March 3, 2017
By Ben Mitchell, Chad Ricker, and Jerry Schwabauer
There is perhaps no building element that typifies contemporary architecture more than the curtain wall. Lightweight and flexible, these glazed assemblies not only allow more usable floor space within a building, but also epitomize upscale, modern construction. Widespread use of the curtain wall has driven various improvements in its functionality and sustainability, and recent improvements have focused mainly on thermal performance and coating technologies.
The North American Fenestration Standard (NAFS), a voluntary standard recently adopted into most Canadian building codes, has streamlined some aspects of the fenestration industry. However, the National Building Code of Canada (NBC) and some provincial codes distinguish between products covered by NAFS and those that are not—and the distinctions may not be intuitive.
For example, curtain wall assemblies are not manufacturer-tested or certified to be NAFS-compliant. Instead, engineers and architects are responsible for designing all individual structures to meet relevant energy efficiency goals. The portion of NBC applicable to curtain walls is Part 5, “Environmental Separation.”
Addressing issues such as air leakage and water penetration, Part 5 sets forth “minimum performance requirements, as well as laboratory and in-situ testing procedures,” according to the National Research Council of Canada (NRC).
Situation-specific testing and field assessments of curtain walls help ensure maximum energy performance in Canada’s cold northern climate. Designers must also provide protection against condensation, since condensation-related mould and moisture can create problems, particularly in sensitive environments such as healthcare facilities. In 2015, the American Architectural Manufacturers Association (AAMA) released AAMA CRS-15, “A Comparison of Condensation Rating Systems for Fenestration,” which compares three common condensation rating systems for the United States and Canada, providing information on how the ratings are calculated to help designers better assess the condensation resistance of fenestration systems.
Understanding the curtain wall
There are two main categories of curtain walls:
1. Stick-built systems, in which all structural components are assembled onsite. This system is optimal for building designs with little repetition among components, where no notable savings can be achieved by factory production.
2. Unitized curtain walls, in which glazed and aluminum components are factory-assembled into panels and shipped to the construction site. This method has several advantages, as the controlled factory setting improves material tolerances, allows faster onsite construction, and works well for tall structures with many repeat components.
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, or a single extruded profile, which is more common for unitized systems. While many curtain wall components are factory-assembled, overall NBC categorizes curtain walls as ‘site-built.’
Contributions to thermal performance
Throughout the 20th century, a series of improvements to glazed systems have reduced the rate of unwanted heat loss and gain for building interiors.
Single-pane window glass was commonly used well into the middle of the century. However, the proliferation of skyscrapers—and the extreme amount of thermal transfer occurring 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 also installed around metal parts such as mullions and pressure plates to insulate them and protect against air and moisture penetration.
In the 1980s, insulating glass (i.e. double panes with inert gas or a vacuum seal between them) enabled further reduction of heat transfer. Following that, low-emissivity (low-e) and other coatings for glass were developed. Together, low-e coatings and insulating glass (IG) units immensely improved the thermal performance of glazed openings, 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 following these improvements was the spacer. In its earliest and simplest configuration, the IG unit consisted of two panes or ‘lites’ separated by an aluminum or metal spacer, which was held in place by seals. IG units used in curtain walls were dual-sealed, with polyisobutyl (PIB) primary sealants applied directly to the glazing and silicone used secondarily to provide a structural component. Spacers were usually U-shaped, with a desiccant placed within the canal to absorb any moisture between the lites.
This construction provided structural strength. However, it also provided a conduit of metal allowing heat and cold into the building, and created a temperature differential between the centre and the edge of the glass, which led to condensation. Replacing the traditional aluminum spacer with warm-edge spacers (i.e. those constructed from materials such as polymers or low-conductivity stainless steel) was a first step toward improving framing. Today, warm-edge spacers are typically an integral part of fenestration systems.
Even after the introduction of IG units and warm-edge spacers, the aluminum profile of the typical curtain wall assembly allowed significant remaining pathways for thermal transfer, since alloyed aluminum is highly conductive. However, aluminum is also prized for its many advantages—most notably, its durability, recyclability, and strength. It is a primary building material in sustainable buildings and those certified under the Leadership in Energy and Environmental Design (LEED) program. Therefore, the fenestration industry turned its attention to mitigating aluminum’s conductivity and, for both stick-built and unitized curtain walls, developed engineered systems capable of interrupting the thermal bridge created by the metal cross-sections of the assemblies.
Thermal bridging and thermal barriers
While the 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, while not representing a direct inverse, are opposites in that a high R-value (representing a high value of insulation) and a low U-value (representing a low amount of heat and 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.
Various types of thermal barrier can be used to maximize these values.
The ‘pour and debridge’ thermal barrier
With this method, the aluminum profile is extruded with a channel designed to hold an insulating polymer. This channel is mechanically abraded to ensure adhesion of the polymer, while a mechanical lock inside the thermal cavity locks the thermal barrier to the aluminum (Figure 1). This allows the maximum separation of the aluminum required for lower U-values, while also providing superior structural strength for the composite.
Polyurethane polymers are then poured as liquid into the thermal barrier channel, where they solidify into a structural component, and the metal floor of the cavity is removed from the bottom of the channel. After this ‘debridging,’ the unit has no continuous run of metal from the building’s interior to its exterior—the polyurethane connects the two separate sides to form a composite. As polyurethane features thermal conductivity more than 1300 times lower than that of aluminum, its thermal benefits can be achieved within a very small amount of space, although the amount of separation should be determined based on the energy efficiency requirements of a given building.
This complete separation of the aluminum profile means the structural abilities of the polymers are of critical importance, since curtain walls (especially those used on tall buildings) must provide shear and tensile strength in strong winds, or even in blast or hurricane situations. In the unitized system, thermal barriers must also accommodate stresses created by thermal expansion and pressure differences. Fortunately, polyurethane offers an exceptional amount of structural strength, particularly shear strength, allowing designers to maximize the span of glazing within the curtain wall framing.
As well as by introducing a polymer to interrupt the thermal bridge, energy efficiency can be increased by manipulating the shape of the aluminum extrusion itself. The latest developments in this area include wide-cavity and dual-cavity construction.
Creating a wider cavity in the frame allows for a lower U-value in most fenestration products. This is outlined in Figure 2, which offers a comparison of the U-value of 2.44 W/m2K (0.43 Btu/[hr-sf-F]) achievable with a thermal isolator to that of a wider cavity. By use of a polyurethane thermal barrier, the U-value can be improved to 2.00 W/m2K (0.35 Btu/[hr-sf-F]), and can be lowered further to 1.70 W/m2K (0.30 Btu/[hr-sf-F]) with the use of
If one debridged cavity performs well, then two will perform even better in many instances. In dual thermal barrier systems (Figure 3), the U-value can improve by as much as 20 per cent depending on cavity size, cavity location, and fenestration type, and greater condensation resistance can also be achieved. Additionally, the dual cavity allows 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-values of less than 1.14 W/m2K (0.20 Btu/[hr-sf-F]).
Polyamide thermal barrier strips
An alternative to the polyurethane-based pour and debridge method, polyamide barriers are pre-extruded, structural plastic insulating strips, usually featuring multi-directional glass fibre reinforcing to improve load transfer. For wall systems utilizing polyamide strips, two separate aluminum extrusions are created—one interior and one exterior—and channels are created in the aluminum profiles to hold the barrier strips. These channels must be knurled or bent to produce ‘teeth,’ which improve the assembly’s shear strength and hold the polyamide strip. Once the polyamide strip is inserted into the aluminum’s channel, the entire assembly is rolled or crimped to create the bond and turn the system into a composite.
These strips allow some of the greatest thermal separation widths available with the use of less metal, which in turn generates savings on resources. They also have a similar co-efficient of expansion and contraction to aluminum, ensuring the overall stability of the system (Figure 4).
Ongoing developments in curtain wall assemblies include alternative designs for pressure plates. Polyamide was used to develop a new pressure-plate system up to 20 per cent more efficient than earlier designs, with a 10 per cent gain in condensation resistance (Figure 5). Pressure plates with polyamide have excellent thermal values, require no special handling or fabrication, and are installed similarly to aluminum pressure plates.
While coatings for aluminum have not changed drastically in recent years, a few new formulations are offering some performance benefits. The rising popularity of powder coatings has also introduced some confusion among
specifiers. The first question typically asked is whether powder or liquid is ‘best,’ and which is most cost-effective. The answer is not a simple one, as there are several considerations.
When powder coatings were first introduced into the architectural market, they were heralded as a superior product and were expected to quickly replace liquid paint. These coatings were gaining visibility at a time when LEED certification was also in the spotlight, and volatile organic compounds (VOCs) were often the only criterion used to measure a coating’s success. The fact powder is VOC-free was its biggest selling point. However, liquid paint has more staying power than many predicted. The adoption of powder coatings has turned out to be an evolution, not a revolution.
What is paint?
Cured paint film is comprised of two principal ingredients: resin and pigment. Regardless of whether a coating is applied as a liquid or a powder, it is the resin and pigment quality that determine the weather resistance and durability properties desirable in an architectural finish. Resin gives the film its adhesion to the substrate, chemical resistance, gloss, and other film properties, while pigment durability confers colour stability.
The difference in coating formulation is for liquid coatings, solvent is added to allow smooth application, while powder has no solvent. This factor is what renders powder coatings VOC-free. Powder coating is typically applied as a single coat, but at a higher cured film thickness than liquid, while liquid coatings are at a lower film thickness and often include a primer.
A common mistake is to specify merely a powder or a liquid coating. This basic description does not denote a specific product or quality, because different coatings are composed of different resin types and different chemistries. Similarly, specifying a coating by brand name can limit one’s options, as it restricts choices to whatever is offered by an individual company.
Choosing a coating
To achieve the desired performance level for a given application, coatings must be specified to meet third-party industry standards. AAMA, for instance, establishes voluntary standards for the fenestration industry, which appear in NAFS.
These are the most commonly referenced coating standards for painted fenestration products. Each has unique requirements—for example, to meet AAMA 2603, a coating is tested for South Florida weathering for only one year, whether in liquid or powder form. These coatings are normally an acrylic or polyester system used in residential or
AAMA 2604 performance testing requires a five-year South Florida exposure. One example of an AAMA 2604 liquid offering is new-generation silicone-modified polyester, a workhorse system in the metal building market with proven performance. Coatings meeting the AAMA 2604 specification generally have a lower cost, and powders falling under this category are super-durable polyesters.
For AAMA 2605 performance testing, 10-year South Florida exposure is required. Longstanding options meeting this performance level include polyvinylidene fluoride (PVDF)—also known as polyvinyl difluoride (PVF2)—and fluoroethylene vinyl ether (FEVE).
With the industry’s shift toward powder being driven primarily by companies’ desire to reduce VOCs, an investment in pollution-control systems for liquid-applied coatings has helped even the playing field between the two types. Painted extrusions must be cured by baking, and it is during this process the solvents—and VOCs—in liquid paint escape into the environment. VOC abatement systems capture more than 98 per cent of VOC emissions from the liquid paint line and send them to a regenerative thermal oxidizer (RTO), where the VOCs are destroyed. An added benefit is the heat from this process is captured and utilized for curing. This helps to lower the amount of carbon fuels required to cure the surface—a process requiring temperatures of 232 C (450 F).
For paint to be compliant with AAMA 2605, a pre-treatment is required, as neither liquid nor powder coatings will adhere to untreated aluminum.Thus, the type of pre-treatment used also becomes an environmental consideration. Traditionally, this consists of a chromate (hexavalent) conversion coating, but since chrome is a heavy metal toxic to people and the environment, its use is highly regulated, and many manufacturers have turned to chrome-free pre-treatments. These meet the performance standard of AAMA 2605, but have limited field-performance history, so some still do not have full confidence in such systems.
Another key to the continued popularity of liquid paint is it allows for mixing at the coater’s site. This makes it suitable for just-in-time production, and means excess paint can be remixed into a new colour, thereby avoiding waste. Finally, in the commercial architectural market, bright mica or metallic colours are very popular, and liquid paints can use a higher level of mica/metallic to give brighter colours. A clear coat containing some of these pigments can be added for additional sparkle, especially in sunlight.
Formulas and application methods for architectural metal coatings continue to evolve. Most companies now offer both liquid and powder options meeting the same AAMA standards. The key when specifying a coating, therefore, is to not jump too quickly to an assumption about which application method or brand is best. Specifiers should evaluate all options, balancing the costs and benefits of each based on a project’s individual requirements.
The energy performance and other sustainable attributes of buildings are also improving. Careful analysis of available options in envelope systems—from framing members to coatings—can help optimize outcomes. Making even minor improvements to efficiency and durability can add up to major gains, and these gains are multiplied when a system is deployed throughout a structure, as curtain walls are in today’s tall buildings.
Ben Mitchell, CSI, is the extrusion coatings sales and marketing manager for AkzoNobel, a global paints and coatings company and producer of specialty chemicals. He has a bachelor’s degree in comprehensive science from Urbana University in Ohio. Mitchell started at AkzoNobel in 1990 as a lab chemist formulating polyvinylidene fluoride (PVDF) coatings, and moved into product management. He can be reached at email@example.com.
Chad Ricker is the market team manager at Technoform Bautec North America. He has an engineering background, having obtained a master’s degree in technology with a concentration in engineering from East Tennessee State University. Ricker has been a part of Technoform’s team for more than a decade, beginning as an engineer and progressing to lead consultative marketing efforts. He can be contacted via e-mail at firstname.lastname@example.org.
Jerry Schwabauer is Azon’s vice-president of sales and marketing, a position he has held since 2000. Schwabauer is active in American Architectural Manufacturers Association (AAMA) and is a frequent speaker about the topic of optimizing thermal performance in commercial fenestration in North America and Asia. He can be contacted at email@example.com.
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