexible bag that can be inflated with a gas, such as helium, hydrogen, nitrous oxide, oxygen, and air. For special tasks, balloons can be filled with smoke, liquid water, granular media (e.g. sand, flour or rice), or light so ...
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 alloy with a very low carbon content (less than 0.08%) in contrast to that of cast iron (2.1% to 4%). It is a semi-fused mass of iron with fibrous slag Inclusion (mineral), inclusions (up to 2% by weight), which give it a ...
and glass such as
The Crystal Palace The Crystal Palace was a cast iron and plate glass structure, originally built in Hyde Park, London, Hyde Park, London, to house the Great Exhibition of 1851. The exhibition took place from 1 May to 15 October 1851, and more than 14,000 exhibit ...
were built, the building blocks of structural understanding were laid for the development of curtain walls.
Oriel Chambers Oriel Chambers is an office building located on Water Street near the town hall in Liverpool, England. It was the world's first building featuring a metal framed glass curtain wall, which has since become a defining feature of skyscrapers aro ...
(1864) and 16 Cook Street (1866), both built in
Liverpool Liverpool is a city and metropolitan borough in Merseyside, England. With a population of in 2019, it is the 10th largest English district by population and its metropolitan area is the fifth largest in the United Kingdom, with a popul ...
,
England England is a 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 and the Celtic Sea to the southwest. It is separated from continental Europe b ...
, by local architect and
civil engineer A civil engineer is a person who practices civil engineering – the application of planning, designing, constructing, maintaining, and operating infrastructure while protecting the public and environmental health, as well as improving existing ...
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 An elevator or lift is a wire rope, cable-assisted, hydraulic cylinder-assisted, or roller-track assisted machine that vertically transports people or freight between floors, levels, or deck (building), decks of a building, watercraft, ...
, 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 List of colossal sculpture in situ, colossal neoclassical sculpture on Liberty Island in New York Harbor in New York City, in the U ...
(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 and largest city of Germany by both area and population. Its 3.7 million inhabitants make it the European Union's most populous city, according to population within city limits. One of Germany's sixteen constitue ...
, 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. It is also often used as a division between double doors. When dividing adjacent window units its primary purpose is a rigid supp ...
, and the polished plate glass was attached to the mullions with asbestos- or fiberglass-modified glazing compound. Eventually silicone
sealant Sealant is a substance used to block the passage of fluids through openings in materials, a type of mechanical seal. In building construction ''sealant'' is sometimes synonymous with ''caulking'' and also serve the purposes of blocking dust, s ...
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 or NYC, is the List of United States cities by population, most populous city in the United States. With a 2020 population of 8,804,190 distributed over , New York City is also the L ...
, in the
United Nations Secretariat Building The United Nations Secretariat Building is a skyscraper within the headquarters of the United Nations in the Turtle Bay neighborhood of Manhattan in New York City. It contains the offices of the United Nations Secretariat, the executive organ ...
(Skidmore, Owings, and Merrill, 1952), was this type of construction. Earlier
modernist Modernism is both a philosophical and arts movement that arose from broad transformations in Western society during the late 19th and early 20th centuries. The movement reflected a desire for the creation of new forms of art, philosophy, an ...
examples are the
Bauhaus The Staatliches Bauhaus (), commonly known as the Bauhaus (), was a German art school operational from 1919 to 1933 that combined crafts and the fine arts.Oxford Dictionary of Art and Artists (Oxford: Oxford University Press, 4th edn., 200 ...
in Dessau (1926) and the
Hallidie Building The Hallidie Building is an office building in the Financial District of San Francisco, California, at 130 Sutter Street, between Montgomery Street and Kearny Street. Designed by architect Willis Polk and named in honor of San Francisco cable car ...
in
San Francisco San Francisco (; Spanish language, Spanish for "Francis of Assisi, Saint Francis"), officially the City and County of San Francisco, is the commercial, financial, and cultural center of Northern California. The city proper is the List of Ca ...
(1918). Ludwig Mies van der Rohe'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 Gregorian ...
. 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 and 53rd Streets, in the Midtown Manhattan neighborhood of New York City. Designed by Ludwig Mies van der Rohe with minor assistance from Philip Johnson, Ely Jacques Kahn, ...
in New York. During the 1970s, the widespread use of aluminium
extrusion Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex c ...
s for mullions began. Aluminum alloys 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 The Omni San Diego Hotel is a four-diamond high-rise luxury hotel and condominium tower in San Diego, California. It is owned and operated by Omni Hotels & Resorts. Hotel Built in 2004 at a cost of $124 million, the 32-story, 511-room Omni San D ...
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 defines quality control as "a part of quality management focused on fulfilling quality requirements". This approach places ...
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 wall detail where the siding (wall cladding) stands off from the moisture- resistant surface of an air/water barrier applied to the sheathing to create a capillary break and to allow drainage and evaporation. The ''r ...
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. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hou ...
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, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
-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 non-building structures. Buildings must conform to the code to obtain planning permiss ...
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 ...
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 In fluid dynamics, vortex shedding is an oscillating flow that takes place when a fluid such as air or water flows past a bluff (as opposed to streamlined) body at certain velocities, depending on the size and shape of the body. In this flow, v ...
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 σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...
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 A structural load or structural action is a force, deformation, or acceleration applied to structural elements. A load causes stress, deformation, and displacement in a structure. Structural analysis, a discipline in engineering, analyzes the e ...
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, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kineti ...
. 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 complex located at 200 N.W. 5th Street in downtown Oklahoma City, Oklahoma. On April 19, 1995, at 9:02 a.m. the building was the target of the Oklahoma City bombing ...
in
Oklahoma City Oklahoma City (), officially the City of Oklahoma City, and often shortened to OKC, is the capital and largest city of the U.S. state of Oklahoma. The county seat of Oklahoma County, it ranks 20th among United States cities in population, a ...
,
Oklahoma Oklahoma (; Choctaw language, Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a U.S. state, state in the South Central United States, South Central region of the United States, bordered by Texas on the south and west, Kansas on the nor ...
, 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 δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' "power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics) ** Aerodynamics, the study of the motion of air ** Analytical dyna ...
load analysis, with full-scale
mock-up In manufacturing and design, a mockup, or mock-up, is a scale or full-size model of a design or device, used for teaching, demonstration, design evaluation, promotion, and other purposes. A mockup may be a ''prototype'' if it provides at lea ...
testing performed prior to design completion and installation. Blast resistant glazing consists of
laminated glass Laminated glass (LG) is a type of safety glass that holds together when shattered. In the event of breaking, it is held in place by a thin polymer interlayer, typically of polyvinyl butyral (PVB), ethylene-vinyl acetate (EVA), lonoplast polyme ...
, 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, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
-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. It is also often used as a division between double doors. When dividing adjacent window units its primary purpose is a rigid supp ...
, 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 ...
s, and through imperfect sealing. The
American Architectural Manufacturers Association American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the "United States" or "America" ** Americans, citizens and nationals of the United States of America ** American ancestry, pe ...
(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 specificati ...
, 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 the unit of measurement of an object's 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 ...
is about one-third that of steel. This translates to three times more
deflection Deflection or deflexion may refer to: Board games * Deflection (chess), a tactic that forces an opposing chess piece to leave a square * Khet (game), formerly ''Deflexion'', an Egyptian-themed chess-like game using lasers Mechanics * Deflection ...
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 which reflects how its points are distributed with regard to an arbitrary axis. The ...
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. Deflection in curtainwall mullions also differs from deflection of the building structure, whether concrete, steel, or timber. Curtainwall anchors must be designed to allow differential movement between the building structure and the curtainwall.


Strength

Strength (or maximum usable
stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
) 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, product, or process. The verb ''to design'' ...
and
analysis Analysis ( : analyses) is the process of breaking a complex topic or substance into smaller parts in order to gain a better understanding of it. The technique has been applied in the study of mathematics and logic since before Aristotle (38 ...
. 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 of heat. This translates into high heat loss through aluminum (or steel) 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 (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 ...
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 may form on the interior of the mullions. This could cause damage to adjacent interior trim and walls. Rigid
insulation Insulation may refer to: Thermal * Thermal insulation, use of materials to reduce rates of heat transfer ** List of insulation materials ** Building insulation, thermal insulation added to buildings for comfort and energy efficiency *** Insulated ...
is provided in spandrel areas to provide a higher R-value at these locations. Thermally-broken mullions with double or triple-glazed IGUs are often referred to as "high-performance" curtainwalls. While these curtainwall systems are more energy efficient than older, single glazed versions, they are still significantly less efficient than opaque (solid) wall construction. For example, nearly all curtainwall systems, thermally-broken or otherwise, have a U-value of 0.2 or higher, which is equivalent to an R-value of 5 or lower.


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 commercially us ...
panels. Infills are also referred to as spandrels or spandrel panels.


Glass

Float glass Float glass is a sheet of glass made by floating molten glass on a bed of molten metal, typically tin, although lead and other various low- melting-point alloys were used in the past. This method gives the sheet uniform thickness and very flat sur ...
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 Insulating glass (IG) consists of two or more glass Paned window (architecture), window panes separated by a space to reduce heat transfer across a part of the building envelope. A window with insulating glass is commonly known as double glazin ...
. 1/4 inch glass is typically used only in 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 readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...
es, such as
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
or
krypton Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
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 U-value. The thermal insulance of a struct ...
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 vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...
, or sound transmission requirements, such as laboratory areas or
recording studio A recording studio is a specialized facility for sound recording, mixing, and audio production of instrumental or vocal musical performances, spoken words, and other sounds. They range in size from a small in-home project studio large enoug ...
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 (optics), the physical property of allowing the transmission of light through a material They may also refer to: Literal uses * Transparency (photography), a still, ...
,
translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions a ...
, 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.


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 Ca2SiO4, also known as calcium orthosilicate and is sometimes formulated as 2CaO·SiO2. It is also referred to by the shortened trade name Cal-Sil or Calsil. It occurs naturally as the mineral larnite. ...
,
granite Granite () is a coarse-grained (phaneritic) 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 cools and solidifies undergro ...
,
marble Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or Dolomite (mineral), dolomite. Marble is typically not Foliation (geology), foliated (layered), although there are exceptions. In geology, the ...
,
travertine Travertine ( ) is a form of terrestrial limestone deposited around mineral springs, especially hot springs. It often has a fibrous or concentric appearance and exists in white, tan, cream-colored, and even rusty varieties. It is formed by a p ...
,
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
, and engineered stone. 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 fibre-reinforced polymer, or in American English ''fiber'') is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass (in fibreglass), carbon (in carbon-fib ...
(FRP) and
terracotta Terracotta, terra cotta, or terra-cotta (; ; ), in its material sense as an earthenware substrate, is a clay-based ceramic glaze, unglazed or glazed ceramic where the pottery firing, fired body is porous. In applied art, craft, construction, a ...
. 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) or louvre (British English; see spelling differences) is a window blind or shutter with horizontal slats that are angled to admit light and air, but to keep out rain and direct sunshine. The angle of the sla ...
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, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HV ...
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 that is used to seal around openings and between joints in a fire-resistance-rated wall or floor assembly. Firestops are designed to maintain the fire-resistance rating of a wall ...
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 system can withstand a standard fire resistance test. This can be quantified simply as a measure of time, or it may entail other criteria, involving eviden ...
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 buildings, traffic tunnels, ships, aerospace vehicles, or submarines, is the fundamental basis and aim of passive fire protection. The idea is to divide a structure into "fire compartments", ...
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 The First Interstate Tower fire was a high-rise fire that occurred on May 4, 1988, at the First Interstate Tower (now Aon Center) in Los Angeles, California, a 62-story, 860 foot (260 m) skyscraper, then the tallest building in the city. The fi ...
in
Los Angeles, California Los Angeles ( ; es, Los Ángeles, link=no , ), often referred to by its initials L.A., is the largest city in the state of California and the second most populous city in the United States after New York City, as well as one of the world' ...
. 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 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
anodized Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called ''anodizing'' because the part to be treated forms the anode electrode of an electr ...
. Care must be taken when cleaning areas around anodized material as some cleaning agents will destroy the finish. Factory applied
fluoropolymer A fluoropolymer is a fluorocarbon-based polymer with multiple carbon–fluorine bonds. It is characterized by a high resistance to solvents, acids, and bases. The best known fluoropolymer is polytetrafluoroethylene under the brand name "Teflon ...
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 A mullion wall is a structural system in which the load of the floor slab is taken by prefabricated panels around the perimeter. Visually, the effect is similar to the stone mullioned windows of Perpendicular Gothic or Elizabethan architecture. ...
*
Insulated glazing Insulating glass (IG) consists of two or more glass window panes separated by a space to reduce heat transfer across a part of the building envelope. A window with insulating glass is commonly known as double glazing or a double-paned window, ...
*
Quadruple glazing Standard quadruple glazed window - openable The quadruple glazing, Q-Air, on Deg 8 building in Oslo, Norway (2020). Renovation brings Ug value of 0,29 W/(m2K) -value 20Quadruple glazing (quadruple-pane insulating glazing) is a type of insulated ...
*
Copper in architecture Copper has earned a respected place in the related fields of architecture, building construction, and interior design.Kireta Jr., Andy (2009). The copper advantage, ''Metal Architecture,'' June 2009; www.metalarchitecture.com From cathedrals ...


References


External links


European Commission's portal for efficient Curtain Walling

EN 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