by Ben Mitchell, CSI, Chad Ricker, and Jerry Schwabauer
With glazed façades dominating urban landscapes, the strides made to improve the energy efficiency of glass are well-documented and generally well-understood. However, much more quietly, the framing members of wall and window systems have also been re-engineered. The performance improvements in these less-discussed components are poised to add up to big gains in efficiency.
Glazed openings have traditionally been major points of unwanted heat loss or gain for building interiors. Exacerbating the problem, alloyed aluminum—the most popular framing material in the industry—is also highly conductive. Nevertheless, aluminum is prized for its many advantages (such as durability, recyclability, and strength), and its conductivity can be reduced by the addition of a thermal barrier. This is important because aluminum is a primary building material in buildings pursuing sustainability, including those seeking certification under the Leadership in Energy and Environmental Design (LEED) program.
While R-value has become a very familiar measurement of thermal insulation, U-value is the key measure when it comes to glazed fenestration. R-value measures resistance to heat transfer, while U-value, or ‘thermal transmittance,’ measures the rate of heat transfer. Therefore, the two numbers are not a direct inverse, but can be thought of as opposites in that a higher R-value (i.e. high value of insulation) and a lower U-value (i.e. low amount of heat and/or cold being transferred across a barrier) is ideal. U-values are commonly used when discussing a system of building components as opposed to a single material.
Past design paths
Single-pane window glass was commonly used well into the 20th century. The proliferation of skyscrapers, and the extreme amount of thermal transfer that occurred over their extensively glazed exteriors, prompted the commercial production of double-and triple-glazed insulating units in the 1940s and 50s. Thermal isolator gaskets were installed around metal parts (e.g. mullions and pressure plates) to insulate them and provide protection against air and moisture penetration.
In the 1980s, insulating glass—double panes with inert gas or a vacuum seal between them—enabled further reduction of heat transfer. Following that, low-emissivity (low-e) coatings for glass were developed. Together, low-e coatings and insulating glass units (IGUs) improved the thermal performance of glazed openings immensely, since the glass itself represents the largest surface area over which thermal transfer occurs.
The most significant thermal path, or bridge, remaining in glazed openings was that of the spacer. In its earliest and simplest configuration, the IGU consisted of two panes, or lites, separated by an aluminum or metal spacer. The spacer was sandwiched between seals that held it between the lites.
IGUs used in curtain walls were dual-sealed; polyisobutylene (PIB) primary sealants were applied directly to the glazing, while silicone was used secondarily and provided structural performance. Spacers were usually U-shaped; a desiccant was placed within the canal to absorb any moisture between the lites.
This construction provided structural strength. However, it also provided a conductive conduit of metal that allowed heat and/or cold into the building, as well as created a temperature differential between the center and the edge of the glass, leading to condensation. Replacing the traditional aluminum spacer with warm-edge spacers (constructed from low-conductivity materials such as polymers or low-conductivity stainless steel) was a first step toward improving framing. Warm-edge spacers are now typically an integral part of fenestration systems.
Even after the introduction of IGUs and warm-edge spacers, remaining paths for thermal transfer were significant, costing building owners countless dollars in climate control. Further, market demand and more rigorous standards for thermal performance continued to grow while curtain walls—with their previously unmatched expanses of glazing—became the dominant design element in contemporary skyscrapers.
Condensation and associated mold and moisture problems (especially in sensitive environments such as healthcare facilities) also became targets for improvement. These health-related factors combined to make the need for framing advancements imperative, and the area addressed next was manipulation of the aluminum profile itself.