Strength In Daylighting

By Madina Mehmood Ali and Seth Warren Rose
From the October 2017 Issue

With the benefit of new polycarbonate sheets, safer and more secure skylight options have recently come on the market. Until recently, commercial skylights were made primarily from either acrylic or high-impact strength polycarbonate for hurricane zones. While polycarbonate is virtually unbreakable, it can yellow over time from ultraviolet (UV) light exposure. Now, the problem of polycarbonate discoloration has been addressed with a new type of polycarbonate lens skylight that remains as clear as glass for many years without sacrificing the strength for which polycarbonate is known.

Today, owners and operators of high performance buildings should consider employing daylighting, including skylights and windows, with controls tied to dimmable efficient electric lights. With excellent color rendering, daylighting is the highest quality light source available for interior environments. And, daylighting caters to the growing number of people who understand how long-term investments in healthier lighting improve occupant well-being.

But even for facility managers who are not prioritizing healthy environments for building occupants, daylighting has economic benefits that go beyond the energy savings. People who work in properly day lit facilities perform better. Students secure higher grades, office workers experience less sick days, and factory employees produce fewer defective products. Occupant benefits have been well documented by the consulting firm Heschong-Mahone Group (now the TRC Company).

Despite its considerable benefits, daylighting is not frequently specified as the primary light source in facilities. In a report, “The Seven Market Obstacles to Daylighting,” Eneref Institute examined the challenge that daylighting technologies face in penetrating the lighting market. A major obstacle is that commercial lighting projects are too often driven by payback calculations of energy, opting for short-term return on investment (ROI) over long-term benefits. Yet, over the last few years, Eneref Institute has gathered a collection of strong endorsements for daylighting from building occupants, facility managers, architects, and engineers alike.

And with today’s polycarbonate, facility managers—especially those with security concerns—can safely turn to polycarbonate skylights. Polycarbonate offers strong impact resistance due to its chemical structure. The ability of structures like carbonates and phenyl groups to dissipate energy is believed to be the main reason polycarbonate can endure heavy force without breaking. Meanwhile, acrylic is brittle and therefore much more likely to be damaged by wind-borne debris and fall accidents.

“With enough impact energy, you can cause any material to fail,” explains Ted Trautman, Ph.D., technical director at Covestro, a manufacturer of polycarbonate materials. “The question is, how much energy does it absorb before it fails? Under the same test, acrylic samples fail, absorbing 25 times less energy than polycarbonate,” he continues.

So-called impact-modified acrylic (IMA) is mixed with styrene butadiene, a rubber that makes the polymer more pliable. While IMA is often substituted for polycarbonate, test results reviewed by Eneref Institute found polycarbonate to surpass the strength of IMA by as much as 50 times.

Says Trautman, “Due to dissimilar testing geometries and impact velocities, some impact tests will rate polycarbonate’s advantage over impact-modified acrylic differently. I’m comfortable saying polycarbonate has 10 times the toughness of impact-modified acrylic. That’s a conservative estimate.”

The U.S. Department of Labor’s OSHA (Occupational Safety and Health Administration) mandates the safety requirements of rooftop skylights. Unlike acrylic, polycarbonate’s impact resistance far surpasses OSHA’s guidelines for fall protection or accidents in skylights.

In Dunn, NC, the construction team for furniture retailer Rooms To Go examined samples of skylight plastics prior to an installation at its warehouse facility. “We felt the polycarbonate was a stronger, more durable product,” says senior construction manager there, Bruce Wallick. “We had product samples in the office, and we stabbed them with a knife, twisted, and bent them. The polycarbonate wouldn’t snap or break.” The facility installed VELUX commercial dome skylights.

Wallick said they selected the polycarbonate skylights for their impact rating over other polymer lenses. “With little structural steel and no fall protection bars on our roofs, the choice for polycarbonate lenses was primarily a safety decision,” he explains.

The significant advance that prevents yellowing of polycarbonate lens skylights is a UV-absorbing “cap layer” that nearly eliminates sunlight damage. By nullifying the effects of UV deterioration, this thin, highly concentrated polymer layer co-extruded and fused onto a solid polycarbonate sheet has changed industry thinking on polycarbonate and its use in daylighting skylights.

Still, polycarbonate suffers from the dated perception of a product that yellows more rapidly than acrylic. “Some architects prefer polycarbonate and often specify it, but if polycarbonate is not required by the architect or code, then acrylic is typically the most common choice,” says Eric Huffman, a senior fellow with Eneref Institute and president of Daylighting Solutions in Rohrersville, MD.

Still, Tim Reidy, an architect with the Hayes Design Group-Architects in Pittsburgh, PA, recommended polycarbonate skylights for the West Allegheny School District in Pittsburgh. “After attending a continuing education course on the importance of fall protection and the changing requirements of OSHA, we introduced the polycarbonate skylights to the district,” he comments. “I’d rather do it right and not worry.”

Since fall protection for acrylic could call for investment in railings and steel mesh—themselves causing roof penetration and light obstruction, polycarbonate was a less expensive option for that school district.

As demonstrated by Hurricane Harvey that recently struck Houston, TX and Hurricane Irma in Florida, storms will continue to increase in intensity, and so will the damage they inflict. The World Meteorological Organization (WMO) has found that the extent of areas affected by extreme weather events has grown and will continue to do so. Kerry Emanuel, MIT hurricane expert, has calculated that Atlantic hurricanes have become 60% more powerful in the last 10 years. The maximum wind speeds of these storms have increased by 25%. Hurricanes Sandy, Irma, Jeanne, Charlie, and Katrina measured wind speeds as high as 200 mph.

“We recommend polycarbonate prismatic skylights, especially in hail zone areas, where strength and safety are key,” explains Pete Shannin, vice president, daylighting product solutions at Acuity Brands.

It is often in the eventuality of damage to acrylic skylights that property owners select polycarbonate. Says Huffman, “When there’s a tornado, hailstorm, or hurricane, or if a school gets broken into through a skylight, people say, ‘How can we get a skylight that won’t break next time?’ That’s when polycarbonate gets really popular—after a tragedy.”

Mehmood Ali is a chemical engineer with Eneref Institute, a research and advocacy organization focused on environmental and social responsibility opportunities as a catalyst for change.

Rose is founding director of the Institute. He is also a special advisor for sustainable development to the Department of Defense at the Pentagon and produces reports on sustainable solutions for the Environmental Protection Agency. Eneref has offices in Philadelphia, London, Nairobi, Bogotá and Manila.

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