History of Window Wall Glazing Systems

Early window wall systems were implemented to create bay windows and utilized face seals. Yet because of the single seal line at the exterior and the absence of a weep system, these early designs were particularly susceptible to air and water infiltration.

Modern window wall systems have advanced to include rainscreens and to span floor-to-floor with slab edge covers, providing water mitigation technologies designed to improve durability and reduce potential mold and sick building syndrome issues. In addition, concerns related to indoor environmental quality have been addressed through upgraded insulated glazing and airtight construction. These improvements support thermal comfort, energy efficiency, and also the acoustic performance of a building.

In general, modern window wall systems provide a good balance between cost and performance, making them a popular choice for many commercial applications. The building envelope reliability of the system is generally greater than that of a storefront system, though not necessarily as strong as that of a curtain wall system.

Performance Requirements of Window Wall Glazing Systems

Very similar to the performance standards required for storefront systems, window wall glazing systems also are evaluated according to:

• ASTM E283 (air infiltration),
• ASTM E330 (structural testing),
• ASTM E331 (static water infiltration),
• AAMA 1503 (U-factor and CRF testing) and
• NFRC 100 (U-factor),
• NFRC 200 (Solar Heat Gain Coefficient & Visible Transmittance) and
• NFRC 500 (Condensation Resistance)

In addition to this, because window walls are installed above the ground floor, they are also evaluated for seismic-related performance according to AAMA 501.4, and glazing fallout per AAMA 501.6.

AAMA 501.4 - Interstory Drift

AAMA 501.4 evaluates the performance of window wall systems under conditions of interstory drift, which occurs during seismic events. Interstory drift refers to the relative movement between floors in a building, a critical factor in high-rise structures where seismic activity can cause significant horizontal displacements. The test involves applying lateral displacement to the window wall system to simulate the expected interstory drift during an earthquake.

To succeed, the window wall system must pass criteria to demonstrate the ability to maintain structural integrity and functionality despite the induced movements.

Key criteria include:

• No glass breakage.
• No significant damage to framing members.
• Maintained ability to resist water and air infiltration.

AAMA 501.6 - Glazing Fallout

AAMA 501.6 addresses the potential for glazing fallout, which refers to the detachment of glass panels from the window wall system during seismic activities. This is a significant safety concern as falling glass can pose hazards to occupants and pedestrians. During the test, the window wall system is subjected to dynamic testing that replicates seismic forces, specifically focusing on the retention of glazing panels.

To pass the standard, the window wall must be able to maintain a secure attachment of glass panels so that none fall out during and after the simulated seismic event.

From a thermal performance perspective, a window wall may be either non-thermally broken, thermally improved, or thermally broken. This variable is largely dependent on the climate zone and thermal performance expectations of the project. Skipping thermal breaks is becoming less popular, in general, as concerns with building performance, energy costs, and operational carbon become more pressing. Thermal provisions may include thermoplastic clips separating the exterior face from the interior framing members, poured and de-bridged pockets, and mechanically crimped struts. As window wall systems are commonly implemented on low- to mid-rise buildings, today’s systems routinely meet static water infiltration requirements of 12-15 psf, notably higher than the standards set for storefront glazing systems.

Window Wall Installation Methods

Window walls are installed from the interior of the building, which enhances safety and ease of installation, especially in high-rise construction. This method reduces the need for exterior scaffolding and minimizes disruption to the surrounding environment.

Window walls are typically designed either with the vertical members spanning the full frame height as in a storefront, or with the head and sill members continuous and vertical mullions spanning between the horizontals. Continuous head and sill member construction lend itself to optimal incidental water management, directing any water that might enter the system down the vertical members to weep out of the glazing system.

When the vertical members span the full frame height, assembly is accomplished much like a screw spline storefront system, in which each bay is snapped together and anchored to the surrounding condition. As mentioned, this approach may be faster and easier but does not support the best water management design.

A common disadvantage of continuous horizontal members is the need for a notch at the top of each vertical member, as necessary, to extend to the assembly surface of the head member. Head and sill members are typically spliced mid-lite for long elevations and anchored using a continuous anchor channel integrated into the sill member.

The structural requirements for window wall systems may be greater than those associated with storefront systems and less than those associated with curtain wall systems, with the mullion depths ranging accordingly. The size of the framing is determined based on the wind load, the span of the verticals, and the spacing between verticals. With portrait-oriented glazing panels, the vertical span of the frame will have a key influence on the required mullion cross-sectional dimensions.

The three main styles of window wall glazing systems are the stick system, unitized system, and panelized system.

Stick System: The stick system involves assembling the window wall components piece by piece on-site. Vertical mullions (sticks) are installed first, followed by the horizontal mullions and transoms. Glass panels and other infill materials are then installed into the assembled framework.

The stick system has several advantages. It allows for adjustments on-site, accommodating any last-minute design changes or site-specific conditions. Also, the components are transported individually, making it easier to handle logistics, especially for large projects.

On the other hand, this style of installation is very labor-intensive and requires on-site assembly, slowing down the construction schedule and increasing labor costs. Quality control can also be an issue as on-site fabrication is dependent on changing jobsite conditions, environmental conditions, and the skill level of the installation crew.

Unitized System: The unitized system consists of large panels pre-assembled in a factory setting, including framing and glazing. Glazing units are typically one or two stories tall and can span the full width of a building bay. Unitized systems tend to be more expensive to produce, but faster and less expensive to install than stick-style glazing systems. The prefabricated units are transported to the construction site and installed as complete sections. The system offers less flexibility for on-site adjustments but increases quality control for thermal, water, and air movement performance.

While factory-controlled conditions ensure consistent quality and precision in the assembly of units, transportation of such large units can be troublesome. The large units require careful handling and additional logistics during shipping and on the jobsite to unload and position the windows safely. This can snag daily construction schedules and potentially invite a costly mishap at the time of installation.

Panelized System: The panelized system is similar to the unitized system but involves larger panels that cover several floors or sections of the building. These panels are pre-assembled in the factory and then installed on-site.

One of the main advantages of panelized systems is performance expectations. The large units have fewer joints and connections compared to smaller unitized panels. As with most building systems, water, air, and thermal performance heavily depends on the quality of sealing joints, penetrations, and the area where two dissimilar materials meet. For panelized window wall systems, these transition areas are addressed in a controlled factory environment ensuring much higher standards of quality control.

Panelized systems are more difficult and challenging to transport than unitized window wall units due to their large size. The trade-off is that installation can go faster due to the size of the units. However, larger lifting devices like cranes will be required to position units for installation.

Photo: Wheaton & Sprague Engineering; courtesy of NGA

The curtain wall system at the William H. Gates Hall, University of Washington Law School, allows ample light into the building while providing a striking appearance on the exterior for students.