When we apply our years of insight gained through forensic failure investigations we learn to design dependable, high performance facades that have long-term sustainability.
To build effective, sustainable building envelopes, it is important to know what does and does not work and to use this knowledge to be better design professionals, manufacturers, and contractors. We see design failures from poorly engineered or constructed assemblies, as well as improper or untested materials used for fabrication such as, coatings, seals, gaskets, glazing systems, aluminum extrusions, etc. and apply these lessons to our design standards on new construction projects.
Due to increasing globalization, assemblies are often designed and fabricated across the globe. Adding to the complexity, the International Building Code (IBC) requires building skins to be highly energy efficient and more resistant to earthquakes and hurricanes. By constantly studying new construction performance and failures, we provide owners, architects, manufacturers and contractors feedback to prevent premature failures and improve products and assemblies.
By presenting examples of our forensic investigations and presenting water caused building damage examples, you will take away a practical understanding of how to avoid typical mistakes while improving building skin design, manufacturing, and assembly.
Today’s presentation will focus on the causes of the increasing number of window and curtain wall assembly failures that we are seeing in so many high-rise buildings. Over the last 20 years many advancements have been made in the construction of building skins including energy efficiency, recyclable content in materials, global manufacturing, air barrier requirements, and other LEED and International Code (ICC) requirements. While changes are necessary to make us more energy efficient, cost efficient, and sustainable, sometimes they result in unexpected consequences. We will examine these changes and how they impact the actual performance of these assemblies from a standpoint of life expectancy, unexpected air and water intrusion issues, coating failure, seals and sealant failure and Insulating Glass Unit (IGU) failure.
2. Curtain Wall Definition
Curtain Walls are 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 at the slab edge, and are typically thin, aluminum framed with glass panels or thin stone panels
that “hang” like a curtain from the building’s structural element. The 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. The inclusion of glass panels in the IGU provides different challenges, which include maintaining internal building temperatures, inhibiting wind and water intrusion, and sustaining the longevity of the building. Remember, the main function of a curtain wall is to keep the outside – outside.
The advent of glazed curtain walls was a marvel when first introduced and American architects embraced them, as did the rest of the world. Offering more interior space than traditional bearing wall construction, they provided less expensive construction, clean lines, greater sightline, and quicker construction. Over a hundred years ago, 1909, one of the first examples of this architecture was built in Kansas City, Missouri as The Boley Clothing Company Building with glazed curtain walls framed by traditional cast iron and terra cotta ornamentation. Architect Louis Curtiss combined the new technology with established period
design elements in this six-story structure that is still used today. Over the course of time, cast iron has given way to aluminum, which is lighter, can be extruded and coated with high performance coatings. Glazing systems have improved with double or triple panes of glass, coating with silver to provide low emissivity and improve thermal efficiency. The common components that make up curtain wall assemblies in high rise buildings include aluminum extrusions, a variety of energy efficient glazing, aluminum panels, stone or brick and glass fiber reinforced concrete (GFRC). These components, generally referred to as skins, are connected to the building either as a curtain wall (slab edge) or over light gauge steel studs extending from floor to slab. Curtain wall assemblies are typically “unitized” or “panelized”, connected to the slab edge or the top of slab by means of steel embeds and clips, engineered to carry their own weight and are further designed to resist lateral wind pressures and both thermal and seismic movement. In a typical light gauge steel framing scenario, the studs and tracks are erected from floor to ceiling with “slip tracks” to allow for floor deflection; the studs are strong enough to handle lateral wind pressure and have lightweight cladding and punched windows. Either type of assembly has to manage water and air infiltration under wind driven pressure, which increases with building
height and allows for floor deflection, seismic forces and wind pressure.
In theory, if the building skins are designed, fabricated, and installed correctly in accordance with good industry practices, it can stand for many centuries, performing as designed without major issues. The empire State Building is over 80 years old was retrofitted with dual, energy efficient glazing in the existing curtain wall skin which is still performing. The components of such assemblies that require maintenance are generally limited to replacing exterior sealants and glazing gaskets. Our forensic studies have shown that even the best designs are subject to failure, and that failure can occur due to many reasons including sub-standard materials and components, improper application of coatings, cutting corners in fabrication or erection, and poor design of assemblies. Failures can often result in air or water intrusion, interior condensation, aluminum coating failures, insulating glass failure, glass breakage and other deficiencies.
3. Water and Air Infiltration
Moisture damage is the most common type of failure in curtain wall sections. Water infiltration can damage interior finishes, create mold and mildew, and degrade indoor air-quality. Repairs to fix building skins are often very expensive, sometimes costing 4 to 10 times the cost of the original construction.
Moisture damage can be classified into two distinct parts. The first being water infiltration while the second is condensation. Air infiltration generally causes energy leakage but can also result in interior condensation by bypassing thermal breaks. Water infiltration creates the largest potential for damage to
buildings. In the USA, water intrusion is responsible for 85% of all construction related lawsuits and can cost contractors, manufacturers and owners Billions of dollars annually.
Generally, the building skin manages water as a “rain screen” design principal. Rain screens carefully manage the water and air infiltration such that water partially enters the “wet” areas of the skin and is managed and effectively expelled. High-rise buildings are subject to higher winds, which create a positive
pressure on the windward side and negative pressure on the leeward side. A rain screen system attempts to maintain the same pressure in the wet zone as the exterior face, with an air barrier on the inside face of the skin. Since the pressure differential is responsible for creating the force that drives water inside, the
rain screen system reduces this differential force. High pressure on the exterior skin can have the effect of forcing air and water through or around the skin assembly. The interior surface of the skin often has an air barrier to prevent or reduce the differential pressure gradient between the interior and exterior face of
the building skin thereby reducing the forces driving water and air through the skin. The primary components used to prevent air and water infiltration in curtain wall skins are seals and sealants.
In-fill type wall construction used for skins utilize weather resistive barriers to create the rain screen and serve as both a water and air barrier. Weather resistive barriers used in high and low rise buildings can include rolled goods like felt or polypropylene types of building paper, to fluid applied barriers that are
applied to the gypsum wall sheathing.
Sealants are known to break away because of poor adhesion or improper application. Add to that the thermal expansion, where the difference of aluminum expansion is 2.5 times greater than that of glass, enabling large relative displacements to cause sealants and seals to separate. A perfectly water-tight
designed curtain wall cannot be maintained by sealants alone. The general rule of thumb when considering curtain wall water penetration is, do 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. The image below illustrates an IGU system with an in-wall drainage system and well-considered sealants working together to remove water from the assembly.
Building skins, especially those constructed like a curtain wall, need to handle many forces such as wind, rain, seismic movement, and temperature differentials. In order to properly ensure an effective curtain wall design, laboratory and field mock-ups are necessary based on ASTM standards and American Architectural Manufacturers Association (AAMA) tests: AAMA 501.1, AAMA 501.2, ASTM E330 (structural), ASTM E331 (water resistance), ASTM E283 (air resistance), ASTM E1105 (field testing).
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 interior finishes. Basic design against condensation is to ensure that the Condensation Resistance Factor (CRF) of a given curtain wall section meets the requirement of the space, which is based on the expected temperature and humidity of the space. Designers should be aware that the CRF is an average and cannot account for cold spaces in the facility which can cause localized condensation.
The Whole Building Design Guide (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. A great way to prevent condensation in curtain wall frames is to use thermally broken aluminum. Thermal breaking is where a piece of plastic is incorporated in the aluminum 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 proven method of limiting condensation is by incorporating within the design, a limited amount of non-thermally-broken aluminum exposed to exterior conditions.
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.
4. The Frame
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 (unitized) in the shop and 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.
5. The Glass
Curtain wall glazing ranges in price, durability, impact resistance, safety, and stability, depending upon the manufacturing process of the glass used. The most common types:
Alastair Pilkington developed float glass in the 1950s, which 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 either 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 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.
IGUs improve thermal performance with double or triple panes of glass, separated by a space filled with air or 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.
6. Glass Failures
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 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. Inclusions can be minimized in tempered glass in order to stop NiS cracking in a curtain wall, by performing a heat soak test, but engineers should consider not using tempered glass in these applications.
Thermal cracking is a noticeable concern that engineers should consider when designing curtain wall assemblies. Thermal cracks occur when large temperature differences in the glass causes high stress within the pane, which cracks the glass. Thermal cracks are easy to detect because they are usually perpendicular to the frame and usually expand through the entire window section. Coatings placed in the glass to reduce the building cooling load often 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 expand 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 likelihood of thermal cracking occurring decreases slightly. 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. Engineers can specify the use of heat strengthened glass, a stronger form of glass, which can take higher thermal stresses, but other alternatives such as a reflective, not absorptive coatings should be considered, if applicable.
7. 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 methods, 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 lead to other problems including deflection as the steel corrodes and expands, causing the aluminum to bow outward.
7.2 Glazing Failure
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 will have failed, permitting air intrusion into the interstitial space and compromising thermal performance, as well as visibility. Glazing seals incorporate a combination of polyisobutylene (PIB) and silicone sealants to hermetically seal the inert air or gas between the dual or triple glazing.
Interior glazing seal failures can occur due to a number of causes including; improper application of sealants, excess or prolonged exposure to moisture and changes in elevation and / or pressure between the inboard and outboard glass.
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.
7.3 Gasket and Seal Degradation
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 water resistant 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 freezethaw cycles, the elastic material degrades, much like an old rubber band. At first, air spaces are 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 and require proper engineering to have the necessary bond strength and waterproofing characteristics. Signs that it’s time to replace perimeter sealants include shrinking or pulling away from the surface, gaps or holes, discoloration and brittleness.
In addition to what we would normally expect as failure causing situations, the advent of new sealants, gaskets and the associated chemicals has proven that designers need to be very aware of the quality and durability of the gaskets being incorporated into these highly energy efficient curtain wall assemblies. We
have discovered that using a high grade rubber is not the only criteria for the window gaskets. Being more ecologically conscious by incorporating recycled materials into new products is commendable for most products, but it can lead to more shrinkage and inadvertent failure. Lastly, competition and profit
goals sometimes drive manufacturers to reduce polymers, UV package, anti-oxidation package, and to add more fillers.
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. Often, the required ¼” minimum dimension is not present to allow wet sealing to work. Where possible, it is best to maintain the original glazing system and start with sustainable gaskets capable of long term performance like silicone.
7.4 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 manufacturer’s guidelines, and improperly manufactured sealants and gaskets, can result in premature failure that is both difficult to access and expensive to repair.
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 manufacturer’s guidelines, and improperly manufactured sealants and gaskets, can result in premature failure that is both difficult to access and expensive to repair.
Flashing detailing 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 not only a danger to people and property below, particularly when adhered using structural glazing tape alone without mechanical attachments, but poor quality control of the products during the manufacturing process can provide potential problems in the
long term. We have found that corrosion of the metal surfaces is accelerated if the proper requirements are not followed. We have seen where a 0.3 Mil paint coating was tested and found to only have 0.1 mil or less.
While the IGU industry has made great strides in the last 30 years to reduce failure, we have seen a rash of new IGU failures over the last 5-10 years. The new failures include flowing PIB sealants, lack of edge deletion of the Low-E coating leading to corrosion of Low-E coatings from the edges of glass and working
its way in.
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 also as a system, with consideration given to the impact of proximal building elements.
8. Conclusions and Lessons Learned
From the descriptions of failures faced in curtain walls and the examples discussed in this report and the presentation, several different conclusions can be inferred. First and foremost, an engineer or curtain wall design professional should be involved in all aspects of curtain wall design and construction. These
professionals should be aware of the proper techniques to prevent failures such as new performance issues plaguing the industry. Understanding waterproofing issues, glass failure issues, installation issues, poor visual performance, and poor thermal performance must be recognized. Appropriate laboratory and
field tests should be performed on all curtain wall sections to ensure proper performance according to the manufacturer’s standards and guidelines. These tests ensure that mistakes caught early on in the project can be corrected or rectified while changes to design are at a less costly stage of construction. Curtain wall failures can be prevented by proper consideration of potential failures and ensuring proper installation and maintenance of curtain wall sections.
8.1 Sealants and Gaskets
We have learned that sealants between the curtain wall and pre-cast concrete walls will fail over time. They can shrink prematurely, de-polymerize, or otherwise fail due to UV, heat and other manufacturing problems. We have found that seals with excessive fillers or non-virgin material breakdown much quicker than original material, possibly because the recycled portion has lived its useful life already and there is no longer the resiliency or sealing capacity that there once was. Gaskets used to seal IGUs may not have the ability to seal the insulating glass unit as originally designed due to shrinkage, deflection or other issues discussed.
8.2 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 and AAMA provide test standards for the evaluation of air and water penetration, as well as structural
performance of glass in curtain wall applications. Field 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 systemic issues 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.
8.3 Considerations for Rehabilitation
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.
Should major rehabilitation or replacement of a curtain wall be indicated, a number of factors should influence the design process.
8.4 Thermal Performance
Fifty years ago, curtain wall systems weren’t as energy-efficient as was solid wall construction. With energy costs and environmental considerations now topping the list of building management concerns, retrofits that increase the efficiency of glazed curtain walls have become an attractive option for improving overall building performance.
Keywords: Building Skin/Envelope/Enclosure, Forensic Investigation, Sustainable Design, Effective Design, Water Penetration, Construction Defects