Glass or plastic: The question for skylights

There is good reason for architects’ recent emphasis on daylighting. Three different studies by the Heschong Mahone Group Inc., a research house in Fair Oaks, Calif., show that natural daylight can increase learning productivity of students, translating in the working world to higher sales in retail stores and improved employee productivity in office buildings. A major part of daylighting in buildings comes from skylights.

With increasing popularity of skylights, more architects, building owners and contractors want to find out about the performance, costs and energy efficiency of different glazing options.

Glazing needs vary according to the demands of each project. Some designs require a clear view through the glazing, while others need natural, daylit illumination of a particular area to help in-crease productivity, limit absenteeism or reduce energy costs. Both glass and plastic products can meet stringent hurricane-resistant standards and offer increased protection in violent storms.

Basic glazing materials

The first option that comes to most people’s minds is glass. Although initially the most expensive option, glass is durable, more scratch-resistant than plastic, and long lasting. Various types of glass provide different degrees of energy efficiency. Insulating glass units, for example, use laminated glass, a spacer and a second sheet of glass to provide a sealed, moisture-free option.

Glass with a low-emissivity coatings can be beneficial in the south to reduce heat gain (lower solar heat gain coefficient) and beneficial in the north to reduce heat loss (better U-factor). Laminated glass, comprised of two sheets of glass with a polyvinyl butyral plastic interlayer, is used in most blast-resistant and hurricane-resistant glazing. The glass may fracture, but fragments remain bonded to the interlayer.

Though glass is less formable than plastics, it can be curved or shaped. Annealed glass is not as strong as heat-treated glass, but it can be cut easily. Heat-strengthened and tempered glass are approximately two and four times stronger than annealed glass, respectively, but can’t be cut. Tempered glass meets the safety glazing codes because it fractures into small fragments when broken.

Acrylic, used in many standard skylights, is the least expensive and most formable of the skylight options. Impact modifiers can be added to acrylic for enhanced durability. However, acrylic is gas permeable, so multidome units cannot be sealed effectively. It is offered in transparent or translucent materials. Transparent acrylics often have a bronze or gray tints, whereas translucent choices usually come in white or patterns. Most people are familiar with the prismatic pattern seen in fluorescent light covers. Acrylic material, manufactured by either a cast or extrusion process, can be easily cut.

Polycarbonates lie between glass and acrylic in terms of cost, are very durable, highly impact-resistant and can be easily cut. They also can be formed, but at a higher cost than acrylic. Manufactured by an extrusion process, polycarbonates come in monolithic sheets, and are also available in multiwall sheets extruded with multiple layers, and the honeycomb effect creates air spaces for more energy efficiency.

Naturally transparent, polycarbonates transmit almost as much light as glass, are heat resistant and have excellent dimensional and color stability. However, they have only fair chemical resistance. Ultraviolet stabilizers are added to help protect polycarbonates from sun radiation, as sunlight degrades its properties.

Fiberglass is a translucent option that features two pieces with an air space for insulation. The pieces create a panel that provides excellent light diffusion.

Fiberglass is not formable but can be shaped and formed inexpensively by bonding it to I-beams. Such translucent panels have high insulating values and feature a variety of grid patterns and color combinations. Fiberglass creates effects that cannot be duplicated with other options.

Translucent panels are virtually shatterproof and impact-resistant, yet lightweight. Ultraviolet coatings can protect translucent panels from weather damage, fiber bloom and discoloration.

What professionals say

David Moss, an independent architect and roof consultant, based in Lakewood Ranch, Fla., works on projects along the East Coast from Florida to New York. Due to the wide variety of climates, Moss’ clients voice diverse concerns. For instance, in Florida, where most schools do not have heating systems, unwanted heat buildup from skylights has become a major concern. Acrylics or polycarbonates proliferate for most of the small skylights, Moss says. The use of hurricane-resistant glass also has become very popular in Florida, he says.

By choosing the right material, Moss creates dramatic changes in lighting. On a job for Genesee Community College, Batavia, N.Y., for instance, he recommended updating six 20-by-120-foot skylights. To replace translucent, bronze-tinted panes, he specified a clear white translucent material. The college reported 75 percent savings in internal lighting after the replacement.

“Other pluses can come from choosing the right infill, “Moss says. “Laminated glass, acrylic and polycarbonate absorb ultraviolet rays.” That reduces fading of furniture, fabrics and carpet.

“Architects and building owners specify skylight infill based on quality, aesthetics and performance,” says Michael Nielsen, president of W.S. Nielsen Skylight Systems in Alpharetta, Ga. From his perspective, “glass is class.” Furthermore, although the most expensive, glass has by far the longest life span. Nielsen says that most of the glass used today is either laminated or heat-treated.

Plastics cost less than glass, and are often specified for different colors or back lighting. “Plastics also come into the picture when lighter loading is an issue,” Nielsen says. Translucent panels are middle-of-the-road in cost and are specified for better insulation performance. The insulation factor makes them popular in the north. In the south, Nielsen says, light transmission and heat-gain management are important when specifying a glazing infill for skylights.

Walter Scarborough, director of specifications for HKS Inc. in Dallas, sees the various glazing products as “different opportunities to give a skylight an aesthetic character,” ranging from the refined elegance of glass tinted to complement an architectural design to fiberglass only for utilitarian purposes. Skylights instill drama in interior spaces, Scarborough says, and the glazing material that will make the strongest statement should be selected.

“Glazed skylights are a design feature whose creative potential is virtually untapped and rarely achieved,” Scarborough says. “I recently saw photographs of a skylight that was a tent-like structure, complete with wooden poles at the center, over a twisting-and-turning hospital walkway in which the glass panels were glazed into the framing in a shingled fashion.” His recommendation is for architects to push the envelope away from the traditional shed shape skylights with clear glass.

Skylight beacon
Located at one of the city’s busiest intersections, the Strom Thurmond Fitness and Wellness Center was designed by The Boudreaux Group of Columbia, S.C., in collaboration with Cannon Design of Los Angeles. The center is a gateway to the University of South Carolina’s Columbia campus and one of its most prominent signature buildings. “The goals of the skylights were to allow for as much natural daylight as possible and to create a literal beacon of light at night,” says Randy Huth, architect, The Boudreaux Group.

“The translucent dome at the Rotunda, the four bays of skylights in the main four-court gym and the skylight in the natatorium accomplished these goals,” Huth says. Two pedestrian bridges with translucent skylights, lighted at night, help to frame the complex.