Glazed Curtain Wall Systems
In a general sense, the term “curtain wall” refers to an exterior wall that does not support loads other than its own weight. Curtain walls are therefore non-structural, and serve solely to protect the building from the elements.
Curtain walls are typically exterior walls that do not carry floor or roof loads, according to the Whole Building Design Guide. Wind loads and the curtain wall’s gravity loads are transferred back to the structural support of the building, and are typically thin, aluminum framed with glass panels or thin stone panels that “hang” like a curtain from the build’s structural element. A typical glazed curtain wall system is often a metal frame (usually aluminum) with in-fills, which can be glass, brick veneer, metal panels, thin stone or precast concrete.
Most Common Failures – water damage, glass damage/breakage, poor thermal performance, poor visual performance, construction and design errors, and installation errors.
Aluminum coating failure shown below.
Moisture damage is the most common type of failure in curtain wall sections. Water infiltration creates premature deterioration of the wall structure, finish damage, biological growth, and decreased interior air-quality (Schwartz, 2001). This situation can be easily prevented in initial curtain wall design and construction of the assemblies and the system installed in the building. Repairs from poor water protection are often expensive to fix, but can be easily mitigated with attention to design and use of effective materials and sealants.
Moisture damage can be classified into two distinct parts; water infiltration and condensation. Water infiltration creates the largest potential for moisture damage to a building. The primary water prevention tool is sealants, which inhibit water penetration into a curtain wall (sealants can break apart or shrink as they age). Sealants are known to break away because of poor adhesion or improper application. Add to that the thermal expansion coefficient, where the difference of metal expansion is 2.5 times that of glass, enabling large relative displacements to cause sealants to break. A perfectly water-tight designed curtain wall cannot be maintained by sealants alone. The general rule when considering curtain wall water penetration is to not rely entirely on sealants. Redundancy in water protection that incorporates an in-wall drainage system, as well as carefully considered sealant, is the best way to avoid water infiltration and to mitigate or minimize water damage.
In order to properly ensure an effective curtain wall design, laboratory and field mock-ups are necessary based on American Architectural Manufacturers Association (AAMA) tests 501.1, AAMA 501.2, ASTM E331, ASTM E547, ASTM E1105 (Viegener and Brown, 2010).
Condensation occurs when the temperature of the glass or aluminum frame in a curtain wall reaches the dew point temperature of the interior space conditions. Water forms on the surface of the glass or aluminum, and can cause damage to the unit. Basic design against condensation in ensuring the Condensation Resistance Factor (CRF) of a given curtain wall section meets the requirement of the space, which is based off of the expected temperature and humidity of the space. Designers should be aware the CRF is an average and cannot account for cold spaces in the facility, which can cause localized condensation. The WBDG states, when designing a curtain wall glass unit in areas where high humidity is required within the space (such as hospitals) or where configurations are abnormal, software modeling is a must, to ensure condensation does not occur. The WBDG also states laboratory tests simulating indoor and outdoor air temperatures and humidity of the space is good practice to see how a glass panel will perform. Specified tests are AAMA 1503.1 and National Fenestration Rating Council (NFRC) 500 (Viegener and Brown, 2010). A great way to prevent condensation in frames of curtain walls is to use thermally broken aluminum. Thermal breaking is where a piece of plastic is incorporated in the frame, which significantly decreases the heat flow in (or out) of a curtain wall. This reduction of heat flow raises the surface temperature of the aluminum, and decreases the possibility of condensation on the aluminum. Another prevention which can be incorporated in design is limiting the amount of non-thermally-broken aluminum exposed to exterior conditions (Viegener and Brown, 2010). Many mistakes have been made in curtain wall waterproofing design in the past. Being informed of proper waterproofing techniques and learning from past mistakes can help prevent future waterproofing failures. The following case studies provide examples of past water damage failures in curtain wall sections.
Composed of steel, aluminum, multi-laminate glass, or other resilient material, the frame is the support grid that holds the glass in place.
Stick systems are curtain walls at their most basic, with individual mullions, or framing elements, assembled in the field.
Unitized systems apply the same design principles as stick systems, but sections of the curtain wall are assembled in the shop, typically along with the infill and are installed as a unit.
Unit mullion systems combine the pre-assembled panels of unitized systems with the multi-story vertical mullions of stick systems. Upright mullions are installed first, with horizontal mullions and glazing installed as a unit.
Column cover and spandrel systems articulate the building frame by aligning mullions to structural columns. Pre-assembled or field-assembled infill units of glass or opaque panels are fitted between the column covers.
Point-loaded structural glazing systems eliminate visible metal framework by incorporating tension cables, trusses, glass mullions, or other custom support structures behind the glass panels. Glazing is anchored by brackets or by proprietary hardware embedded in the glass.
A comparison of a unitized system versus stick assembly in the images below.
Curtain wall glazing ranges in price, durability, impact resistance, safety, and stability, depending upon the manufacturing process. The most common types:
Float glass was developed in the 1950s by Alastair Pilkington, whose breakthrough float process enabled production of the large glass sheets that characterize curtain wall construction. Molten glass is fed into a bath of tin, where it flows along the surface, forming smooth glass with even thickness.
Annealed glass undergoes a controlled heating and cooling process that improves its fracture resistance. Despite its improved durability, annealed glass can break into sharp pieces, and many building codes limit its use in construction.
Tempered glass is chemically or thermally treated to provide improved strength and shatter resistance. On impact, tempered glass shatters into tiny pieces that are less likely to cause injury than are larger shards.
Heat-strengthened glass and chemically strengthened glass fall between annealed and tempered glass in terms of strength. Unlike tempered glass, strengthened glass can be sharp when shattered, so it is best suited to areas with limited access. Scratches in strengthened glass have also been shown to compromise its strength.
Laminated glass bonds two or more sheets of glass to an interlayer of plastic, generally polyvinyl butyral (PVB), which holds the glass in place if broken. Laminated glass is often specified for curtain walls in hurricane-prone regions or in areas requiring blast protection.
Insulating glazing units (IGUs) improve thermal performance through the use of double or triple panes of glass, separated by a space that is filled with air or with an inert gas.
Spandrel glass, which is darkened or opaque, may be used between the head of one window and the sill of the next. To create the illusion of depth at spandrel areas, transparent glass may be used in a shadow box, with a metal sheet at some distance behind the glass.
Glass failures in curtain walls can be split up into several different categories. Nickel Sulfide (NiS) inclusions, thermal cracking, and damage from impact are the most common types of glass damage. NiS inclusions, also known as “glass cancer”, are imperfections incorporated in the glass when it is manufactured. NiS remains at high temperatures, after the rest of the glass has cooled. After the NiS cools, the inclusions expand in volume and crack the glass. This effect is most commonly seen in tempered glass. In order to stop NiS inclusions from cracking in a curtain wall, the engineer should consider not using tempered glass, or perform a heat soak test (Gromowski, 2010).
Thermal cracking of glass is another concern which the engineer should consider when designing a curtain wall. Thermal cracks occur in the glass when large temperature differences in the glass cause high stresses within the pane, forcing the glass to crack (Chowdhurt and Cortie, 2007). Thermal cracks are easy to detect because they perpendicular to the frame and usually expand the whole window section (McCowan and Kivela, 2011). Failures are more likely to occur when an absorptive coating is placed on the glass. These coatings are put in place to reduce the cooling load of the building, but can come at a cost to the glass integrity because they absorb solar radiation and keep it stored in the glass. The stored energy increases the temperature of the glass, and can cause it to expanded unevenly, creating a crack. The more effective (i.e. more sun absorbed) the more likely the glass is to crack. If the glass support allows some movement, the likeliness of a thermal crack occurring slightly decreases. In order to properly design for absorptive coatings, an engineer should consider the stresses which will be induced in the glass by the sun and the coating and see if the stresses will likely cause the glass to crack. Also, an engineer can specify the use of heat strengthened glass, a stronger form of glass, which can take higher thermal stresses. Other alternatives such as a reflective, not absorptive coatings should be considered, if applicable.
Causes of Distress and Failure
Like all building elements, curtain walls have their weak points. Knowing what to look for, how to extend the service life of a curtain wall system, and when it’s time to retain a consultant are critical to avoiding costly and disruptive failures. Although issues vary with frame material, construction method, and glazing type, there are some common concerns that design professionals look for when evaluating the condition of a curtain wall system.
Aluminum has many advantages as a curtain wall framing material, but it has the distinct disadvantage of deflecting approximately three times as much as steel does for a given load. Even when the amount of deflection doesn’t compromise the strength of the aluminum members, it still may pose a danger in that the glass may be forced out of place. To protect against excess deflection, mullions are extruded into shapes that maximize the area moment of inertia, or resistance of a particular cross-sectional shape to bending stress. Wide-flange elements, such as I-beams, have particularly high area moments of inertia, which is why this profile is used so often in construction.
To reduce deflection in a curtain wall assembly, without adding excess depth to the frame profile, steel reinforcement may be added to aluminum mullions. This method protects the steel from exposure to the elements, while taking advantage of its load-bearing properties. However, water penetration into a steel-reinforced system can also lead to deflection as the steel corrodes and expands, causing the aluminum to bow outward.
Condensation on glass curtain walls may be an indication that the relative humidity of the interior is too high, and an adjustment to heating and cooling equipment is necessary. However, condensation may also point to failure of the curtain wall system. If moisture is observed between panes of glass in an IGU, the hermetic seal may have failed, permitting air intrusion into the interstitial space and compromising thermal performance, as well as visibility.
Hairline cracks in glass may indicate excessive thermal loading, particularly if the glass has a coating, such as a low-e film or tint. When the sun strikes the glass, it heats the exposed portion of the pane, causing it to expand. The unexposed edges remain cool, creating tensile stress that can lead to cracking, particularly in glass that has not been heat-strengthened or tempered.
Nickel sulfide (NiS) inclusions can cause glass to shatter suddenly, sometimes years after installation. All glass has microscopic imperfections, or inclusions, that result from the manufacturing process. Generally speaking, these are of little concern. The exception is NiS inclusions in tempered glass, which have led to a number of dramatic glass failures. As glass is heated during the tempering process, NiS converts to a compressed (alpha) phase. When the glass is cooled rapidly to temper it, the NiS lacks sufficient time to return to a stable low-temperature (beta) phase. Over months or years, the trapped NiS transitions to the beta phase, expanding as it does so. The resultant pressure leads to micro-cracks in the glass, which can propagate until the glass structure is thoroughly compromised, and the glass shatters in what seems to be a spontaneous breakage. In an existing structure, ultrasound, laser imaging, or heat soak testing may be used to identify NiS inclusions; however, such test methods can be labor-intensive and expensive. For buildings with multiple glass failures, the pros and cons of full glazing replacement should be weighed against the costs of testing and isolated replacements.
Gasket and Seal Degradation
The image shows gaps created by gasket failure, which admit air and moisture into the system.
A common cause of curtain wall problems is failure of the gaskets and seals that secure the glazing. Gaskets are strips of synthetic rubber or plastic compressed between the glazing and the frame, forming a watertight seal. Gaskets also serve to cushion the glass and to accommodate movement due to wind, thermal, or seismic loads.
As they age, gaskets begin to dry out, shrink, and crack. Subjected to ultraviolet radiation and freeze-thaw cycles, the elastic material degrades, much like an old rubber band. At first, air spaces created by the shrinking, dried gaskets admit air and moisture into the system, leading to condensation, drafts, and leaks. As the gaskets further disintegrate, they may loosen and pull away from the frame. Without the support of flexible gaskets, the glass loses stability and may shatter or blow out. For this reason, it is important to maintain and routinely replace gaskets to keep the curtain wall system operational and safe.
In lieu of compression gaskets, some curtain wall systems use structural sealant, usually a high-strength silicone product, to secure the glass to the frame. Like gaskets, sealants have a finite service life. Signs that it’s time to replace perimeter sealants include shrinking or pulling away from the surface, gaps or holes, discoloration, and brittleness.
Typical EPDM Mullion Gasket Shrinkage
15/16’’ glazing installed produces a loose seal and leaks.
Design or Construction Defects
As with any type of construction, curtain walls are subject to the shortcomings of human capability. Material failure and age-related deterioration may be common causes of curtain wall distress, but many catastrophic and costly failures are attributable to avoidable errors.
Missing, incorrectly applied, or otherwise deficient sealants at frame corners and other intersections can lead to serious water infiltration issues. Failure on the part of the contractor to follow manufacturers’ guidelines, and on the part of the design professional to provide sufficient oversight, can result in water damage that is both difficult to access and expensive to repair.
Flashing detailing, too, requires fastidious attention to prevent leaks at intersections between the curtain wall and other building elements. Without detailed contract documents that fully describe and illustrate perimeter flashing conditions, along with coordination between the curtain wall installer and construction manager during installation, flashings may not be adequately tied or terminated, permitting water to enter the wall system.
Poorly installed trim covers and accessories can pose a danger to people and property below, particularly when adhered using structural glazing tape alone, without mechanical attachments. Construction sequencing is of particular importance, as swing staging and scaffolding can damage or displace mullion covers. Maintenance activities may also be responsible for loose trim elements.
Unforeseen structural interactions among building elements may lead to failure if the curtain wall has not been properly engineered. Inadequate provision for differential movement, as well as incorrect deflection calculations, may be responsible for cracked or broken glass, seal failure, or water intrusion. Glass and framing must be evaluated not only independently, but as a system, with consideration given to the impact of proximal building elements.
Finally, haphazard erection techniques may be responsible for premature curtain wall failure. Any of the above construction defects may result from unskilled and poorly supervised installation. Sloppy sealant application, for example, can block drainage outlets at the glass perimeter, or weep holes, trapping water inside the wall system. Scratches to glass during installation may diminish its strength and durability, and improperly applied window films also may decrease the lifespan of the glass.
Evaluation and Testing
If leaks, deflection, etched glass, or other issues have become a concern, an architect or engineer should conduct a systematic evaluation of the curtain wall system, beginning with close visual inspection. ASTM International provides test standards for the evaluation of air and water penetration, as well as structural performance of glass in curtain wall applications. Tests for water penetration, such as ASTM E1105, use a calibrated spray rack system with a positive air pressure differential to simulate wind-driven rain. ASTM E783 specifies test procedures for determining field air leakage at specific pressures.
Glazing that displays systematic scratches or other defects after installation may need to be evaluated for structural integrity. In such cases, a representative sample of glass units may be removed and tested under laboratory conditions. ASTM E997 is one test method for determining the probability of breakage for a given design load.
Considerations for Rehabilitation
Shrinking of neoprene exterior gaskets is a common concern, and it is not always easy to fix. Although some curtain wall systems, such as those that incorporate pressure bars, may permit gasket replacement without glazing removal, by and large it is difficult or impossible to replace gaskets without removing the glass. Wet sealing, which involves cutting out worn gaskets and adding perimeter sealant, may be an option; however, wet sealing does not generally result in a reliable water barrier, and it creates an ongoing maintenance demand. Where possible, it is best to maintain the original glazing system.
Anodized aluminum frames should be cleaned as part of a routine maintenance program to restore an even finish. For powder coats, fading and wear can be addressed with field-applied fluoropolymer products, although these tend to be less durable than the original factory-applied thermoset coating. Other coatings on the market aim to improve durability, but their track records and maintenance requirements should be considered prior to application.