Second, high-performance windows need to exhibit superior thermal control related to heat transfer, solar transfer and air infiltration. Energy codes and ASHRAE 90.1 are quite commonly known for helping to establish minimum energy performance but buildings that are pursuing certification or demonstration of superior energy performance will need windows that perform beyond the minimum. The National Institute of Building Sciences (NIBS) produces the Whole Building Design Guide (WBDG). Within it they suggest a number of strategies to improve energy performance in buildings, including installation of high-performance windows. They define this strategy further by several specific tactics:

• Specify frame and sash materials fabricated with low thermal conductivity
• Specify windows with a whole-unit U-factor less than 0.49 (greater than R-2.1). Go to the next level with super-windows that have a whole-unit U-factor less than 0.25 (greater than R-4.0)
• Avoid divided-lite windows to reduce edge losses

In selecting and specifying high-performance window systems, it is appropriate to use recognized national standards to establish benchmarks and targets for achieving the desired results. There are two very important organizations that provide a great deal of insight and direction in this regard:

The National Institute of Building Science (NIBS) identifies ways to make windows in buildings truly high performing. Photo courtesy of Manko Window Systems, Inc.

 

The American Architectural Manufacturers Association (AAMA)

By their own definition, "AAMA is the source of performance standards, product certification and educational programs for the fenestration industry. AAMA's membership is comprised of window, door, skylight, curtain wall and storefront manufacturers, suppliers and test labs." Among the standards developed by AAMA, the performance-based and material-neutral North American Fenestration Standard or NAFS (AAMA/WDMA/CSA 101/I.S.2/A440), is referenced in current International Building Codes as the basis for third-party testing and certification of windows, doors and skylights. This standard identifies four Performance Classes which each contain their own set of performance requirements related to use and location within a building. Within each class, a specific Performance Grade is identified corresponding to a design pressure range expressed in pounds per square foot (psf) or pascals (Pa) of pressure exerted during testing. This testing simulates the effect of wind and rain pressure on a window system when it is installed in a building. To qualify for a given performance grade (PG), a representative specimen of a manufactured product must pass all required performance tests for the following:

a) Operating force (if applicable)

b) Air leakage resistance

c) Water penetration resistance

d) Uniform load deflection test

e) Uniform load structural test

f) Forced-entry resistance (if applicable)

Note that each Performance Class is identified by "entry level" or "gateway" minimum Performance Grade levels to simplify the task of matching window system performance to project requirements. These AAMA Performance designations are as follows:

• R class (Residential)—commonly used in one- and two-family dwellings and designed to withstand pressures of at least 15 psf (720 Pa)
• LC class (Light Commercial)—commonly used for low- and mid-rise multifamily dwellings or other buildings where larger sizes and higher loading requirements are expected and designed to withstand pressures of at least 25 psf (1200 Pa)
• CW class (Commercial)—commonly used in low- and mid-rise buildings where larger sizes, higher loading requirements, limits on deflection and heavier use are expected and designed to withstand pressures of at least 30 psf (1,440 Pa)
• AW class (Architectural)—commonly used in high-rise and mid-rise buildings to meet increased loading requirements and limits on deflection and in buildings where frequent and extreme use of the fenestration products is expected and designed to withstand pressures of at least 40 psf (1,920 Pa). This class of window system in particular is most able to achieve the status of "high-performance" windows due to the higher capabilities.

When selecting a particular Performance Class, there is also the option to specify the pressure requirements above the minimum gateway requirements listed above (in increments of 5 psf) up to 100 psf maximum except in category AW which has no stated maximum (practical limits exist). So, for example, a light commercial system could be specified at the minimum Performance Grade level with the designation class LC-PG25 or an optional increased level of performance as class LC-PG35 indicating that passing a pressure test of 35 psf is required. The designation may also additionally include the size tested and any specific product type related to operable or fixed-in-place windows.