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 1930'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.
Water penetration resistance is a function of glazing details (see Glazing), frame construction and drainage details, weather stripping 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.
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.
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 1910.66, CFR 1910.28 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.
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.