How to avoid common channel glass pitfalls

March 5, 2018

Photo courtesy Temple Beth Sholom, designed by Berliner Architects

By Jeff Razwick
Channel glass first made waves in the design community for its ability to capture and transfer large amounts of diffuse daylight deep into interior spaces without glare or excessive shadows. Today, linear, U-shaped, translucent cast-glass channels are making headlines for their ability to create an esthetic not possible with conventional windows or glass block. Ontario’s Bradford West Gwillimbury Leisure Centre, designed by Salter Pilon Architecture, is one good example (Figure 1).

The slender, linear glazed segments wrap around the multi-use recreation facility’s east exterior wall, creating an artfully curving channel glass façade. The façade is formed of a mixture of standard cast glass and clear channel glass. The former diffuses light through its textured surface while also obscuring vision, and the latter adds a second layer of visual interest and provides occupants with access to daylight and views to nature. These varied channel glass segments are set in a serpentine configuration, which break up the building’s rectangular shape and serve as a surround for a lobby, meeting room, and multi-purpose room. By day, the façade transmits light into the leisure centre’s interior rooms. By night, its backlit form helps draw people inside.

As design professionals continue to push the form and function of channel glass in projects like the one in Bradford West Gwillimbury, proper specification and use has never been more important. Due to the material’s distinctive nature, building teams may not be aware of how it differs from more traditional glazing materials. This can lead to missteps during the design and specification process that jeopardize its visual appeal and performance capabilities. As a result, it is critical to understand and avoid the following potential pitfalls when using channel glass.

Failing to account for variances in local code requirements
Architects must consider a number of building codes when designing façades, partitions, and glazed areas with channel glass. The application may need to meet stringent energy efficiency and daylighting standards while withstanding potentially high wind loads and rain exposure. Another assembly may have specific acoustic, light transfer, and privacy requirements.

Figure 1: A channel glass system wraps around a multi-use recreation facility’s east exterior wall, creating a dynamic façade.
Images courtesy Technical Glass Products

When evaluating codes, one challenge is the fact safety requirements, deflection limitations of the building structure, acoustic standards, and insulating requirements differ depending on the building height, the occupancy group, and a given channel glass system’s location in a building. To further complicate the matter, best practices and code requirements vary nationally, regionally, and sometimes even locally. Channel glass in the building envelope of a library located in one of Canada’s coastal regions may have vastly different requirements than one in a landlocked region. As such, whether a building team is familiar with the material or new to it, checking local codes and verifying the specified product has been tested to the appropriate standards—for factors ranging from seismic and wind-induced inter-storey drift to forced-entry and impact-safety resistance—is critical.

While it is the responsibility of the architect, owner, or other design professionals to know and design to local code requirements, the subcontractor or channel glass supplier can serve as a great resource for navigating local codes and providing design support. Suppliers can aid in providing continuing education and resources, as well as narrow the lens on channel glass solutions that balance code, esthetic, and functional goals. For example, manufacturers can apply coatings, polycarbonate laminates, films, and other surface treatments to the material, creating a beautiful glazed system that meets stringent hurricane, sound, and thermal insulating requirements.

Figure 2: This detail illustration shows the dead air-filled space in dual-glazed channel glass.

Overlooking design constraints
Channel glass systems are heralded for their design adaptability. They consist of self-supporting, U-shaped cast-glass channels mounted in a perimeter frame—often, extruded aluminum. The versatile segments are available in varying colours, translucencies, and textures, and are cast in up to 7-m (23-ft) lengths. They can be installed in both vertical and horizontal orientations in exterior and interior applications, as well as in curved and serpentine configurations.

While channel glass’s vast array of configurations provides designers with exceptional flexibility, not all permutations are available in all combinations. Understanding product boundaries upfront can save critical time and energy during the design phase.

Dual-glazed channel glass planks must be installed in standard framing
Dual-glazed channel glass planks are always mounted in a standard aluminum perimeter frame. This allows the channels to be set in an overlapping fashion to provide extra strength. It also helps reduce heat transfer by creating an insulated dead air-filled space and thermal gap (Figure 2). Since single-glazed applications are not limited to standard framing, they can help expand design freedom, as discussed in greater detail later in this article.

Horizontally hung channel glass segments are typically dual-glazed and limited to 4-m (13-ft) lengths
As mentioned, unsupported cast-glass channels can span up to 7 m without interruption in vertical applications. However, horizontally hung channel glass segments are typically dual-glazed and limited to 4-m (13-ft) lengths, or the length specified in the manufacturer’s or supplier’s wind load charts for high-design-load areas (Figure 3). This precaution ensures horizontal channel glass segments have adequate structural support. In some instances, it is possible to extend the width beyond what is normally achievable. This often requires custom wind/dead load clips at the intermediate point.

Figure 3: A library usesa horizontally glazed channel glass system to emit a soft glow from interior lights.

Channel glass in curved or serpentine configurations must be installed vertically
To create a curved or serpentine channel glass system, the aluminum framing is stretch-formed into a radius. Glazing subcontractors then place the cast-glass channels around the framing to form a curved or serpentine configuration. While the planks can be dual- or single-glazed depending on project needs, they can only be installed vertically. The cast-glass channels would need to be bent to achieve a curve in a horizontal configuration, which is not done.

While elliptical shapes are popular, the geometry of a true ellipse is not something most stretch-formers can economically produce. Instead, designing the shape as individual arcs with the minimum number of unique radii required to approximate the true elliptical shape is often preferred by many material suppliers and will provide designers with a similar esthetic at lower cost.

Figure 4: In the building shown above, single-glazed channel glass planks are held together in a stunning staggered pattern with custom clips, allowing for natural ventilation and visual interest at the street level.

Mixing up single- and dual-glazed channel glass criteria
Channel glass elevations are either single- or dual-glazed. Given the product’s versatility, one may assume the decision to specify single- or dual-glazed channel glass is a matter of design preference. However, project requirements drive this decision.

Single-glazed channel segments are typically specified in interior and exterior feature walls, including:

These feature wall applications typically have less stringent imposed loads and performance criteria, allowing the planks to be set in various customized arrangements in order to achieve a specific esthetic or enhance light transfer. This is particularly beneficial for vertical, single-glazed applications, since they do not require intermediate mullions. Custom clips can hold the channel glass planks in place, allowing for stunning staggered patterns or natural air ventilation (Figure 4).

Comparatively, it is typical to specify dual-glazed channel glass segments when the planks function as part of the building envelope. The overlapping channels help to bolster the system’s strength against imposed wind loads, which allows them to be used in place of traditional curtain walls and storefronts. The dual-glazed configuration also helps manage heat transfer, and is ideal when channel glass is used as a light-transmitting wall where heat gain or loss is a concern. For example, adding insulating materials in the gap created by the nestled channels, as well as low-emissivity (low-e) coatings, can reduce the solar heat gain co-efficient (SHGC) to as low as 0.31 and the U-value to 0.21 while still preserving light transfer.

Not accounting for project customization with fixed elements
Linear channel glass segments come in various face widths, ranging from 232 to 331 mm (9.13 to 13 in.), with consistent flange depths. While the differing widths make it easier for architects to design within set parameters, it is still crucial to account for project customization when working with fixed elements—both for the channel glass system and conditions surrounding the assembly.

For example, constraints such as doors, vision windows, building columns, and other fixed elements may in fact need to move slightly to achieve a full plank look. If it is impossible to move these elements in a building, the channel glass system may be able to move around these objects with appropriate planning in the design or construction phases.

Figure 5: A channel glass tower incorporates stepped transition, creating a striking anchor feature for a mixed-use development.

One way to accomplish this is to alter the spacing of the glass joints. The installation team can adjust the glass at the head or jamb since the extruded aluminum perimeter frame typically allows for flexibility in glass bite (i.e. how much of the frame depth is taken up by the glazing). The installation team can also create a stepped transition (i.e. an area where the glass lengths decrease from the adjacent planks in order to ‘step’ around a fixed element such as a beam or doorway) to achieve the appropriate configuration (Figure 5). This requires using flange-cut pieces (i.e. L-shaped pieces of channel glass) along with a mix of standardly available plank widths.

Given the detailed customization available for these solutions, it is necessary to engage the manufacturer or supplier early during the design phase. If this is not done during the tender phase, it will instead occur at the time the product is ordered, but such a course of action will require correcting certain design assumptions that were made without the manufacturer’s involvement. This could lead to delays in product delivery or in a design esthetic that was not originally desired.

It is also critical to leave room for custom work and budget in time for technical support when using channel glass in the following applications:

For example, when specifying channel glass in assemblies with an extreme slope, it is important to address safety, as well as air and water management. Specifically, since channel glass is a ventilated system and not an insulated glass unit (IGU), it is necessary to allow condensation or other moisture to drain from the sill. The greater the slope of the glazing, the more difficult this becomes. While there is often a solution, early and appropriate planning is key.

Figure 6: By splitting the channel glass elevations, the design team was able to create a soaring glass wall esthetic on this project.

Involving the structural engineer of record late in the project life cycle
When designing with channel glass, it is critical to receive the input of the structural or professional engineer as early as possible during the project life cycle. This person’s direction on anchoring the channel glass assembly to surrounding structural elements and the allowable structural loads imposed on the glass has the potential to change the fabrication and installation approach. It may also change the design of the glass, or even the building itself.

One might consider channel glass specified in sweeping vertical installations. While the cast-glass channels can span up to 7 m (23 ft), design-imposed wind loads will ultimately determine the channels’ maximum length. As such, one should consult with the structural engineer of record early in the project development phase to determine how design loads may impact the desired esthetic, and if alterations are necessary. If wind loads do limit the length of single- or dual-glazed channel glass segments and constrain the design intent, the project team will then have adequate time for problem solving.

For instance, the design team may be able to split the elevation to achieve desired outcomes (Figure 6). This requires adding a structural member within the large framed opening at or below the upper height limit for the desired wind load, creating two separate openings to independently support the channel glass perimeter framing and emulate the esthetic of a tall, sweeping glazed wall. Framing should always be installed plumb, level, and square.

Alternately, in instances of dual-glazed configurations, the addition of a wind load anchor (a small multiple-part clip that secures the inboard and outboard glass planks at each joint while tying back to a structural member internal to the building envelope) can provide the desired glass height in a single framed opening while keeping the bending allowances of the glass below their critical values.

Whatever the solution, early engagement of the structural or professional engineer of record is the best way to avoid costly, last-minute project alterations or impractical solutions that fail to achieve critical performance and design goals.

Improperly weather-sealing the framing
To create the desired esthetic, it may be tempting to reduce the size of perimeter sealant joints. The reality is appropriately sized sealant joints help weatherproof the glazing system by not allowing water or air to pass around the frame or between the channels. Since the sealant type and joint depth define how much allowable movement the joint can withstand, it is crucial to research this component or consult with the sealant manufacturer.

It is also important to seal accessories such as flashings, shims, expansion joints, splice joints, and trim so they do not interfere with the perimeter joints. Manufacturer guidelines are designed to aid in this process. Improperly sealed accessories can affect proper sizing and movement of the system’s primary and secondary seal.

For exterior installations, the weep system of the sill framing must also be properly sealed at the ends, as well as be baffled at the exterior weep exit points per the manufacturer’s instructions.

Figure 7: Properly sealed channel glass helps prevent potential maintenance problems in a university setting.
Photo courtesy Technical Glass Products/NBBJ

Improperly sealing the glazing
Since there are numerous glass joints in a given channel glass assembly, a potential pitfall is omitting sealant joints either by accident or by design to save on costs or achieve a specific look.

For example, when designing exterior channel glass walls, it is common practice to try to leave out the sealant on the interior side of wall (Figure 7). However, on exterior channel glass walls, it is essential to seal every glass-to-glass joint and every glass-to-framing joint, both inside and out. Failure to do so will draw warm air and dust into the system on cold days, resulting in condensation trapped within the mounted channel glass cavity. The condensation will then collect, streak down, and leave marks on the inside of the glass. Additionally, bugs may penetrate any unsealed openings. These undesirable conditions form a long-term maintenance problem, since it is not possible to clean the interior cavity without removing one side of the glass.

Similarly, for interior channel glass partition walls, design teams may try to eliminate the glass sealant altogether. However, true interior channel glass elevations also need to be sealed. At a minimum, sealant must be located at channel glass head and sill connection points. Unsealed channel glass will move, rattle, or whistle, often from HVAC. This can cause breakage from glass-to-glass contact. It may also compromise the integrity of the channel glass edges, causing it to break if someone pushes along the glass planks with enough force, as may occur from a person falling.

Staggered, overlapping single-glazed applications may not require sealant at each joint. Glazing tape may also be used in some situations in lieu of silicone sealant. Typically, this tradeoff occurs in areas that are not accessible to seal and therefore not seen by the public. To confirm sealant practices in these instances, it is best to check with the manufacturer or supplier.

Perimeter sealants are usually high-performing silicones, which allow for movement of the building and frame system. Sealant manufacturers typically provide adhesion tests and input on appropriate joint sizing. Glass-to-glass joints are generally sealed with a translucent, one-part moisture-curing silicone.

Overlooking transitions between conditioned and unconditioned spaces
Occasionally, channel glass designs will take an exterior dual-glazed wall from its dual-conditioned space (fully inside the building) and either carry it further into a fully conditioned interior space, or fully out into an exterior unconditioned space (facing the elements).

Design teams typically assume this is done via a continuous uninterrupted channel glass assembly; however, the head and sill frames must be separate and spliced together. This creates two independent frame systems. Specifically, a glass isolation joint at the splice is required, creating two distinct cavities of space—exterior from interior. The result can minimize thermal bridging, while also giving the illusion the glass continues seamlessly from the inside to the outside.

Not accounting for sufficient lead time
Custom channel glass segments take time to produce. While many manufacturers and suppliers have efficient production and distribution systems, building these materials into the project schedule as if they were an off-the-shelf item is a formula for project delays.

To hit project construction schedules—particularly for channel glass assemblies with planks in various widths, lengths, or custom cuts—it is essential for builders to talk with manufacturers or suppliers early during the project construction phase or, if possible, design phase. They can confirm opening sizes and glass dimensions, providing a realistic material lead time frame to help keep the project on schedule.

Ignoring maintenance needs
As with any material, it is important to design channel glass assemblies in such a way that routine annual maintenance, such as cleaning and perimeter joint inspections, can be done. Asking the following questions in the specification process can help ensure the design does not compromise maintenance needs:

As architect Louis Kahn once said, “A great building must begin with the unmeasurable, must go through measurable means when it is being designed, and in the end must be unmeasurable.” When channel glass systems are appropriately designed and specified, their versatile, artistic form can help architects make this statement a reality.

Jeff Razwick is the president of Technical Glass Products (TGP), a supplier of fire-rated glass and framing systems and other specialty architectural glazing. He writes frequently about the design and specification of glazing for institutional and commercial buildings, and is a past chair of the Glass Association of North America’s (GANA) Fire-rated Glazing Council (FRGC). Razwick can be reached via e-mail at[9].

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