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Cellular Polycarbonate Glazing
Norm Bonenfant
As the design industry looks to reduce energy consumption, glass’ most appealing attribute is its ability to allow natural light to enter a structure. Still, glass can be heavy and prone to breakage/vandalism, so one should look at all alternative materials and possible glazing solutions for a given project—including polycarbonate plastic panels. After all, not many other building products can be delicate enough to transmit light, while possessing enough durability to withstand hurricane-blown debris.
Some interesting glazing products have left their mark in recent years, including acrylic (as explored in the article on page 5). Most designers are familiar with acrylic plastic products, with acrylic plastic’s virtues lying in its clarity and ultraviolet (UV)-resistant attributes. Another way to bring soft, diffused natural sunlight into a structure is through translucent roof panels—’sandwiches’made of an aluminum grid core onto which fiberglass-reinforced polyester (FRP) facings are glued. (These translucent panels often rely on fiberglass insulation between the inner and outer skins to reach the insulating level required for most contemporary applications.)
Another insulation method is to choose a glazing material comprising multiple walls (with multiple cells) to achieve the required rating. Employed successfully in Europe for more than 15 years (and growing in popularity here), cellular polycarbonate is currently used in everything from airplane windows to compact discs because of its strength and toughness.
Years ago, many architects/engineers (A/Es) associated polycarbonate plastic with inexpensive plastic products and considered it prone to yellowing. However, technology has improved. Thin UV-resistant coatings can be applied to polycarbonate plastic when polycarbonate is extruded, offering enhanced protection for performance and aesthetics. Furthermore, the cellular plastic structure’s light-diffusing characteristic can leave any scratch or dirt almost undetectable. When cleaning is desired, a simple hosing or pressure-washing usually suffices.
Attributes and advantages
Polycarbonate panels were among the first window glazing materials certified under Florida’s Miami-Dade County building codes. In lab settings, successfully hurricane-tested polycarbonate storm panel windows can resist the impact of a 2.4-m (8-ft) long 2×4 fired from an air cannon at 55 km/h (34 mph). In another test of the polycarbonate plastic panel’s strength, a polycarbonate barrel-vault skylight was impact load and high-pressure tested to 19,727 Pa (412 psf)—the equivalent to 571-km/h (355-mph) winds.
Improved technology in cellular polycarbonate has led to new polycarbonate panel profiles, which are wider, thicker (ranging from about 6 mm to 41 mm [0.25 in. to 1.6 in.]), and have as many as seven polycarbonate cells (eight walls). Color options include green, blue, clear, mist, opal, and bronze tints.
In addition to impact resistance, polycarbonate panels can offer opportunities for daylighting—a key component of the sustainable building movement. Daylighting is not merely a matter of bringing in the sun’s light—efficient systems must also insulate well. U-values as low as 0.16 can be achieved through cellular polycarbonate panel systems comprising double polycarbonate panels with an air space between the cellular polycarbonate sheets. Another ‘green’ attribute is that polycarbonate plastic can be a recyclable thermoplastic.¹
Construction and retrofit
Generally, a skylight made of 16-mm (0.6-in.) cellular polycarbonate panels can be cheaper than a conventional skylight glazed with an insulating glass unit (IGU). When the skylight is of curved design, the potential price difference can become quite dramatic.
A significant portion of these savings result from the possibility for reduced labor costs, as polycarbonate panels can be lighter and easier to work with than some other materials. This is particularly true when working with plastic products manufacturers who completely pre-fabricate all components, even to the extent of inserting and gluing the gasketing.
Cellular polycarbonate panel systems are typically installed by glaziers, sheet metal workers, or even ironworkers or carpenters. Cellular polycarbonate panel installation generally depends on the nature of the job and framing system.
In terms of renovation work, it is important to note one cannot simply retrofit an existing glass skylight or wall system with cellular polycarbonate panels, since the rabbets of traditional framing systems are often insufficient. Furthermore, these conventional systems not only lack controlled pressure on the gasketing, but also a special low-friction surface treatment. A typical system also includes aluminum framing members overlapping at the intersections for tighter weathersealing and improved appearance.
A common retrofit is to simply replace the entire glass skylight with a polycarbonate plastic version, sometimes employing the existing framing for its structural value.
One particularly interesting polycarbonate plastic retrofit was recently executed in Los Angeles (Salesian Boys and Girls’ Club). The existing skylight over a gymnasium was admitting so much solar energy (i.e. heat) that it became too hot to play basketball indoors during the summer.
The solution was a cellular polycarbonate plastic skylight ‘overglazing,’ which reduced the solar heat load by about two-thirds. This substantial decrease was made possible by the aluminum ‘dust’ mixed in with the resin from which the cellular polycarbonate glazing was made, which helps reflect solar energy. Through this polycarbonate plastic retrofit, the owner gained a completely new weathering surface, and roughly doubled the U-value of the skylight. For this particular project, the cellular polycarbonate overglazing added only about 0.05 kPa (1 psf) to the overall weight—not enough to be an issue for its steel structure below.
Standards and tests
The test methods for cellular polycarbonate are the following:
Other tests, such as ASTM International D 635, Standard Test Method for Rate of Burning and/or Extent and Time of Burning of Plastics in a Horizontal Position, and ASTM E 84, Standard Test Method for Surface Burning Characteristics of Building Materials, are used to determine weatherability and resistance to fire or smoke generation.²
Cellular polycarbonate panel manufacturers (as well as monolithic polycarbonate sheet suppliers) often issue warranties that address erosion and light transmission loss. Most polycarbonate panel system manufacturers also warrant their polycarbonate panel framing systems against deterioration and leakage.
Debunking misconceptions
Design professionals who have embraced polycarbonate glazing appreciate the way light plays up and down the flutes, and the way polycarbonate panels glow when lit from behind. This glow can be further enhanced by incorporating fine glass beads within the polycarbonate polymer’s resin. Light passing through the beads gives the polycarbonate panel an iridescent appearance.
Although the design and construction community is embracing polycarbonate plastic as a viable glazing option, some misconceptions persist. For example, it is conceivable polycarbonate glazing may have experienced some discoloration, strength, and scratch problems in its infancy, but as with any technology, polycarbonate processes and products evolved and advanced. Many criticisms of polycarbonate plastic can often be attributed to improper framing and installation. As with any building component, engineering the right polycarbonate panel system for the project dramatically reduces wear and noise.
Five years ago, it is unlikely anyone would have used cellular polycarbonate as a ‘feature wall’ (i.e. a wall standing out some 0.6 m [2 ft] from a building’s main wall), but this is exactly what has been done at a building in London, United Kingdom. The multicolored polycarbonate panels provide a thermal envelope and dramatic exterior which glows when lit from behind.
Cellular polycarbonate is also being used to form all four walls of an approximately 21 x 40-m (70 x 130-ft) tennis court building in Englewood, New Jersey (see photos on page 17). The individual polycarbonate wall panels are around 12 m (38 ft) long and have no aluminum mullions or other typical framing. Instead, they are joined by polycarbonate battens, which snap the polycarbonate panels together to form a total polycarbonate wall. While polycarbonate plastic is not a new material, it is one for which design possibilities are still being discovered.
Norm Bonenfant sold polycarbonate structural sheets (PCSS) for more than seven years as the national marketing manager for Extech/Exterior Technologies. He sits on the membership committee of the Small Manufacturers Council and is currently employed by SGT Distributors.
1 Recycling is not available in all areas.
2 For more on plastics and fire ratings, see the article on page 10.
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