Parsing performance
The more recent generalized use of the ‘window wall’ term has begun to blur the differences and distinctions between different glazing system products. As designers, it is important to understand not only the differences in attachment and detailing between glazing system product types, but also the performance expectations and possible product limitations. This way, designers can set appropriate performance criteria early based on the glazing being considered. For contractors and owners, it is similarly important to realize these differences when evaluating glazing system products and substitutions for window wall applications.
Windows
Window products are manufactured products evaluated and divided into performance classes (AW, CW, LC, R) and grades (ranging from 15 to 100) per AAMA 101, Voluntary Performance Specification for Windows, Skylights, and Glass Doors. Windows are typically specified by the performance class and grade that meets the building code and project requirements. These ratings take into account typical performance criteria, such as design pressure, structural test pressure, air leakage resistance, and water penetration resistance.
For example, an AW-80 fixed window is a Commercial-grade window with a performance grade of 80. In other words, it has been tested to meet a design pressure of 3840 Pa (80 psf), and has an allowable air leakage rate of 0.5 L/s*m2 (0.1 cfm/sf) when tested at a static pressure differential of 300 Pa (6.27 psf). It also has a static water penetration resistance test pressure of 580 Pa (12 psf), which is equivalent to 15 percent of the design pressure.
It is important to note the rating applies only to individual window units of a tested size or smaller. This performance class and grade does not apply to a series of individual windows mulled together, as is typical in window wall applications. AAMA 450, Voluntary Performance Rating Method for Mulled Fenestration Assemblies, provides guidance for designing and evaluating mulled window assemblies. However, as indicated in the title, this standard is voluntary and not always implemented or followed for mulled window assemblies.
Storefront and curtain walls
Alternatively, storefront and curtain wall systems are typically designed for project-specific applications, based on the fenestration opening size and span, glazing components and configurations, and the allowed air leakage rate and water penetration resistance to meet the desired performance criteria. Project specifications should typically include a list of performance criteria or requirements for storefront or curtain wall products from which to design and evaluate the proposed assemblies. Typically, storefronts and curtain walls are capable of achieving taller spans, resisting high design pressures, and having different air infiltration and potentially more stringent water penetration performance requirements than windows.
For example, a curtain wall product shall:
- be designed and tested in accordance with ASTM E330, Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights, and Curtain Walls by Uniform Static Air Pressure Difference, to meet most service-level wind pressure/suction loads indicated in a project’s performance requirements;
- have a typical allowable air leakage rate of
0.33 L/s*m2 (0.06 cfm/sf) when tested at 300 Pa (6.24 psf) in accordance with ASTM E283, Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen; and - have a typical static water penetration resistance test pressure of up to 730 Pa (15 psf) when tested in accordance with ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference.
Curtain wall products are also typically tested for a dynamic water penetration resistance of up to 730 Pa (15 psf) in accordance with AAMA 501.1, Standard Test Method for Water Penetration of Windows, Curtain Walls, and Doors Using Dynamic Pressure.
Substituting one for the other
Obvious differences in appearance and performance between window products and storefront/curtain wall products can restrict interchanging these systems in window wall applications. For example, taller floor-to-floor spans achievable with a single curtain wall mullion may result in more bulky sightlines if using window products with reinforced mulled joints to achieve the same span. However, even subtle differences between window products and storefront/curtain walls—such as how they are evaluated, specified, and attached—can limit or complicate interchanging these systems as part of value engineering exercises or substitutions.
For instance, in the product examples cited in the previous paragraphs, both the AW-80 window and the generic curtain wall may meet project requirements, but if the former is substituted for the latter in a window wall assembly, the AW-80 windows may no longer have the same performance rating they had as individual units, unless tested per AAMA 450. Without performing this testing, the project team may not be able to confirm beforehand whether the series of window products of a certain size, mulled together in a window wall application, meets the performance criteria for the original curtain wall product or for the project.
In addition to the aforementioned performance criteria, field quality control testing is often specified and performed on glazing systems while completing installation, and is prudent for window products installed in window wall applications that have not been previously tested per AAMA 450. Waiting to conduct performance verification testing of window products until installation is ongoing (i.e. concurrent with field quality control testing) can leave the product team attempting to repair, redesign, and retest the window wall assembly if testing failures are encountered, potentially costing the project team additional time and money. This article’s authors recommend verification testing be performed on a mockup installation as early as possible during a project so performance issues or necessary supplemental detailing can be identified prior to wholesale installation of window wall products.
When specifying and evaluating field quality control or verification testing, it is important to keep in mind window and storefront/curtain wall products are evaluated by different standards, regardless of whether or not they are installed in a window wall application. Window products are covered by AAMA 502, Voluntary Specification for Field Testing of Newly Installed Fenestration Products, whereas storefront and curtain wall products are covered by AAMA 503, Voluntary Specification for Field Testing of Newly Installed Storefronts, Curtain Walls, and Sloped Glazing Systems.
