Maximizing Building Performance with Steel Curtain Walls and High-Performance Glazing
Devin Bowman, General Manager, Technical Glass Products
Today, demand for sustainable architecture and efforts to improve a building’s energy-efficiency are driving design decisions across the building and construction industry. Consequently, building codes and standards are becoming more stringent, many of which require reduced heat transfer in structures. To meet these demands, architects and engineers are exploring creative solutions that enhance a building envelope's energy performance while balancing their design vision. Steel curtain walls can serve as an important piece of the larger puzzle in meeting efficiency, offering both visual appeal and elevated performance for storefronts, offices, institutions and more.
With three times the strength of aluminum, modern steel frames allow for larger free spans with narrower profiles. This, in turn, enables design teams to both expand the viewing area and permit a greater percentage of light to enter through to interior occupied spaces. To balance the increase in daylight with heat transfer, steel can support the weight of high-performance glazing, including heavy triple-glazed IGUs. Plus, when paired with low-emissivity (low-E) coatings, steel can help architects and designers optimize the U-value of the curtain wall. With air-tight framing capabilities and thermal conductivity that is significantly lower than aluminum’s, steel framing systems can maximize the performance of a glazing system.
The U-value of a curtain wall
To boost a glazing assembly’s energy efficiency, design teams often combine framing systems with low-emissivity (low-E) glass, insulated glass units (IGUs) or other energy-efficient glazing. While this improves center-of-glass performance values, most framing materials have a high thermal conductivity compared to other elements of the building envelope. This leaves a weak spot for heat transfer where the captured or retained glass edge meets the supporting frames.
This is why the thermal conductivity of any profile plays an important role in the U-value of curtain wall system, especially at the junction where glazing meets the supporting frames. In the past, professionals using aluminum framing have long had to rely on thermally broken systems (separation between inner and outer frames) to reduce the heat flow associated with aluminum’s high thermal conductivity (118 Btus per hour). But today thermally broken systems are no longer the only solution available to design professionals.
Steel’s thermal conductivity of 31 Btus per hour is approximately 74 percent less than aluminum’s, which is equivalent to that of a thermally broken aluminum frame. These values underscore steel’s ability to minimize heat transfer in a curtain wall, creating more energy-efficient building envelopes. In fact, some advanced steel curtain walls do not require traditional thermal breaks due to their design detailing. With less metal required to support the glass, the glazing system further reduces the pathway for heat transfer.
When paired with 1 in. thick clear IGUs with low-E coatings, independent testing agencies have recorded steel curtain wall’s modeled U-values to be between 0.29 to 0.39. The thermal performance significantly surpasses that possible with many aluminum curtain walls.
Steel frames support high-performance glazing
Another way steel frames are advancing energy-efficient building design is through its support for high-performance glazing. To meet forward-thinking building performance standards and energy codes, architects and designers may need to specify double- and triple-glazed systems. These assemblies help balance natural light admission with energy costs. But due to the size and loads of these configurations, similar-sized traditional framing may struggle to support them—requiring either a reduction in glass lite dimensions or shorter free spans, which would increase the metal area.
Steel’s inherent strength allows it to easily support heavy triple- and quadruple-glazed units, with infills up to 3 inches thick and weights up to 23 pounds per square foot (lb/sf). Its load-bearing capacity easily surpasses a triple-glazed unit’s standard thickness of 1.7 in. and weight of 10 lb/sf, helping maximize the assembly’s performance with ease.
Underscoring the potential of a high-performance steel curtain wall, computer simulations of steel framing with 1-inch IGUs comprised of clear glass and non-gassed airspace plus triple glazing achieved U-values as low as 0.19.
Modern steel curtain walls are more airtight and mitigate leakage
Reducing air leakage is another crucial factor in improving building performance and energy efficiency. Typically, uncontrolled air leakage disrupts indoor temperature, forcing mechanical systems to work harder to compensate for heat loss or gain, translating to increased energy consumption.
Advanced steel curtain wall systems excel in mitigating air infiltration. Firstly, steel’s coefficient of thermal expansion is about 12 x 10-6 meters per meter 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. In application, 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 full gasketing, modern steel curtain walls restrict 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 6.24 lb/sf, a steel curtain wall’s air leakage has been consistently measured at 0.01 cfm/sf or less of wall area. The gasketing covering the full face of the steel frames in the glazing pocket makes this possible by isolating the steel from coming in contact with any air possibly 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 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.
Steel curtain wall balance daylight goals with energy performance
As a curtain wall framing material, steel is a versatile choice that allows design teams to dissolve boundaries, maximize daylight penetration and frame uninterrupted views to enhance occupant experience—all while elevating building performance. Steel curtain wall’s capabilities can support design teams achieve credits for LEED requirements under multiple categories, including ‘Energy and Atmosphere’ and ‘Indoor Environmental Quality (EQ)’.
Learn more about TGP’s wide range of architectural steel curtain wall systems for your projects.
Devin Bowman is General Manager of Technical Glass Products (TGP) and AD Systems. With over 20 years of industry experience, Bowman is actively involved in advancing fire- and life-safety codes and sits on the Glazing Industry Code Committee (GICC).
