Typical Curtainwall Framing Member
Glazed framing systems have undergone tremendous improvements over the metal framed, single-paned energy pigs of yesteryear. The introduction of dual glazed insulating units, low-emissivity coatings, and thermally improved framing systems have resulted in huge energy-saving performance. A single glazed, non-thermally-broken aluminum framing system has a U-value of around 1.0. By comparison, an insulating glazing unit with a low-e coating glazed into a thermally broken aluminum frame can achieve a U-value in the neighborhood of 0.40, considered to be about the best affordable, achievable performance for most commercial glazing systems. The rising costs of energy, more stringent energy code requirements, and sustainable building awareness have worked together in pressuring the industry to do better. A new wave of affordable, glazed framing system offerings with U-values in the low 0.20s will soon be available to project teams in meeting new glazed framing systems requirements.
MEASURING ENERGY PERFORMANCE
Until fairly recently, the energy performance of glazed framing systems wasn’t paid much attention to by energy codes, design professionals or building owners. Most of the U.S. has been content with using the Center of Glass U-value of the glazing unit alone in performing energy calculations of buildings. The problem with this is that the frame around the glazing can be one of the biggest contributors to a system’s energy performance. An insulating glazing unit with a U-value of 0.29 glazed into a poorly performing framing system can result in an overall Frame+Glass U-value of 0.60. Obviously, an energy calculation done for a building using only the COG value does not produce an honest or realistic result.
Enter the National Fenestration Rating Council. Formed in 1989, the NFRC began its mission to develop a national, uniform fenestration energy rating system. The newly formed organization agreed that it would develop test procedures and certification for only whole-product performance, putting an end to misleading center-of-glass U-values. Lengthy debate about number and types of tests, size of test samples, led eventually to a fair, accurate, and credible energy rating certification system for residential and non-residential fenestration products. NFRC certification requirements have been finding their way into city and state energy codes for commercial glazing systems, and is already required for residential glazing in the International Energy Code.
The NFRC Web site lists hundreds of certified fenestration products but the list is almost entirely for residential, factory-fabricated window units. The commercial NFRC certification process is more complicated, and costly, for manufacturers because of the many possibilities of framing systems and glazing unit combinations, and a requirement that certified systems be physically tested against computer calculated performance. If the physical test results are too far afield from the calculated results, the system is unable to obtain certification.
The NFRC has recently launched a pilot program for an alternative certification path for commercial framed glazing systems called the Component Modeling Approach Product Certification Program. The software used in calculating a systems performance is also being made available to the public “for design, development, and similar non-certification purposes.” Component modeling certification is generated using performance data from the three primary components that make up a glazed framing system: 1) glazing, 2) glass spacer, and 3) frame, which are combined to obtain an overall performance rating. The program is scheduled to be fully implemented in January 2010.
This alternative method will allow manufacturers to quickly ascertain a system’s overall performance using any component combination without having to undergo more costly physical testing for each. The software will also be a valuable tool for designers, allowing them to mix and match glazing system component combinations during design, with certainty about final performance.
FRAMED GLAZING SYSTEM COMPONENTS
Commercial framed glazing systems are typically field assembled with extruded aluminum framing members, insulating glazing units and dry glazed with compressible gaskets that hold the glazing unit into the framing members. Terms commonly used to describe these systems include curtainwall, storefront, ribbon wall, and window wall.
Extruded aluminum is by far the most common material used for framing members. Aluminum is corrosion resistant, strong, easy to fabricate into required shapes, has nearly the identical expansion coefficient of glass, and can be finished with extremely durable baked on coatings. It is almost the perfect framing material, except for one thing: thermal conductivity. Aluminum is among the poorest choices of materials for resistance of heat flow. The thermal conductivity table of commonly used glazed framing systems below illustrates aluminum’s poor thermal performance:
Thermal conductivity or heat transfer coefficients, of some common materials expressed as 1 W/(mK) = 1 W/(moC) = 0.85984 kcal/(hr moC) = 0.5779 Btu/(ft hr oF).
Air 0.024
Aluminum 237 (pure) 120 - 180 (alloys)
Argon 0.016
Brass 109
Copper 400
Carbon steel 54
Fiberglass 0.04
Glass 1.05
Nylon 6 0.25
PVC 0.19
Softwoods (fir, pine) 0.12
Stainless steel 16
THERMAL SEPARATION
To address aluminum’s poor thermal performance, manufacturers have developed ways to thermally separate portions of the aluminum framing at the plane of the glazing unit with non-metal spacers, the state-of-the-art material being glass-reinforced polyamide nylon. Some manufacturers use PVC as a thermal break material, which is good for thermal performance, but does not perform as well structurally and therefore limited it its use.
The illustration above shows a cross section through a standard curtainwall framing member showing the framing and thermal break:
The glazing is pressed into the framing member from the exterior with a metal pressure plate attached to the frame through the thermal break. One curtainwall manufacturer in the U.S., Kawneer, will soon be introducing a version of its 1600 Wall curtainwall framing system that substitutes the metal pressure plate with a fiberglass pressure plate, reducing the system’s U-value by nearly 25 percent, while adding as little as 10 percent in additional cost.
Manufacturers have also experimented with different thermal break configurations, the most energy efficient and radical being multi-compartment design. This type of exotic component adds significant cost to the system, however, which is often only justified for use on projects in the most extreme climates. Systems utilizing this type of thermal break offer U-values from 0.30 to as low as 0.15 (dual- and triple-glazed units, respectively). Kawneer’s Isoweb 5500 is one such system.
INSULATING GLAZING UNITS
Dual-glazed insulating glazing units have been the rule for use in glazed framing systems for decades. Advances in low-e coating technology have resulted in excellent COG U-values while simultaneously allowing the passage of significant amounts of visible light for good day lighting, and rejection of unwanted solar heat gain (expressed in value as Solar Heat Gain Coefficient). An average COG U-value for an insulating glazing unit with two 1/4-inch thick lites, a high performance low-e coating, an aluminum edge spacer, and 1/2-inch air space is around 0.30 with a VT of 70 percent and a SHGC of 0.38. Greater performance can be achieved by using gas infill (such as Argon), using two low-e coatings, and using less conductive spacers (stainless steel is a commonly used as a replacement to aluminum).
Triple glazed IGUs have been around since the development of dual glazed IGUs. The first triple glazed units simply incorporated another glass light into the system that, while increasing the unit’s thermal performance, also increases weight and thickness of the IGU, and reduces visible light transmittance. Another type of “triple-glazed” IGU uses a transparent, plastic film that is suspended between the interior and exterior glass lites. These units perform as well as, and often better than, standard glass/glass/glass triple glazed units without the weight (and in some cases thickness) penalty. Two potential detractors for suspended film IGUs are cost-at 40 to 60 percent more than standard dual and triple glazing, and durability.
One popular U.S.-made suspended film product, Heat Mirror by Southwall Technologies, incorporates a low-e coating onto the film, which degrades rapidly if the glazing seal fails. IGU seals for this type of film are critical and the Achilles’ heel with this type of suspended film product. Another company offering suspended film glazing units, Visionwall, uses Swiss technology, which suspends an uncoated film with a spring-loaded mechanism and does not degrade if the seal fails. The manufacturer also claims that the method for suspending the film within the IGU is superior to all others, and does not fail.
Triple glazed framing systems can easily achieve U-values in the neighborhood of 0.11 to 0.20, SHGCs in the 0.30 range and visible light transmittance between 55 to 65 percent.
For even better performance, a proprietary warm edge spacer can be used in lieu of metal. Warm edge spacers are superior to metal edge spacers for thermal performance, durability, condensation resistance, gas infill retention and structural strength. Edgetech’s “Super Spacer” is one such example. Cost is greater for warm edge spacers but the biggest impediment in using these products may have more to do with whether or not the glazing unit fabricator can provide such a product.
CONCLUSION
The best way to improve building enclosure energy performance with regard to glazing systems is, of course, to limit the amount of glazing. The best performing window is a wall. Walls easily achieve R-values of 15 and higher. The best glazed framing system gets us to about R-10. Having said that, designers have the parts and pieces to create much more energy efficient glazed framing systems over what is traditionally specified and installed. Many of these offerings come at a very reasonable increase in cost, and a substantial return on investment. As energy codes become more restrictive, energy costs continue to rise, and green buildings become the rule rather than the exception, this new wave of highly efficient glazed framing systems is likely to also become the rule. W&C
Sidebar: LEED CREDENTIALS REVISITED
In my July 2009 article “Green Building Credentials-How Credible Are They?” I reported that current LEED AP holders would be required to jump through some hoops to hold onto their credential:“To retain LEED AP status, Legacy LEED APs will be required to sign a USGBC disciplinary policy agreement, meet the continuing education requirements, and pay a biannual ‘maintenance’ fee.”
Having thought better of this, the USGBC has recently decided that current LEED AP holders will need to do nothing to retain their credential. For more information, go to the USGBC Web site and download “CMP Enrollment Guide (for LEED APs without specialty) Valid for September 2009.” At the end of the document, you will find the following:
“All LEED APs without specialty (those credentialed under the LEED AP NC, CI, or EB exam tracks) will continue to hold the credential in perpetuity whether they choose to enroll in CMP or not. Additionally, if at any point, a LEED Professional who was credentialed under the LEED AP NC, CI, or EB exam tracks fails to maintain their LEED AP with specialty (or LEED Green Associate) credential, the may use the LEED AP without specialty title and logo again.
“LEED APs without specialty who choose not to enroll will continue to appear as a LEED AP without specialty in the LEED Professional Directory without completing any credentialing maintenance or paying any fees. They may continue to use the title of LEED AP with no specialty designation afterward.
“LEED APs without specialty may choose to enroll at any point during their enrollment window. After this period, if LEED APs without specialty want to become LEED APs with specialty, they must apply and take both parts of the LEED AP exam and are responsible for all applicable fees.”
