As seen in Commercial Building Products (January/February 2011)
Glazing Option Resists Hurricane Forces
Channel glass systems protect buildings from high winds yet allow architects design flexibility.
By Jeff Razwick
Hurricane-rated channel glass systems provide protection against storms while meeting an architect's aesthetic goals.
Few would dispute that hurricane-related building codes and product certifications are beneficial when it comes to glazing. The wrong type of glass can be broken or shattered by high winds and wind-borne debris. Witness the commercial buildings throughout New Orleans after Hurricane Katrina and along the Gulf and Atlantic coasts after any number of other storms.
Stringent building codes call for protective glazing, which is beneficial when a storm strikes. Yet those same codes can severely crimp an architect's design ideas.
Hurricane-rated glazing options have been limited, offering minimal choices for surface treatments, textures, and glass lite sizes. Yet glazing, a key component of the building envelope, affects aesthetics, energy efficiency, condensation, sound performance, and impact resistance.
A hurricane-impact channel glass system is one alternative to use on buildings in coastal regions. Such a system consists of dual-glazed, vertically oriented, and self-supporting U-shaped cast-glass channels. Available in lengths to 10 feet for hurricane- or impact-rated applications, the framing and narrow glass channels allow for sleek formations and large areas of glazing that can be stacked over floor slabs to create extended expanses of glass.
Hurricane-approved channel glass systems that meet Miami-Dade County and state of Florida requirements (the benchmark of hurricane codes) allow for the transfer of natural light while helping protect buildings and occupants against severe storms. The translucent channels, with a polycarbonate laminate and mounted in an extruded-metal perimeter frame, capture diffuse light (a component of daylighting strategies) and balance it with the privacy concerns of building occupants.
Daylight without the heat gain
The composition of channel glass captures sunlight while avoiding excess solar heat gain or loss, compared with conventional window assemblies. Large glass areas can be susceptible to creating a terrarium effect, raising interior temperatures to uncomfortable levels in summer months or hot climates. Conversely, in coastal regions where clouds can linger, glazing can contribute to heat loss, given its lower thermal performance compared with surrounding building materials.
The configuration of channel glass systems, however, helps reduce heat transfer. Individual glass channels are mounted in an overlapping fashion, which creates an insulating space filled with dead air. The result is a typical solar heat-gain coefficient of 0.69 and a U-value of 0.50. Using thermally broken frames to reduce heat bridging or specifying insulating aerogel for the enclosed space between the channels further enhances energy performance. The use of aerogel can reduce the U-value to as low as 0.28.
Another channel glass benefit is the ability to reduce glare and avoid the spotty illumination of interior spaces. Direct sunlight can be harsh and overly focused, diminishing productivity gains by building occupants. However, texturing and the translucent quality of channel glass itself diffuse daylight for soft, even lighting of interior spaces. This is particularly effective when channel glass is used for floor-to-ceiling glazing or extended glazed areas, which helps eliminate contrasting shadows.
Because effective daylighting requires thorough project design, it is essential to pair channel glass with other building systems. The U.S. Dept. of Energy, Washington, explains that realizing the benefits of daylighting requires coordination between window design and placement, along with electric lighting controls. Failure to do so can result in a net energy loss from lighting and increased mechanical cooling costs.
When correctly designed and installed, channel glass has the potential to contribute toward LEED certification credits from the U.S. Green Building Council, Washington, including "daylight and views" and "optimize energy performance."
Two design challenges tend to arise with typical glazed systems. First, protective glass films can block out too much daylight and produce excessive exterior reflectivity, according to the National Institute of Building Sciences, Washington. Not only does this detract from occupant comfort and visibility, it also diminishes exterior aesthetics. Second, for applications using glass block in place of windows, supplemental protective devices, such as shutters, are frequently required. These can hinder the sleek, contemporary lines that some architects desire.
These issues do not present themselves when using channel glass systems. They allow for the transfer of daylight while serving as dramatic focal points.
Because hurricane-rated channel glass systems are tested for large and small missile-resistance and cyclic loading, they can provide adequate protection without films or shutters. The systems can also be used to create visual statements. Cast-glass channels are available in a wide range of textures, translucencies, and colors to vary the level of opacity and expand aesthetic choices. At night, a channel glass screen with lights set between it and the primary thermal wall can produce a backlit effect or illuminate street fronts.
Effective aesthetics and daylighting in hurricane-prone regions require special attention to the glazing. With new glazing systems, architects have greater freedom to meet life-safety goals without sacrificing other design needs. Hurricane-rated channel glass is one solution that can help buildings resist Mother Nature, while preserving beauty and occupant comfort.
Jeff Razwick is a vice president for Technical Glass Products (TGP), Snoqualmie, WA.
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