In addition to these values, manufacturers have the option of testing for and displaying values for two other significant factors:

Air Leakage (AL) is indicated by a rating expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). Since significant heat loss and gain can occur by infiltration through cracks or joints in the window assembly it is often very appropriate to specify a maximum AL rating for a window assembly. The lower the AL rating the lower the amount of air that passes through the window assembly. While the NFRC lists this as an item to report, it should be noted that this testing protocol has not yet been fully implemented, particularly in commercial windows. AAMA ratings are currently more relevant for this characteristic.

Condensation Resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation. NFRC notes that "while this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation formation." CR is expressed as a number between 0 and 100.

With these standards as a basis we can look in more detail at specific window system types and how they can meet high performance criteria.

Single-story storefront system used in a common light commercial retail setting Photo courtesy of Manko Window Systems, Inc.

 

Storefront Window Systems

Storefront systems take their name from retail settings, where they are commonly used for large, single floor height openings of glass. These systems were designed for light commercial single-story retail viewing and readily incorporate entrances for quick access to interior product displays. They are often used on interior applications such as shopping malls or schools as well as exterior light commercial applications. They typically span floor to floor or floor to head condition without passing in front of any intermediate anchorage conditions. Most storefront designs utilize small, non-obstructive, aluminum framing shapes that present "flush glass appearance" where glass seems to disappear into the framing system. Factors influencing the selection of this type of system include the following:

Storefront Structural Performance: As a light commercial system, storefront systems are typically limited to a maximum height of 12 feet due to the limited capacity of the framing and glazing system. Since the primary attachment is at the top and bottom (i.e. head and sill), the width of a properly designed system can theoretically stretch on indefinitely. The wind load resistance or Performance Grade of storefront systems generally cannot withstand more than 40 psf, so specifying anything higher class will not likely be attainable. Note that AAMA has established performance tests designed for storefront systems that are intended to simulate expected field use which are very similar to tests used for pre-glazed windows. However, there are notable differences that make a direct comparison difficult. For example, storefront test sizes are typically larger (144 in. wide by 120 in. tall) than pre-glazed windows (1/3 – 1/2 storefront test size). Further, storefront test pressures are generally limited and can be expected to be less than those available on many CW and AW pre-glazed windows. Hence testing does not support specifying storefront systems with a high-performing CW or AW class rating, rather LC class would be more appropriate.

Storefront Thermal Performance: Typically, storefront systems are not always offered with truly high-performance thermal characteristics. Nonetheless, when specifying, select the highest performing glass available that is appropriately coated and treated to suit the thermal needs of the buildings and local conditions. The local glazier and installer may or may not be able to use non-conducting insulating foam spacers between layers of insulated glass instead of metal, but specifying the best available means to minimize thermal bridging between glass layers along the glass edges is appropriate. Similarly, the thermal properties of the frame must be managed to create true thermal breaks between the interior and exterior portions of the frame based on the design of the storefront frame. Finally, due to the field installation, air infiltration rates may vary widely from site to site, not only within the storefront system, but around the exterior where it meets the adjacent building materials. Examination of the sealant around all edges is warranted and in some cases, even field testing to determine any areas of air infiltration breaches.