{"id":18322,"date":"2016-01-12T10:26:42","date_gmt":"2016-01-12T15:26:42","guid":{"rendered":"http:\/\/www.constructionspecifier.com\/?p=18322"},"modified":"2026-01-12T15:17:08","modified_gmt":"2026-01-12T20:17:08","slug":"thermal-efficiency-in-glazed-curtain-wall-systems","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/thermal-efficiency-in-glazed-curtain-wall-systems\/","title":{"rendered":"Thermal efficiency in glazed curtain wall systems"},"content":{"rendered":"
\"Close-up<\/a>
Photo courtesy Technoform<\/span><\/figcaption><\/figure>\n

This article was originally published in September 2016. It has been refreshed and updated by Chad Ricker to further expand on the subject, including updates in industry standards and codes related to thermal efficiency in glazed curtain wall systems.<\/em><\/p>\n

With glazed facades 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.<\/p>\n

Glazed openings have traditionally been major points of unwanted heat loss or gain for building interiors. Exacerbating the problem, alloyed aluminum\u2014the most popular framing material in the industry\u2014is 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.<\/p>\n

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 \u2018thermal transmittance,\u2019 measures the rate of heat transfer. U-values are commonly used when discussing a system of building components as opposed to a single material.<\/p>\n

Past design paths<\/h2>\n

Single-pane window glass was commonly used well into the 20th<\/sup> 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.<\/em> mullions and pressure plates) to insulate them and provide protection against air and moisture penetration.<\/p>\n

\"Street-level<\/a>
The 33-story JW Marriott Nashville in Tennessee was designed by architectural firms Smallwood, Reynolds, Stewart, Stewart & Associates, and Arquitectonica. The building\u2019s elliptically shaped, high-performance envelope was specified and constructed with an aluminum-framed window wall system featuring polyamide thermal barriers. Photo courtesy Chad Baumer Photography\/Technoform\/YKK<\/span><\/figcaption><\/figure>\n

In the 1980s, insulating glass\u2014double panes with inert gas or a vacuum seal between them\u2014enabled 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.<\/p>\n

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.<\/p>\n

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 box or U-shaped; a desiccant was placed within the spacer to absorb any moisture between the lites.<\/p>\n

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.<\/p>\n

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\u2014with their previously unmatched expanses of glazing\u2014became the dominant design element in contemporary skyscrapers.<\/p>\n

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 thermal improvement of the aluminum profile itself.<\/p>\n

\"Exploded<\/a>
High-performance thermal curtain wall. <\/span>
Image courtesy Tubelite<\/span><\/figcaption><\/figure>\n

Understanding the curtain wall<\/h2>\n

There are two main categories of curtain walls. The first comprises the \u2018stick-built\u2019 systems, in which all structural components are assembled at the project jobsite. Stick-built systems are optimal for building facades with little repetition among components, where architectural designs are more complex and few economies of scale can be reached by factory production.<\/p>\n

Unitized curtain walls, on the other hand, involve having glazed and aluminum components factory-assembled into panels and shipped to the construction site. Good for tall structures that use repeat components, this method is advantageous in that the controlled factory setting improves quality and installation to complete the enclosure faster.<\/p>\n

The extruded aluminum members comprising the curtain wall frame can be two separate pieces (one for the building interior and one for the exterior) joined at pressure points, as is common in stick-built systems. They can also be a single extruded profile, which is more common for unitized systems. With either method, engineered systems to interrupt the thermal bridge created by the metal cross section must be installed. Two systems include the pour-and-debridge thermal barrier and polyamide thermal barrier strips.<\/p>\n

The pour-and-debridge thermal barrier<\/h2>\n

For this method of thermal barrier, the aluminum profile is extruded with a channel designed to hold an insulating polymer. The channel is mechanically abraded to ensure adhesion of the polymer. A mechanical lock inside the thermal cavity locks the thermal barrier to the aluminum to provide superior structural strength for the composite. (Structural strength is extrusion- and cavity-specific; there is a great deal of data available depending on the extrusion type.) This allows the maximum separation of the aluminum required for lower U-factors while providing structural strength.<\/p>\n

Polyurethane polymers are poured as liquid into the thermal barrier channel (or cavity) where they solidify into a structural, thermal component. Finally, the metal bridge (or floor) is removed from the bottom of the aluminum channel during the debridging operation. The result is a true, non-metal-to-metal structural aluminum and polymer composite. By eliminating the conductive pathway of the interior to the exterior of the curtain wall framing, the energy efficiency of the building is significantly improved.<\/p>\n

Polyurethane polymers feature thermal conductivity 1300 times lower than aluminum and the thermal benefits occur within a very small amount of separation. The width and type of thermal barrier separation should be determined based on the given building\u2019s energy use or reduction requirements. (Once the thermal performance is determined [usually by U-factor], the manufacturer will select the appropriate thermal barrier and the required separation to achieve the desired performance.)<\/p>\n

\"Thermal
A:<\/strong> Curtain wall profile with thermal isolator. <\/span>
B:<\/strong> Curtain wall profile with polyurethane thermal barrier improves thermal efficiency by interrupting the heat flow through the extrusion. This also works to interrupt the heat flow in desert climates. Simulations conducted with – 18 C (0 F) outside and 21 C (70 F) inside temperatures. <\/span>
Image courtesy Azon<\/span><\/figcaption><\/figure>\n

A complete separation of the aluminum framing with an encapsulated polymer core maintains critical structural abilities to avoid strains and forces. Curtain walls \u2014especially those used on tall buildings\u2014are considered structural fa\u00e7ade elements as part of a building\u2019s wall system and must provide shear and tensile strength to sustain loads from high winds or even blasts or hurricanes. Thermal barriers must also accommodate stresses in the unitized curtain wall system that are created by thermal expansion and pressure differences. Fortunately, polyurethane polymers offer an exceptional amount of structural strength, particularly shear strength, allowing designers to maximize the span of glazing within the curtain wall framing. (Manufacturers of thermal barrier composites should be able to provide expected shear values, and structural testing, based on the type of extrusion selected.)<\/p>\n

In addition to the introduction of a polymer to interrupt the thermal bridge, energy efficiencies can be gained by manipulating the shape of the aluminum extrusion itself. The latest developments in this area include wide cavity and dual cavity construction.<\/p>\n

Wide cavity<\/h3>\n

Creating a wider cavity in the frame achieves a lower U-factor in most fenestration products. One can compare the U-factor of 2.44 W\/m2<\/sup>K (0.43 Btu\/[hr-F-ft]), achievable with a thermal isolator (Figure 1a) to that of a wider cavity, achieved by the use of a polyurethane thermal barrier (Figure 1b). In this example, the U-factor is improved to 2 W\/m2<\/sup>K (0.35 Btu\/[hr-F-ft]), and can be lowered to 1.7 W\/m2<\/sup>K (0.3 Btu\/[hr-F-ft]), assuming the use of performance glazing.<\/p>\n

\"Cutaway<\/a>
Dual thermal barrier curtain wall composite.<\/span>
Image courtesy Azon<\/span><\/figcaption><\/figure>\n
\"Cutaway<\/a>
Mechanical lock profile.<\/span>
Image courtesy Azon<\/span><\/figcaption><\/figure>\n

Dual thermal barrier<\/h3>\n

If one debridged cavity performs well, then two will perform even better in many instances. In dual thermal barrier systems, the U-factor can improve by as much as 20 percent, depending on cavity size and location, and fenestration type. Greater condensation resistance can also be achieved. Additionally, the dual cavity allows for wider span openings for greater glass area and, consequently, increased daylighting. Dual thermal barrier designs also allow for the use of triple-glazing for U-factors of <0.20.<\/p>\n

Polyamide thermal barrier strips<\/h2>\n

Polyamide barriers represent an alternative to the polyurethane-based pour-and-debridge method. They are pre-extruded, structural polyamide insulating strips that usually have multidirectional glass fiber reinforcing to improve load transfer.<\/p>\n

For wall systems utilizing polyamide strips, two separate aluminum extrusions are designed\u2014one interior and one exterior\u2014and channels in the aluminum profiles are created to hold the barrier strips. These channels must be knurled to produce teeth that improve the shear strength of the assembly, as well as hold the polyamide strip and prevent shrinkage. (In many curtain wall applications, the shear strength of the composite is typically not critical because the composite I value comes from the interior tube. Shear values for composite assemblies using strips generally achieve between 3560 N and 9785 N for 100-mm (800 and 2200 pounds of force for 4-in.) section. Additionally, there is thermal cycling data in which there is minimal drop in shear after field exposure or thermal cycling.) Once the polyamide strip is inserted into the aluminum\u2019s channel, the entire assembly is rolled, or crimped, to create the mechanical bond and turn the system into a composite.<\/p>\n

\"Section<\/a>
Curtain wall polyamide pressure plate. <\/span>
Image courtesy YKK AP<\/span><\/figcaption><\/figure>\n

Advantages of the polyamide thermal barrier strips include:<\/p>\n