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.
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.