By Chuck Knickerbocker

Glazing systems play a significant role in connecting interior spaces to the outdoors, which has made them a popular option for today’s sleek and modern building facades. Their ability to transfer daylight also makes them beneficial for interior applications. While naturally heralded for these attributes, glazing systems are increasingly required to do more as sustainable construction and energy efficiency take centre stage. As a result, building and design professionals are turning to high-performance building envelopes to reduce operational carbon.  To meet this demand, glass and framing manufacturers have developed products that improve U-values, reduce solar heat gain, and enhance thermal performance.

With an ever-growing array of options available, building and design professionals might be challenged with selecting materials and systems that balance visual appeal with performance. To ease their process, this two-part series will delve into the performance of two beneficial glazing systems—steel curtain wall and channel glass, starting with the former.

Solving design challenges

Canadian building and design professionals frequently use curtain walls for their ability to create monumental building entrances, providing beautiful views of the outdoors as well as natural lighting for spaces inside. Given the prominent role glass plays in achieving these goals, it is easy to see why much of the conversation centres on the glass itself. However, despite glass having a significantly larger area in a curtain wall, the framing is key in helping establish numerous design and performance outcomes. After all, the material and size of the framing significantly impact a curtain wall’s structural strength.

Consider aluminum—the historical framing material for curtain walls. The material is lightweight, versatile, and more corrosion-resistant than traditional steel framing, which is beneficial, considering much of Canada is subject to freezing winters and heavy snowfall.

However, with a Young’s modulus € of approximately 69 million kPa (10 million psi), it may not be able to support the required loads associated with high-performance glazing, simply due to their size, thickness, and/or weight. Likewise, increasing the free span size may lead to thicker and bulkier frame profiles than is desirable to ensure structural integrity. Unfortunately, larger frame profiles limit the total area of glazing possible.

As often happens in building construction, materials used in the past have metamorphed into other variations of themselves, regaining popularity, as they did with wood and stone. This is now happening with steel, making it a viable alternative to aluminum for curtain wall framing systems.

Advanced technology offers weather sealing and moisture protection

Steel is an inherently stronger material than aluminum, with a Young’s modulus € of approximately 207 million kPa
(29 million psi). As a stronger material, it can support larger free spans at various locations throughout (not just at the podium level) of a building’s design.

Despite its strength advantage, the Canadian design-build sector rightfully preferred corrosion-resistant materials such as aluminum over steel since it could not stand up to catalysts of corrosion such as moisture and air.

However, today’s steel framing can be combined with innovative technology to eliminate any concern with regard to corrosion. The material can be powder- or wet-coated to match the desired colour scheme, using any coating suitable for Architecturally Exposed Structural Steel (AESS). The author recommends they approach the coating manufacturers or suppliers about using specific coatings on steel if there are any questions. Canadian building professionals can also use exterior caps or interior back mullions from stainless steel to match other building elements and suit local climate requirements. Manufacturers also provide high anti-corrosion protections, such as double-sided pre-galvanization, which is coated with a durable primer and finished with colour to enhance weather protection.

While these protective mechanisms are beneficial in their own right, some manufacturers now offer gaskets that isolate the water and prevent it from coming into contact with the steel components. In such systems, a continuous gasket, typically made from extruded silicone, fills the gap between curtain wall components and keeps water off the tops of insulated glass (IG) units. The water is directed to the verticals, where it is then weeped out of the glazing systems. The glazing pocket is also free of metal, supporting condensation resistance.

To further ensure protection, installers seal the lapped gasket joints at horizontal-to-vertical connections to prevent water intrusion to the steel back members and interior occupied spaces. Another benefit of using steel systems is they do not require zone damming at each glazed lite to manage water flow, unlike aluminum pressure plate systems. However, the full gasketing fabrication method without zone damming still provides a water penetration resistance of 718 Pa (15 psf) or more, meeting ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference.

Modern techniques and installation systems have improved steel curtain wall assemblies, ensuring they effectively protect against both dynamic and static water penetration which helps reduce the risk of corrosion. These overlaps and specific joint treatments also provide a layered barrier to help maintain air tightness within the envelope.

Manufacturing advantages help utilize steel’s strength

Beyond corrosion concerns, traditional steel framing members were not as adaptable to different designs, preventing their widespread use as a primary material. Today, modern steel fabrication techniques overcome these challenges, resulting in previously unimagined esthetic possibilities for curtain walls. For instance, in the cold-roll forming process, flat steel sheets or continuous coils undergo a series of rolling operations to achieve project-specific profiles. This process enables the creation of complex shapes and larger sections from thin-gauge carbon or even stainless steel, with thickness ranging from 24 gauge 0.60 mm (0.02 in.) to 6 mm (0.23 in.).

Further, laser cutting and welding processes provide even greater flexibility. These processes involve taking long, flat carbon or stainless plates with lengths ranging from 11.8 to 15 m (38 to 49 ft), and thicknesses up to 38 mm (1.5 in.), and cutting them into bars or strips of the necessary width for shaping. Once cut, these bars are assembled into the desired shapes (rectangles, channels, T’s, angles, square tubes, and I-beams), and the joints are welded using lasers. This method allows steel profiles to meet custom width and depth requirements, and in turn, enables curtain wall system depths to be tailored per specific project specifications. Unlike traditional steel forming methods, the laster cutting/welding process allows these profiles to have sharp corners 0.5 mm (0.02 in.) rather than rounded ones, allowing teams to create corner joints with no visible weld beads or fasteners.

These steel formation processes also allow for the development of larger sections of thin-gauge carbon or stainless material with longer member lengths. Steel, when shaped or formed in this advanced manner, is much more versatile. By combining design flexibility with strength, it makes enduring and expansive glazing facades possible.

Narrow yet stronger framing system

In addition to production techniques, steel’s material properties offer specifiers significant advantages over conventional materials in large curtain wall designs. Using the Young’s modulus € numbers earlier, steel is approximately three times stiffer than aluminum. This means steel framing systems can better resist deformation and deflection under wind and other loads. Consequently, the stiffness of steel allows framing profiles to be much narrower in width and depth than aluminum’s for the same load specifications.

As an example, if building and design professionals aim to meet design criteria for a typical two-storey curtain wall, steel frames can accommodate a 45 x 146 mm (1.75 x 5.75 in.) profile, whereas aluminum would require a 64 x 200 mm (2.5 x 8 in.) profile. The profile size of an aluminum frame in such a case would be 25 to 50 per cent larger than a steel framing system. The reduction in steel profile size without a compromise in material strength allows Canadian teams to build more expansive curtain walls with larger glazing areas.

Greater glazing spans with fewer supports

Due to their strength, steel profiles can also support much larger glass panes than what is possible with aluminum. Moreover, the aforementioned cold roll-formed and laser-welded steel processes help capitalize on steel’s strength and bolster its ability to handle a greater wind load. This enables building and design teams to achieve larger free spans and larger daylight openings between framing members without the need for additional supporting members. For example, consider a 6 m (20 ft) long steel or extruded aluminum mullion with the same cross-sectional properties, using 1.4 Pa (30 psf) in a 1.5 m (5 ft) module as the design load (discounting allowable deflection limits momentarily). In this scenario, an aluminum mullion deflects 111.6 mm (4.4 in.), whereas a steel mullion deflects about 38.51 mm (1.5 in.).

These properties allow building and design professionals to use smaller system shapes to realize larger free spans and glass lites, facilitating uninterrupted views of the outside and greater illumination for interior spaces.

Thinner frame profiles have a smaller surface area for the sun to move across during the day, resulting in smaller shadow projections. Understanding steel’s performance capabilities, designers for Vaughan Metropolitan Centre Station in Toronto used it to meet project goals efficiently. The steel system supported large free spans of glazing to allow generous amounts of daylight to fill the pavilion and support light transfer to the below-ground concourse.

Prioritizing designs with energy efficiency

Steel’s ability to increase glazing spans and areas is one of the simplest ways for Canadian building and design professionals to maximize daylight area in building design. However, this can make a building more vulnerable to heat transfer, posing a challenge to attaining energy efficiency. While pairing framing systems with glazing solutions such as low-emissivity (Low-E) coatings improves centre-of-glass (COG) thermal values, they may still be susceptible to summer heat gain and winter heat loss.

With Canada’s predominantly subarctic climate and long history of setting standards for energy efficiency in new construction, many building and design professionals have preferred thermally broken aluminum frames. By incorporating thermal breaks (i.e. separations between the inner and outer frames), they can help reduce the heat flow associated with the material’s high thermal conductivity (i.e. approximately 118 Btus per hour). However, thermally broken aluminum frames are not the only high-performance alternative available to Canadian professionals.

Thermally efficient glazing systems

Steel’s thermal conductivity is approximately 74 per cent less than aluminum (i.e. approximately 32,7000 joule
[31 BTUs/hr]). This is equivalent to that of thermally broken aluminum frames. Moreover, some advanced steel frames do not require a traditional thermal break due to profile designs. Steel frames without a thermal break require less metal to support the glazing than traditional aluminum frames. Therefore, they reduce and resist heat transfer. The computer simulations were conducted on steel curtain wall systems according to the National Fenestration Rating Council (NFRC) 100 Procedure for Determining Fenestration Product U-factors.

These simulations used 25.4 mm (1 in.) insulating glass units (IGUs) with clear low-e coatings. The results showed the combined system achieved U-values of 1.65-2.21 watt per square metre and Kelvin (W/m²•K)[0.29-0.39 Btu per hour per square foot per degree Fahrenheit (Btu/h•sf•F) (depending on the glass), significantly surpassing the thermal performance of many aluminum curtain walls.

Steel supports high-performance glazing

To further mitigate heat transfer in curtain walls, building and design teams often turn to double- and triple-glazed systems, and/or a variety of glass thicknesses to help balance the natural light admission with energy costs. In similar scenarios, traditional framing systems might struggle to support these configurations due to their size and loads, requiring either a reduction in glass lite dimensions or free spans (which would increase the metal area).

Steel’s inherent strength allows it to support heavy triple- and quadruple-glazed units, with glazing infills up to 76 mm (3 in.) thick and weights up to 112.3 kg/m² (23 lb/sf). This easily surpasses a triple-glazed unit’s thickness of 45 mm (1.7 in.) and weight of 48.8 kg/m² (10 lb/sf), helping bolster a building’s thermal efficiency with ease. Underscoring this point, in NFRC 100 computer simulations of steel frame materials with IGUs comprised of clear glass and non-gassed airspace plus triple glazing, recorded U-values as low as 1.078 W/m²•K
(0.19 Btu/h•sf•F). However, it is important to note these are approximate values only. Actual values will vary depending on specific glass and framing combinations per project requirements.

Mitigating airtight leakage

In addition to high-efficiency glazing, mitigating air leakage can be crucial to building performance and improving energy efficiency. Typically, uncontrolled air leakage disrupts comfortable indoor temperature. Consequently, the mechanical systems need to work harder to compensate for heat loss or gain, translating to increased energy consumption. Understanding its role in achieving energy efficiency, the National Building Code of Canada (NBC) emphasizes the importance of reducing air leakage.

13.1 x 10^-6 in./in./F or 23.6 x 10^-6 m/m/C

metres per metres per degree Celsius (m/m/C)

11.7 x 10^-6 m/m/C

Modern steel curtain walls can mitigate air penetration infiltration, which can be especially beneficial in cold Canadian climates. There are two factors at play here. Firstly, steel’s coefficient of thermal expansion is about 12 x 10-6 metres per metre per degree Celsius (m/m C), which is much closer to that of glass (9) and concrete (10) than aluminum’s coefficient of 23.6. Similar expansion rates to neighbouring materials reduce the strain placed on one material in case of temperature fluctuations. For example, when there is a high-temperature differential between the framing material and the supporting structure, the components will expand or contract differently, which can create gaps in the envelope and cause air leakage. The compatibility of the physical properties between framing materials is also crucial to minimizing risks of sealant failure and other instances that affect thermal performance.

Secondly, when paired with the full gasketing, modern steel curtain walls promise almost no air penetration. When tested (per ASTM E283/E283M, Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Skylights, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen) with a pressure differential of 30.47 kg/m² (6.24 lb/sf), a steel curtain wall’s air leakage has been consistently measured at 0.05 L/s•m² (0.01 cfm/sf) or less of wall area. Currently, the NBC and National Energy Code of Canada for Buildings (NECB) limit air leakage for fixed windows and other fenestrations at 0.2L/s•m² (0.04 cfm/sf). This means that steel-framed curtain walls can well meet and exceed these performance requirements.

The gasketing on the front of the steel frames makes this possible by isolating the steel from coming in contact with any air that may be present in the glazing channel. Together with perimeter detailing around the curtain walls or windows and select infill panels (glass, metal panels, etc.), the steel-framed curtain wall can have an almost impenetrable air and water barrier, as the test results above underline.

Naturally, tighter systems that mitigate heat transfer are better at keeping conditioned air inside a building while not allowing untreated exterior air into building interiors. This means lower loads on mechanical systems that do not need to work as hard to heat or cool interior spaces, improving energy efficiency. This way, steel-framed curtain walls support a project and team’s sustainability goals.

Sustainability considerations and LEED ratings

From optimizing energy performance to contributing toward daylight and views, steel curtain walls help building and design teams meet sustainability goals and various LEED requirements, which are especially relevant to Canadian green building initiatives.

With steel-framed curtain walls, green building professionals can earn points in the “Energy and Atmosphere” category by optimizing energy performance beyond standard requirements. Since high-efficiency glazing systems may improve building performance and reduce excess load on the HVAC systems, they can contribute to mitigating excessive energy use. Additionally, steel’s lower thermal conductivity compared to aluminum also decreases energy demands by reducing heat transfer between inside and outside.

Steel frames can also help earn points in the “Indoor Environmental Quality (EQ)” category, which aims to connect occupants with the outdoors, reinforce circadian rhythms, and reduce electrical lighting use by introducing daylight. There are three methods to accomplish this goal. The initial method involves conducting yearly computer simulations to determine spatial daylight autonomy and average spatial daylight autonomy, with a minimum value of 40 per cent. The second option requires computer simulations for illuminance during the equinox, while the third option involves directly measuring illuminance. Both the second and third options have a minimum threshold of 55 per cent for regularly occupied spaces.

Since steel mullions support large free spans, they assist building and design teams in meeting daylight goals. Additionally, steel mullions come in various sizes and shapes, such as box, I-beams, and T-shapes. Compared to aluminum, steel T-shapes are thinner, effectively allowing better light penetration deep into building interiors by increasing the glazing size and reducing shadows to improve the quality of space. Larger glazing areas also offer better unobstructed and quality views, which can earn more LEED points in a category that seeks at least 75 per cent of all regularly occupied floor areas to have a clear view of the outdoors through vision glazing. These factors make steel a compelling choice for environmentally conscious projects to maximize energy efficiency and advance sustainability goals.

In addition to evaluating the operational carbon of the building envelope to make strides toward a more sustainable construction practice, professionals are also examining the embedded energy and recyclable content within metal frames. Among available options, primary steel manufacturing consumes 2,800 kg (6,173 lb) of carbon dioxide equivalent per tonne (kgCO2e/tonne), while aluminum has an embodied carbon rate of around 8,300 kg (18,298 lb) CO2e/tonne. Both these materials are 100 per cent recyclable. However, recycled content in steel frames typically ranges between 15 to 25 per cent, whereas it is possible to utilize about 30 per cent of recycled aluminum in framing profiles. One of the reasons for this low recyclable content is that the global supply of scrap steel and aluminum (low carbon feedstock) is limited, albeit to a different extent.

Looking beyond design and performance

When specifying curtain walls, professionals often consider not only the design potential and performance but also the installation complexity, value for money, and long-term maintenance.

Typically, steel curtain walls are designed as stick wall systems, where fabricated and finished parts are shipped to the site for direct installation. Once the frame is in place, glaziers install the glazing. That said, it is also possible to unitize steel frames to a very limited extent, although it is less common when compared to aluminum extrusions. This is because the heavier weight of steel profiles requires stronger connections if assembled before installation. Plus, shipping unitized steel frames is not recommended over great distances, due to the vibration the framing is subject to in transit. Adding to the complexity, pre-glazing steel-framed unitized wall assemblies before field installation can negate the functionality of the single-glazing gasket on the face of the framing member. As a result, the installation process would require a field-applied wet seal between units, fundamentally negating the desired “unitized” approach.

Whether using a partly unitized or stick system, steel-framed curtain wall installation can be beneficial when evaluating total costs. Although steel framing generally runs 20 to 25 per cent higher than aluminum frames with the same spacing, this difference can be easily offset by steel’s ability to support larger lites. When looking at the larger picture, steel frames enable less framing members to be fabricated and erected, and can mean fewer field connections to the structure that have to be installed. A study found that increasing vertical mullion spacing from 1.219 m (4 ft) on centre (OC) to 1.524 m (5 ft) and 1.829 m (6 ft) OC, reduced framing costs by 15 to 20 per cent at each interval. While these values do not include erection costs, there are clear advantages in that regard as well. For example, a 9.144 m (30 ft) column spacing typically requires six 1.524 m (5 ft)  modules or five 1.829 m (6 ft) modules. This means at least one less vertical component to purchase, fabricate, ship, and install, along with one fewer module of horizontals to furnish and install, thus reducing the number of glass lites to install. These reductions facilitate labour savings and offset field installation costs.

Additionally, efficient field installation directly impacts long-term maintenance. Precise installation of the continuous gasket, mentioned above, is crucial to preventing air and water infiltration through a steel curtain wall system. Reduced exposure to elements can reduce maintenance requirements and prolong the service life of the glazing system. For projects exposed to high-speed winds, where air tightness is a concern, installers can use pressure plates as a two-line resistance strategy for air and water penetration resistance. In these systems, the pressure plates hold the lites in place and help maintain adequate pressure on the glass, gaskets and framing. This creates efficient seals that resist air and water penetration, not only enhancing a curtain wall’s performance but also reducing the need for intensive maintenance over the long term.

The balancing act

As Canadian building and design teams increasingly look for high-performance system solutions, specifying steel-framed curtain walls can help them address performance priorities without sacrificing visual appeal—after all, the exterior defines a building’s appearance.

To achieve the best of both worlds, it is crucial to consider how steel curtain walls fit within the overall building enclosure. This involves evaluating how a specific system interacts with perimeter details and surrounding materials, examining anchorage details, and verifying deflection to ensure structural integrity. Ironing out these details early in the design and specification process, through collaboration with steel framing manufacturers, can result in precise fabrication and seamless installation.

Moreover, consulting and adhering to the manufacturer or supplier’s installation guidelines is crucial for optimal performance. Assuming standard practices may lead to improper installation, particularly regarding air and water integrity, and could void the manufacturer’s warranty. Fortunately, many manufacturers offer design and installation support, which can help ensure enhanced energy efficiency and the enduring beauty of curtain walls for years to come.

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Author

Chuck Knickerbocker is the curtain wall manager for Technical Glass Products (TGP), a supplier of fire-rated glass and framing systems, along with specialty architectural glazing products. With more than 40 years of curtain wall experience, Knickerbocker has successfully worked with numerous architects, building owners, and subcontractors from development of schematic design through installation.