Thermal Performance …
More than Just U-Factor Ratings
by John Lewis

With green building initiatives in the news, it is interesting to focus on what “green” means to the fenestration industry. In addition to the well-known positive benefits of natural daylighting and ventilation, one of the greatest contributions that fenestration products can add to the greenness of a building project is their ability to influence energy efficiency. 

The industry began focusing on that aspect more than 30 years ago. Since then, window energy performance standards have been developed by AAMA and by the National Fenestration Rating Council (NFRC). AAMA 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections, developed in 1980, was the first standard to establish a uniform and repeatable thermal performance testing protocol. Developed during the late 1980s, NFRC 100 derives a thermal rating based primarily on computer simulations to determine window U-factors. NFRC ratings, recognized by the U.S. Department of Energy and the Energy Star^® Windows program, are the principle means of assessing residential windows for U-factor performance. AAMA 1503 tends to be favored in the commercial and architectural sectors, as its tests are the preferred means for verifying project-specific designs, not just mass-produced product lines. 

Meanwhile, technology has evolved in many ways that provide greater options for window manufacturers to build energy efficiency into their products.

Alternatives to drive down the U-factor have evolved well beyond simple two-lite glass with a “dead air” space between. Inert gas (argon or krypton) infills, low-emissivity (low-E) metallic coatings that reflect heat inward or outward depending on the glass surface to which they are applied, as well as the advent of new framing materials, have improved performance greatly. 

Glass Alternatives
Glazing materials have evolved to offer a number of options, including the well-known low-E glass, whose surface coating reflects much of the heat normally transmitted through clear glass. Reflective surface films control solar gain and tinted heat-absorbing glass soaks up a hefty portion of incoming solar energy. 

Warm-edge technology for insulating glass units is a recent development in energy-efficient design. Warm-edge spacers use less conductive materials whose insulating properties increase the inside-edge temperature by 10 degrees Fahrenheit or more under the same set of conditions, thus reducing heat transfer around the window perimeter. 

Framing Factors
An expanding variety of high-performance thermal barrier materials between the interior and exterior segments of the frame (such as “poured and de-bridged” barriers, thermoplastic inserts, neoprene gaskets, and structural silicone separations) have enhanced thermal performance of traditional framing materials. 

It’s also important to recognize that energy efficiency is affected by more than Energy Star ratings. The air infiltration rate, an indicator of the product’s “tightness” (rated by testing such as that conducted through the AAMA Certification Program) is another well-recognized factor in window energy efficiency. To reduce these energy-robbing air leaks around window frames and sash, today’s window manufacturer can choose from a variety of weatherstripping options to seal the contact area between the fixed and movable sections of the window. The trick is not to make the window so “tight” that it is difficult to open and close. For this reason, maximum operating force is a testing requirement specified in AAMA/WDMA/CSA 101/I.S. 2/A440-05.

In addition, installation quality—which affects whether the window product performs as designed—has been addressed as a major factor in window energy efficiency. Various standards and guidelines are available for best practices regarding installation, some of which are available from AAMA. And, though less directly related to, but pertinent, to this discussion other factors, such as structural integrity, condensation resistance, sound control and water leakage, are critical to the overall product performance.

Stay Tuned
Simply put, window materials technology, design capabilities, standards, testing protocols and installation requirements continue to evolve to exceed earlier expectations for energy efficiency. Developments in the research and development pipeline are even more exciting, including those that could allow building occupants to control their windows’ response to heat and light and even generate energy to supplement other sources through the use of building-integrated photovoltaics.

It is our role to help ensure that these developments and new alternatives are brought to market in such a way that they are evaluated easily by architects and specifiers, feasible for manufacturers and of benefit to building owners.

John Lewis serves as technical director for the American Architectural Manufacturers Association in Schaumburg, Ill. He may be reached at jlewis@aamanet.org. Mr. Lewis’ opinions are solely his own and not necessarily those of this magazine.

DWM
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