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For the defensive walls around castles and towns, see Curtain wall (fortification)
A building project in Wuhan, China, demonstrating the relationship between the inner load-bearing structure and an exterior glass curtain wall Curtain walls are also used on residential structuresA curtain wall is an exterior covering of a building in which the outer walls are non-structural, instead serving to protect the interior of the building from the elements. Because the curtain wall façade carries no structural load beyond its own dead load weight, it can be made of lightweight materials. The wall transfers lateral wind loads upon it to the main building structure through connections at floors or columns of the building.
Curtain walls may be designed as "systems" integrating frame, wall panel, and weatherproofing materials. Steel frames have largely given way to aluminum extrusions. Glass is typically used for infill because it can reduce construction costs, provide an architecturally pleasing look, and allow natural light to penetrate deeper within the building. However, glass also makes the effects of light on visual comfort and solar heat gain in a building more difficult to control. Other common infills include stone veneer, metal panels, louvres, and operable windows or vents.
Unlike storefront systems, curtain wall systems are designed to span multiple floors, taking into consideration building sway and movement and design requirements such as thermal expansion and contraction; seismic requirements; water diversion; and thermal efficiency for cost-effective heating, cooling, and interior lighting.
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Oriel Chambers, Liverpool, England. Designed by the architect Peter Ellis and built in , it is the world's first building to feature a metal-framed glass curtain wall. 16 Cook Street, Liverpool, . Extensive use is made of floor-to-ceiling glass, enabling light to penetrate deeper into the building, thus maximizing floor space. Glass curtain wall of Bauhaus Dessau,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.[citation needed]
The development and widespread use of structural steel and later reinforced concrete allowed relatively small 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.[citation needed]
Post-and-beam and balloon framed timber structures effectively had an early version of curtain walls, for their frames supported loads that allowed the walls themselves to serve other functions, such as keeping weather out and allowing light in.[citation needed] When iron began to be used extensively in buildings in late 18th-century Britain, such as at Ditherington Flax Mill, and later when buildings of wrought iron and glass such as The Crystal Palace were built, the building blocks of structural understanding were laid for the development of curtain walls.[citation needed]
Oriel Chambers () and 16 Cook Street (), both built in Liverpool, England, by local architect and civil engineer Peter Ellis, are characterised by their extensive use of glass in their facades. Toward the courtyards they boasted metal-framed glass curtain walls, which makes them two of the world's first buildings to include this architectural feature.[1] Oriel Chambers is listed in the Guinness Book of Records as the earliest such building.[2] The extensive glass walls allowed light to penetrate further into the building, utilizing more floor space and reducing lighting costs. Oriel Chambers comprises 43,000 sq ft (4,000 m2) set over five floors without an elevator, which had only recently been invented and was not yet widespread.[3] The Statue of Liberty () features a thin, non-load-bearing copper skin. Extensive use of glass became required for large factory buildings to allow light for manufacture, sometimes making it seem like they had all glass facades.[4]
An early example of an all-steel curtain wall used in the classical style is the Kaufhaus Tietz department store on Leipziger Straße, Berlin, built in (since demolished).[5]
Some of the first curtain walls were made with steel mullions, and the polished plate glass was attached to the mullions with asbestos- or fiberglass-modified glazing compound. Eventually silicone sealants 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 landmarks of curtain wall design as it came to dominate construction were the very different systems used by the United Nations Headquarters and the Lever House completed in .[4]
Ludwig Mies van der Rohe's curtain wall is one of the most important aspects 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 860-880 Lake Shore Drive Apartments. 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's curtain wall appeared on all of his subsequent high-rise building designs, including the Seagram Building in New York.
The widespread use of aluminium extrusions for mullions began during the s. 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 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.[6]
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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.
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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 onsite. The drawbacks of using such a system is reduced structural performance and visible joint lines down the length of each mullion.
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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 within an interior climate-controlled environment. The economic benefits are typically realized on large projects or in areas of high field labor rates.
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A common feature in curtain wall technology, the 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.
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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.
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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 is defined as the weight of structural elements and the permanent features on the structure.[7] 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 is a normal force acting on the building as the result of wind blowing on the building.[8] 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-prone regions. For each project location, building codes specify the required design wind loads. Often, a 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 around corners and the effects of surrounding topography and buildings.
Seismic loads in a 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 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 (76 mm) of relative floor movement without glass breakage or water leakage.
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.[9]
Thermal loads are induced in a curtain wall system because aluminum has a relatively high coefficient of thermal expansion. 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.
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 in Oklahoma City, Oklahoma, 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.[10]
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 load analysis, with full-scale mock-up testing performed prior to design completion and installation.
Blast resistant glazing consists of laminated glass, which is meant to break but not separate from the mullions. Similar technology is used in hurricane-prone areas for impact protection from wind-borne debris.
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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, through weep holes, 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.[11]
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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 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 E 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 E 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.
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One of the disadvantages of using aluminum for mullions is that its modulus of elasticity is about one-third that of steel. This translates to three times more 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 that 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 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 curtain wall mullions also differs from deflection of the building structure, whether concrete, steel, or timber. Curtain wall anchors must be designed to allow differential movement between the building structure and the curtain wall.
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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 and analysis. 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.
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Condensation forms on the glass curtain wallRelative 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. These are barriers between exterior metal and interior metal, usually made of polyvinyl chloride (PVC). These breaks provide a significant decrease in the 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 may form on the interior of the mullions. This could cause damage to adjacent interior trim and walls.
Rigid insulation 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" curtain walls.[12] While these curtain wall systems are more energy-efficient than older, single-glazed versions, they are still significantly less efficient than opaque (solid) wall construction.[13] For example, nearly all curtain wall 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.[14]
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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 panels. Infills are also referred to as spandrels or spandrel panels.
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Glass curtain wall on the hotel Andaz in Singapore at sunsetFloat glass 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 (6.4 mm) monolithic and 1 inch (25 mm) insulating glass. 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 (13 mm) airspace. The air inside is usually atmospheric air, but some inert gases, such as argon or krypton, may be used in order to offer better thermal transmittance values. In Europe, triple-pane insulating glass infill is now common. In Scandinavia, the first curtain walls with quadruple-pane have been built.
Larger thicknesses are typically employed for buildings or areas with higher thermal, relative humidity, or sound transmission requirements, such as laboratory areas or recording studios. In residential construction, thicknesses commonly used are 1&#;8 inch (3.2 mm) monolithic and 5&#;8 inch (16 mm) insulating glass.
Glass may be used which is transparent, translucent, 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.
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Thin blocks (3 to 4 inches (76 to 102 millimetres)) 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, granite, marble, travertine, limestone, and engineered stone. To reduce weight and improve strength, the natural stone may be attached to an aluminum honeycomb backing.
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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 (FRP) and terracotta. Terracotta curtain wall panels were first used in Europe, but only a few manufacturers produce high quality modern terracotta curtain wall panels.
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A louver 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 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.
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Most curtain wall glazing is fixed, meaning that 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.
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Firestopping 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 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) 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 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.[15] 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.
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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.
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Aluminum frames are generally painted or 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.[16] 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.
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A curtain wall is defined as thin, usually aluminum-framed wall, containing in-fills of glass, metal panels, or thin stone. The framing is attached to the building structure and does not carry the floor or roof loads of the building. The wind and gravity loads of the curtain wall are transferred to the building structure, typically at the floor line. Aluminum framed wall systems date back to the 's, and developed rapidly after World War II when the supply of aluminum became available for non-military use.
Curtain wall systems range from manufacturer's standard catalog systems to specialized custom walls. Custom walls become cost competitive with standard systems as the wall area increases. This section incorporates comments about standard and custom systems. It is recommended that consultants be hired with an expertise in custom curtain wall design for projects that incorporate these systems.
The following are brief descriptions of commonly used curtain wall framing methods and components.
Curtain walls can be classified by their method of fabrication and installation into the following general categories: stick systems and unitized (also known as modular) systems. In the stick system, the curtain wall frame (mullions) and glass or opaque panels are installed and connected together piece by piece. In the unitized system, the curtain wall is composed of large units that are assembled and glazed in the factory, shipped to the site and erected on the building. Vertical and horizontal mullions of the modules mate together with the adjoining modules. Modules are generally constructed one story tall and one module wide but may incorporate multiple modules. Typical units are five to six feet wide.
Curtain walls can also be classified as water managed or pressure-equalized systems. See Moisture Protection below.
Both the unitized and stick-built systems are designed to be either interior or exterior glazed systems. Interior and exterior glazed systems offer different advantages and disadvantages. Interior glazed systems allow for glass or opaque panel installation into the curtain wall openings from the interior of the building. Details are not provided for interior glazed systems because air infiltration is a concern with interior glazed systems. Interior glazed systems are typically specified for applications with limited interior obstructions to allow adequate access to the interior of the curtain wall. For low rise construction with easy access to the building, outside glazing is typically specified. For high-rise construction interior glazing is sometimes used due to access and logistics of replacing glass from a swing stage.
In exterior glazed systems, glass and opaque panels are installed from the exterior of the curtain wall. Exterior glazed systems require swing stage or scaffolding access to the exterior of the curtain wall for repair or replacement. Some curtain wall systems can be glazed from either the interior or exterior.
Typical opaque panels include opacified spandrel glass, metal panels, thin stone, and other materials, such as terra cotta or FRP (fiber-reinforced plastic).
Vision glass is predominantly insulating glass and may have one or both lites laminated (see Glazing), usually fixed but sometimes glazed into operable window frames that are incorporated into the curtain wall framing.
Spandrel glass can be monolithic, laminated, or insulating glass. The spandrel glass can be made opaque through the use of opacifiers (film/paint or ceramic frit) applied on an unexposed surface or through "shadow box" construction, i.e., providing an enclosed space behind clear spandrel glass. Shadow box construction creates a perception of depth behind the spandrel glass that is sometimes desired.
Metal panels can take various forms including aluminum plate, stainless steel or other non-corrosive metal, thin composite panels consisting of two thin aluminum sheets sandwiching a thin plastic interlayer, or panels consisting of metal sheets bonded to rigid insulation, with or without an inner metal sheet to create a sandwich panel.
Thin stone panels are most commonly granite. White marble should not be used due to its susceptibility to deformation due to hysteresis (thin stone is not covered in this chapter).
The curtain wall often comprises one part of a building's wall system. Careful integration with adjacent elements such as other wall claddings, roofs, and base of wall details is required for a successful installation.
Face-sealed, water-managed and pressure-equalized rainscreen systems are the three systems that are available. Normally, pressure-equalized rain screen systems provide the highest levels of resistance to air and water infiltration, with water-managed systems the next most reliable.
Pressure-equalized rain screen systems function by blocking all of the forces that can drive water across a barrier. See the article on Moisture Protection for a complete explanation of how pressure-equalization resists water passage. As related to curtain wall systems, PE rain screen systems design the inside face of glass and the inside face of the glazing pocket and the interconnecting gasket or wet seal as an airtight barrier. The outside face of glass, exterior glazing materials and the outer exposed face of aluminum framing function as a rain screen, shedding water away. Between the exterior rain screen and the interior air barrier a pressure-equalization chamber is formed in the glazing pocket, which serves to reduce water penetration by eliminating (equalizing) the pressure difference across the rain screen that tends to force water into the system. Minor amounts of water that may penetrate the system are weeped harmlessly to the exterior.
Water-managed systems appear similar at first glance, incorporating drains and weeps from the glazing pocket, but no effort is made to create an air barrier or "zone-glaze" each glass or spandrel unit, and therefore a larger amount of water is forced into the system and must be weeped away. Also, since no air barrier exists, the pressure differential between the glazing pocket and the interior may be strong enough to force water vertically higher than interior gaskets, resulting in leaks. Weep holes in a water-managed system function largely to drain water that enters the glazing pocket while weep holes in a pressure-equalized system function primarily as vents to allow air movement between the exterior and glazing pocket. Weeping of water is only a secondary function. Note that the easiest way to recognize a pressure-equalized rain screen system is yo note that the that glazing pocket around each individual unit of glass is isolated air tight from adjacent units, most obviously with plugs or seals at the gaps between screw splines at mullion intersections. Detailing of spandrels, shadow boxes and interface with adjacent construction must maintain the continuity of the air barrier and rainscreen to function properly with a pressure-equalized rainscreen curtain wall framing system.
Some aluminum curtain wall systems are still designed as face-sealed barrier walls. They depend on continuous and perfect seals between the glass units and the frame and between all frame members to perform. The long-term reliability of such seals is extremely suspect and such systems should be avoided.
Overall curtain wall thermal performance is a function of the glazing infill panel, the frame, construction behind opaque (spandrel and column cover) areas, and the perimeter details.
Curtain wall frame conductance is a function of the frame material, geometry and fabrication (e.g. thermal break).
Aluminum has a very high thermal conductivity. It is common practice to incorporate thermal breaks of low conductivity materials, traditionally PVC, Neoprene rubber, polyurethane and more recently polyester-reinforced nylon, for improved thermal performance. Some "poured and debridged" polyurethane thermal breaks shrink and stress forms in the thermal break when the exterior aluminum moves differently from the interior aluminum due to temperature differences. Back-up mechanical attachment of the two halves of the frame is recommended (e.g. skip debridging or "t-in-a box"). A true thermal break is ¼" thick minimum and can be up to 1" or more, with the polyester reinforced nylon variety. Some curtain wall systems incorporate separators that are less than ¼", making them "thermally improved". The deeper thermal breaks can improve thermal performance and condensation resistance of the system.
Some curtain wall systems utilize "pressure bars" (also referred to as "pressure plates") that are fastened to the outside of the mullions to retain the glass. These systems frequently include gaskets that are placed between the pressure bar and mullions and function as thermal breaks and help with acoustic isolation. These systems require special care in design and construction to ensure continuity of the gaskets at horizontal and vertical transitions. Gaskets are also used to cushion the glass on the interior and exterior faces of the glass. The problem with gaskets is that they tend to be stretched during installation and will shrink back to their original length in a short time; they will also shrink with age and exposure to ultraviolet radiation. There is usually a gap in the gasket at the corners after shrinkage occurs. With a properly designed system the water that enters the system at the gasket corners will weep out through the snap cover weep holes. To mitigate shrinkage of gaskets back from the corners the use of vulcanized corners and diagonally cut splices are recommended.
Thermal performance of opaque areas of the curtain wall is a function of insulation and air/vapor barriers. Due to the lack of interior air adjacent to opaque curtain wall areas, these areas are subject to wide swings in temperature and humidity and require careful detailing of insulation and air/vapor barriers to minimize condensation. Some curtain wall systems include condensation drainage provisions, such as condensate gutters, that are intended to collect and weep condensate from spandrel areas to the exterior; such condensate gutters and weeps are a violation of the air barrier of the curtain wall unless they are outboard of the backpan. See discussion of back pans below.
At the curtain wall perimeter, maintaining continuity of the air barrier reduces airflows around the curtain wall. Integration of perimeter flashings helps ensure watertight performance of the curtain wall and its connection to adjacent wall elements. Proper placement of insulation at the curtain wall perimeter reduces energy loss and potential condensation issues. Insulating the mullions in a spandrel area may lead to excessive condensation in cold climates unless it can also be assured that humid air from the interior will never come in contact with the mullions. The spandrel area is typically not heated, thus the interior environment does not warm the mullions and offset the migration of the cold temperatures deep into the wall. In the vision area the interior heat helps to mitigate the cold and prevents condensation. For this reason, do not insulate between the interior portion of mullions and adjacent wall construction either.
Water penetration resistance is a function of glazing details (see Glazing), frame construction and drainage details, weatherstripping and frame gaskets, interior sealants (for operable windows, see Windows), and perimeter flashings and seals. Water can enter the exterior wall system by means of five different forces: gravity, kinetic energy, air pressure difference, surface tension, and capillary action. To mitigate water infiltration, all of these forces must be accounted for in the system design.
Unlike discontinuous windows, which are smaller units and can rely to a high degree on sill flashings to capture frame corner leakage, curtain walls cover large expanses of wall without sill flashings at each glazed opening. Water penetration of curtain wall frame corners is likely to leak to the interior and/or onto insulating glass below. Watertight frame corner construction and good glazing pocket drainage are critical for reliable water penetration resistance.
Key visual features of curtain walls are glazing appearance (see Glazing) and sightlines. Sightlines are defined as the visual profile of the vertical and horizontal mullions. The sightlines are a function of both the width and depth of the curtain wall frame. Lateral load resistance requirements (wind loads, spans) generally dictate frame depth. Where narrow sightlines are desired, steel stiffeners inserted into the hollow frame of aluminum extrusions can help reduce frame depth.
The acoustic performance of curtain walls is primarily a function of the glazing and internal seals to stop air leakage (covered elsewhere). The sound attenuation capability of curtain walls can be improved by installing sound attenuating infill and by making construction as airtight as possible. Incorporating different thicknesses of glass in an insulated glass unit will also help to mitigate exterior noise. This can be accomplished by increasing the thickness of one of the lites of glass or by incorporating a laminated layer of glass with a noise-reducing interlayer, typically a polyvinyl butyral or PVB.
Back pans are metal sheets, usually aluminum or galvanized steel, that are attached and sealed to the curtain wall framing around the perimeter behind opaque areas of a curtain wall. In cold climates insulation should be installed between the back pan and the exterior cladding in order to maintain the dew point outboard of the back pan so that the back pan acts as an air and vapor barrier. Back pans provide a second line of defense against water infiltration for areas of the curtain wall that are not visible from the interior and are difficult to access. Water infiltration in opaque areas can continue for extended periods of time causing significant damage before being detected. Back pans also are to be preferred over foil vapor retarders in high performance and humidified buildings as convection currents short-circuiting the insulation can cause condensation, wetting and ultimately failure of these spandrel areas.
Shadow box construction creates the appearance of depth behind a transparent lite of glass by incorporating a metal sheet into the curtain wall behind the lite. The metal sheet should be at least two inches behind the glass and may be painted or formed to create a texture, but reflective surfaces add the most visual depth to the wall. Insulation should also be installed behind the shadow box if interior finishes prevent room air from contacting this area. The system should be designed to collect any condensation that may collect on the exterior side of the metal sheet and drain it back to the exterior. Shadow boxes present a variety of challenges related to venting the cavity behind the glass, that can allow dirt on surfaces difficult to clean, or sealing the cavity and risking excessive heat build-up. Either way, the cavity may be at temperatures significantly above or below interior conditions with only thermally conductive aluminum between them. This can lead to condensation or surfaces so hot they can burn. Careful detailing can provide a method to thermally isolate the cavity from the interior. An interior back pan behind the insulation is desirable as well, to avoid condensation on the metal shadow box from the interior.
Curtain wall systems must transfer back to floor structure or intermediate framing both their own dead load plus any live loads, which consist primarily of positive and negative wind loads but might also include a snow load applied to large horizontal areas, seismic loads, maintenance loads and others. Unfortunately, the curtain wall will likely demonstrate movement caused by thermal changes and wind significantly different than movement of the building structure. Therefore the connections to anchor the curtain wall must be designed to allow differential movement while resisting the loads applied.
In stick-framed aluminum curtain wall, vertical mullions commonly run past two floors, with a combined gravity/lateral anchor at one floor and a lateral anchor only at the other. The splice between the vertical mullions will also be designed to allow vertical movement while providing lateral resistance. In large areas of stick framed curtain wall, a split vertical mullion will be introduced periodically to allow thermal movement. Note that this movement slightly distorts the anchors at the vertical mullions. Individual units of glass must accommodate the movement of the surrounding aluminum frame by sliding along glazing gaskets, distorting the gaskets or a combination of both. The movement of the glass within the frame and the movement forced in the anchors tend to induce additional stresses into a stick framed system.
Unitized curtain wall systems accommodate the differential movement between the structure and the thermal movement of the frame at the joints between each curtain wall unit. Because these units are frequently custom designed, the amount of movement to be accommodated can be carefully engineered into the system. Anchoring of unitized curtain wall typically consists of a proprietary assembly with three-way dimensional adjustability. The anchors occur at each pair of vertical mullions along the edge of slab or spandrel beam. Frequently, unitized systems span from a horizontal stack joint located at approximately desk height up to the anchor at the floor line above and then cantilevering past the floor to the next horizontal stack joint. The stack joint is designed to resist lateral loads while the two floor anchors resist gravity and lateral loads. One of the two floor anchors will allow movement in plane with the unitized system.
Fire safing and smoke seal at gaps between the floor slab-edge and the back of the curtain wall are essential to compartmentalize between floors and slow down the passage of fire and combustion gases between floors. A substantial ½" thick minimum poured smoke-seal is required to separate air return and supply plenums from each other, and for infection control in hospitals. Laboratory-tested fire rated assemblies may be required in unsprinklered buildings by some codes as Perimeter Fire Containment Systems when the floor assemblies are required to be fire-resistance rated. The ratings of the Perimeter Fire Containment System must be equal to or greater than the floor rating. These systems provide confidence that the materials used for perimeter containment remain in place for the specified duration of the required rating in a fire event.
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. Knock-out panels are identified by a non-removable reflective dot (typically two inches in diameter) located in the lower corner of the glass and visible from the ground by the fire department.
Buildings in cold climates have struggled throughout the ages with ice and snow formations that slide, fall, or get windblown from their roofs, ledges, and window sills, causing harm to people and damage to property below. Refer to the Resource Page on Considerations for Building Design in Cold Climates.
The curtain wall should be designed for accessibility for maintenance. Low-rise buildings can generally be accessed from the ground using equipment with articulated arms. For high rise construction the building should be designed for swing stage access for window cleaning, general maintenance, and repair work, like glass replacement. Davits and fall arrest safety tieback anchors should be provided on the roof and stabilization tie-offs provided on the face of the wall to comply with OSHA standards CFR .66, CFR .28 and ANSI/IWCA I-14.1 "Window Cleaning Safety Standard".
Curtain wall leakage, both air and water, can contribute to IAQ problems by supplying liquid water and condensation moisture for mold growth. This leakage can often remain concealed within the wall system and not become evident until concealed wall components experience significant deterioration and mold growth, requiring costly repairs.
Common curtain wall durability problems include the following:
Glazing failures (see Glazing). Glazing problems specific to curtain wall construction include visual obstruction from condensation or dirt, damage to opacifier films from material degradation, condensation and/or heat build-up, and IGU issues/laminated glass issues.
Failure of internal gaskets and sealants from curtain wall movements (thermal, structural), prolonged exposure to water (good drainage features reduce this risk), heat/sun/UV degradation (age). Repairs (if feasible) require significant disassembly of curtain wall. If restoration of internal seals is not physically possible or not economically feasible, installation of exterior surface wet sealing at all glazing and frame joints is often performed.
Failure of exposed gaskets and sealants, including perimeter sealants, from curtain wall movements (thermal, structural), environmental degradation. Repairs require exterior access.
Aluminum frames are inherently corrosion resistant in many environments if anodized and properly sealed or painted with baked-on fluoropolymer paint. Aluminum frames are subject to deterioration of the coating and corrosion of aluminum in severe (industrial, coastal) environments and galvanic corrosion from contact with dissimilar metals. Frame corner seals constructed using sealant are prone to debonding from prolonged contact with moisture and from thermal, structural, and transportation movements.
Curtain walls and perimeter sealants require maintenance to maximize the service life of the curtain walls. Perimeter sealants, properly designed and installed, have a typical service life of 10 to 15 years although breaches are likely from day one. Removal and replacement of perimeter sealants requires meticulous surface preparation and proper detailing.
Aluminum frames are generally painted or anodized. 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.
Exposed glazing seals and gaskets require inspection and maintenance to minimize water penetration, limit exposure of frame seals, and protect insulating glass seals from wetting.
The best strategy for sustainability of curtain walls is to employ good design practices to ensure the durability (maximum service life) of the installation and to use systems that have a good thermal break and high R-value (values as high as R-7 are possible with triple-glazed systems). Also, the use of low-e and spectrally selective glass coatings can significantly reduce energy loads and improve comfort close to the wall.
Aluminum and steel frames are typically recycled at the end of their service life. Salvage and demolition contractors generally require a minimum of 1,000 sq ft or more of window/curtain wall to make material recycling economical (smaller amounts are generally disposed as general trash). Recycling is less economical if the aluminum is contaminated with sealants, fractured glazing, etc., as salvage companies pay considerably less for the material. There is a limited market for salvaged steel and wood frames.
Select a curtain wall with a demonstrated track record in similar applications and exposures. Verifying track records may require significant research by the designer. ASTM E provides guidance.
Review laboratory test results of systems or similar custom systems for air, water, and structural resistance, heat transmission, condensation resistance, sound transmission, and operability. Verify that tests pertain to the system under consideration and not a version of the system with the same product name but of different construction.
Curtain wall design should start with the assumption that external glazing seals, perimeter sealant joints and curtain wall sills will leak. The following summarizes recommended features:
Pressure Plate Glazing: In this system the glass and infill panels are installed from the exterior, typically against dry gaskets. The outer layer of gaskets is installed and the gaskets are compressed against the glass by the torque applied to fasteners securing a continuous pressure plate. The plate is later typically covered with a snap-on mullion cover. This system provides reasonable performance but is susceptible to leaks at corners or joints in dry gaskets. For improved performance four-sided gaskets can be fabricated at additional cost or wet sealants can be installed to provide a concealed interior toe bead or exposed interior cap beads. Pressure plate glazing allows the easiest method to seal an air barrier from adjacent construction into the air barrier of curtain wall system.
Interior Dry Glazing: In this system the glass and infill panels are installed from the interior of the building, eliminating the need for substantial scaffolding and saving money. The frame is fixed and exterior dry gaskets are installed. Typically only the top interior mullion has a removable stop. The glass unit is slid into a deep glazing pocket on one jamb far enough to allow clearing the opposite jamb and is then slid back into the opposite glazing pocket and then dropped into the sill glazing pocket. The removable interior stop is installed and finally an interior wedge gasket is forced in. Sometimes this method is called "jiggle" or "wiggle" glazing because of the manipulation necessary to get the glass into place. Performance is slightly reduced because dry metal to metal joints occur at the ends of the removable stop at a point that should properly be air and watertight. Wet sealant heel beads will improve performance and some systems include an extra gasket to form an air barrier seal. Installation of spandrel panels may need to be installed from the exterior.
Structural Silicone Glazing: In this system the glass or infill unit is adhered to the frame with a bead of silicone. Outer silicone weather seals supplement the structural seal. Unitized systems are frequently structural silicone glazed, especially if four-side SSG is desired. Two-sided SSG, with pressure plate glazing or wiggle glazing on the other two sides is acceptable to be field installed.
Butt-Glazing: SSG is frequently mistakenly referred to as butt-glazing. True butt-glazing has no mullion or other back-up member behind the joint and relies solely on a sealant, typically silicone, between the glass units to provide a perfect barrier seal.
AAMA's Curtain Wall Design Guide provides guidance on window selection for condensation resistance. Establish the required Condensation Resistance Factor (CRF) based on anticipated interior humidity and local climate data and select a curtain wall with an appropriate CRF. Designers should be aware that the CRF is a weighted average number for a curtain wall assembly. The CRF does not give information about cold spots that could result in local condensation. Projects for which condensation control is a critical concern, such as high interior humidity buildings, require project-specific finite element analysis thermal modeling using software such as THERM. Careful analysis and modeling of interior conditions is required to accurately estimate the interior temperature of the air at the inside surfaces of the glass and frame. Curtain walls that are set well outboard of perimeter heating elements will have air temperatures along their interior surface that are significantly lower than the design wintertime interior temperatures. Thermal modeling of the building interior using Computational Fluid Dynamics (CFD) software can help establish a reasonable estimate for air temperatures at the inside surfaces of the glass and frame. These interior air temperatures are inputs for the thermal modeling software. Include lab mock-up thermal testing in addition to CFD modeling for analysis of project-specific conditions. Unusual or custom details, such as copings, deep sills, projected windows, spandrel areas and shadow box can dramatically alter performance.
Use thermally broken or thermally improved aluminum frames for best performance. At the perimeter of the curtain wall, the thermal break must be properly positioned with respect to the wall system/insulation to avoid exposing the aluminum frame inboard of the thermal break to cold air ("short circuiting" the thermal break). Special insulation provisions may be required where curtain walls project beyond adjacent cladding systems (e.g., an insulated perimeter extrusion or metal panning).
Consider frame geometry for thermally conductive aluminum frame materials. Minimize the proportion of framing exposed to the outdoors.
Refer to AAMA for descriptions of test method, parameters and equipment for determining U factors and CRF's for window products. Refer to NFRC 100 for U Factor and NFRC 500 for condensation resistance.
The use of glazed curtain walls can present challenges in balancing the desire for more natural daylight versus addressing the heat gain typically associated with such systems. Occasionally, there are concerns relating to having too much uncontrolled daylight, sometimes referred to as glare. The challenge is to strive for the highest visible light transmittance (VT) and the lowest solar heat gain coefficient (SHGC) while not preventing the glass from being too reflective when viewed from both the exterior and the interior, while controlling glare. This glass performance data are obtained from data using the Lawrence Berkeley National Laboratory (LBNL) Window 5.2 program with Environmental Conditions set at NFRC 100 criteria. NFRC 200 is used to determine the VT and SHGC values while the solar optical properties are determined using NFRC 300. Typically, for products more widely available on the market, the aforementioned values are readily available from glass manufacturers/fabricators.
Aluminum: Class I anodic coatings (AAMA 611, supersedes AAMA 606, 607 and 608) and high performance factory applied fluoropolymer thermoset coatings (AAMA ) have good resistance to environmental degradation.
Unitized systems are typically custom designed. There isa wide range of systems on the market from manufacturers that provide varying levels of reliability. Unitized systems range in performance ability from industry standard to high performance walls. It is thus recommended that projects specifying unitized curtain wall systems incorporate a team member who has a breadth of experience in designing and working with unitized systems.
Unitized systems are typically pressure equalized rain screen systems. The units should be completely assembled in a factory and shipped to the site for installation on the building. The units are placed on the floors, bundled in crates, using the tower crane and lowered into place using a smaller crane or hoist owned by the glazing contractor. The mullion dimensions tend to be slightly larger than a stick system due to their open section as compared to the tube shape of a standard stick curtain wall section. The advantages of the unitized system derive from the more reliable seals achievable from factory construction and the reduced cost of labor in the factory versus that of high rise field labor. Units can be assembled in a factory while the structural frame of the building is being constructed. Where stick systems require multiple steps to erect and seal the wall, unitized walls arrive on the site completely assembled allowing the floors to be closed in more quickly. Unitized systems also require less space on site for layout thus providing an advantage for urban sites with space limitations.
Unitized systems generally rely on rain screen design principles and gaskets and/or the interlock of mating frames for moisture protection at joints between adjacent modules. The interlocking vertical mullions will typically have two interlocking legs. One leg will be in the plane just behind the glazing pocket and the other at the interior face of the mullions. The interlocking leg in the plane of the glazing pocket will be sealed by gaskets and is the primary line of defense against water and air infiltration. More robust systems will also include a gasket at the interior interlock. Systems whose connecting legs lock also compromise the ability of the system to accommodate movement. Some unitized designs are sensitive to small irregularities in the spacing of adjacent modules; for example, if the module joints are slightly out of tolerance, gaskets may not be properly compressed and moisture protection may suffer. Robust designs include multiple lines of defense, realistic tolerances and adjustability for erection of modules.
The four-way intersection refers to the location where four adjacent units meet. This is where field labor must seal between adjacent units to achieve a weather tight wall. The interlocking legs of the horizontal mullions are the most critical interface of a unitized system. Water that infiltrates the interlocking vertical mullions drains to the interlocking horizontals that must collect and divert this water to the exterior. The top horizontal mullion of a unit incorporates upstanding vertical legs that mate with cavities in the bottom horizontal of the unit above. These upstanding legs have gaskets that seal against the walls of the bottom horizontal. Some designs provide one upstanding leg that provides one line of defense against air and water infiltration. More robust systems will provide two upstanding legs with gaskets on both legs. A splice plate or silicone flashing that is installed at the top of the two adjacent units as they are erected on the building is typically required.
The vertical mullions of unitized systems typically anchor to the slab edge as they pass by. The stack joint is the horizontal joint where units from adjoining floors meet. Placing the stack joint at the sill of the vision glass (typically 30" above the floor) will minimize the dimension of the vertical mullions. This positioning utilizes the back span of the mullion above the anchoring point at the slab to counteract the deflection of the mullion below the slab. Also placing the stack joint above the floor provides a more convenient location for field workers to achieve the critical seal at the four-way intersection.
While two story spans are feasible, the weight of the unit is doubled which may require increased structural capacity to accommodate the increased load. Wind load bracing should be incorporated at the single span height to avoid increasing the vertical mullion dimension to accommodate the increased span. Steel can be added to a unitized system to increase its spanning capability. However, unlike a stick system which has an integral hollow shape, the split mullions must be allowed to move independently to accommodate the building movement thus complicating the introduction of steel. Large units may also increase transportation costs from the factory to the site and erection costs of placing the units on the building.
Thermally broken unitized systems are available, utilizing similar technology as that used in stick curtain wall systems.
The service life of even the most durable curtain wall may be shorter than that of durable adjacent wall claddings such as stone or brick masonry. Therefore, the design of the curtain wall and perimeter construction should permit curtain wall removal and replacement without removing adjacent wall components that will remain.
The service life expectancy of components that are mated with the curtain wall into an assembly should match the service life expectancy of the curtain wall itself. Require durable flashing materials, non-corroding attachment hardware and fasteners, and moisture resistant materials in regions subject to wetting.
Laboratory testing: For projects with a significant amount of custom curtain wall, require laboratory testing of a mock-up curtain wall prior to finalizing project shop drawings. Have a curtain wall consultant present to document mock-up curtain wall construction and verify mock-up performance. Specify that laboratory tests are to be conducted at an AAMA Accredited Laboratory facility.
Field Mock-up: For all curtain walls, stock or custom, require construction and testing of a field mock-up representative of the wall/window assembly. This is best scheduled prior to the release of shop drawings for window production, so that there is an opportunity to make design changes based on the test performance of the field mock-up. Specify that field tests be conducted by an independent third party agency accredited by AAMA.
Field testing of curtain walls: Require the field testing of curtain walls for air infiltration and water penetration resistance, for quality assurance of curtain wall fabrication and installation. Require multiple tests with the first test on initial installations and later tests at approximately 35%, 70% and at final completion to catch problems early and to verify continued workmanship quality. Require additional testing to be performed if initial tests fail.
Shop drawing coordination: Require curtain wall installation shop drawings showing all adjacent construction and related work, including flashings, attachments, interior finishes, and indicating sequencing of the work.
Curtain wall systems, especially unitized systems, require expertise on the part of the building designer, the manufacturer, the fabricator, and the installer. For all but the simplest of systems, the designer should consider engaging an outside consultant, if such expertise is not available on the staff.
The following details can be viewed online in Adobe Acrobat PDF by clicking on the PDF to the right of the drawing title.
The details associated with this section of the BEDG on the WBDG were developed by committee and are intended solely as a means to illustrate general design and construction concepts only. Appropriate use and application of the concepts illustrated in these details will vary based on performance considerations and environmental conditions unique to each project and, therefore, do not represent the final opinion or recommendation of the author of each section or the committee members responsible for the development of the WBDG.
Note: the following S-series details are courtesy of Richard Keleher Architect
Typical Elevation&#;Stick-Built Curtain Wall-Pressure Equalized-Outside Glazed (Figure S &#; 1) PDF
This elevation shows a typical stick-built curtain wall set in a punched opening in a masonry cavity wall.
Curtain Wall Head&#;Stick-Built System-Pressure Equalized-Outside Glazed (Figure S &#; 2) PDF
Curtain Wall Jamb&#;Stick-Built System-Pressure Equalized-Outside Glazed (Figure S &#; 3) PDF
Curtain Wall Sill&#;Stick-Built System-Pressure Equalized-Outside Glazed (Figure S &#; 4) PDF
Intermediate Mullion&#;Stick-Built System-Pressure Equalized-Outside Glazed (Figure S &#; 5) PDF
Isometric of Finished System&#;Stick-Built System-Pressure Equalized-Outside Glazed (Figure S &#; 6) PDF
Isometric of Curtain Wall Sill&#;Stick-Built System-Pressure Equalized-Outside Glazed (Figure S &#; 7) PDF
Isometric of Vertical Curtain Wall Mullions&#;Stick-Built System-Pressure Equalized-Outside Glazed (Figure S &#; 8) PDF
Elevation of Horizontal Pressure Plate&#;Stick-Built System-Pressure Equalized-Outside Glazed (Figure S &#; 9) PDF
Note: the following U-series details are courtesy of The Facade Group
Isometric of System Assembly Unitized Curtain Wall System (Figure U&#;1) PDF
This elevation shows a typical unitized curtain wall assembly hung from the edge of the floor slab.
Isometric of Open Stack Joint Unitized Curtain Wall System (Figure U&#;2) PDF
Isometric of Completed Stack Joint Unitized Curtain Wall System (Figure U&#;3) PDF
Vision Glass Jamb Unitized Curtain Wall (Figure U&#;4) PDF
Unit Stack Joint Unitized Curtain Wall (Figure U&#;5) PDF
Intermediate Horizontal Unitized Curtain Wall (Figure U&#;6) PDF
Jamb at Spandrel Area with Anchorage to Slab Unitized Curtain Wall (Figure U&#;7) PDF
Unit Anchor to Slab Edge Section Unitized Curtain Wall (Figure U&#;8) PDF
"Smart" Curtain Walls, like smart windows, control visible light transmittance by employing electrochromic or photochromic glass coatings; see the discussion in Glazing. Double-skin systems, which employ a ventilated space between the inner and outer walls are rare in the U.S., but have been constructed in Europe and Asia where energy costs are much higher. Similar in concept to air-flow windows, the ventilated space is intended to conserve energy by modulating the temperature conditions inboard of the curtain wall. During the heating season, the space acts as a buffer between the exterior and interior, and can be used to temper outdoor supply air. During the cooling season, warm interior air is exhausted into the space. There is currently discussion among building science experts that, at least for cold climates, a less expensive way of achieving energy savings might be through the use of curtain walls with high (over R-6) insulating values. Point-supported glass, structural glass mullions and use of tension structures are recent technologies.
Functional / Operational&#;Ensure Appropriate Product/Systems Integration
Building Envelope Design Guide&#;Glazing, Building Envelope Design Guide&#;Windows, See appropriate sections under applicable guide specifications: Unified Facility Guide Specifications (UFGS), VA Guide Specifications (UFGS), Federal Guide for Green Construction Specifications, MasterSpec®
NOTE: Photographs, figures, and drawings were provided by the original author unless otherwise noted.
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