Glazing Materials

There are numerous choices when it comes to the actual glazing material used. Transparent glass has been considered the default choice by many, but in fact there are many different types of glass with different properties and characteristics. Some of those traits apply to things like material strength, durability, and suitability for different applications. Many are relevant to daylighting, including things like the percentage of visible light transmittance (VLT) and the ability of the glass to diffuse light or not. Keeping in mind that more light is not always better, the LT rating of different glass products allows designers to choose how much daylight to let in or not. Similarly, transparent glass allows for good views but may not always be the best for daylighting. In many cases, translucent glazing that creates a more even, diffuse light distribution within the building is often better, as glare is more easily eliminated since direct sunlight shining through transparent glass creates high contrasts that can be visually distracting or counterproductive. Hence, the most successful daylighting strategies commonly combine a mix of transparent glazing where views to the exterior are needed or desirable plus translucent glazing for more uniform, even daylighting overall.

Photos courtesy of Kalwall® Corporation

Glass and polycarbonate panels are two commonly used glazing materials for daylighting in buildings, each with their own characteristics and limitations.

Glass is not the only glazing material available today. In fact, glass brings limits and some challenges to daylighting in terms of weight, vulnerability to breakage, low thermal performance, and cost. As such, manufacturers of glazing products have emerged with two prominent alternatives to glass. The first is the use of polycarbonate panels that are readily available as a commodity. Polycarbonate glazing is typically formed as an extruded product with two smooth outer faces separated by continuous ribs that provide strength and rigidity to the panels. The thickness of the panel can vary based on the manufacturer and the structural needs of the panel. The ribs give it a general translucent quality, even when clear polycarbonate is used. As a design material, it is also available in a variety of colors, such as whites, reds, blues, greens, and greys. From a thermal performance standpoint, double-glazed polycarbonate is about the same as double-glazed glass, although polycarbonate may have limitations in high-impact situations.

Fiberglass reinforced polymer (FRP) is another common glazing option that is very effective for daylighting applications. It is typically comprised of a thin sheet of material that is enhanced as needed to suit the conditions it may be exposed to either on the outside or inside of buildings, including high-impact and windborne debris. This can include formulations for general performance issues, such as UV blocking, weathering, structural reinforcing, or fire resistance. It can also include options particularly suited to daylighting, such as light transmittance levels, color, texture, or finish. In particular, it is possible to specify FRP that is specifically manufactured with integral resin that is tuned to be spectrally accurate in terms of light coloration related to daylight, thus assuring a greater correlation to sunlight and circadian rhythms.

In comparing these two glazing materials, polycarbonate and FRP, there are a few similarities for daylighting in that they both provide lighter weight, attractive, translucent alternatives to glass. However, there are some very notable distinctions between the two of them as well. Some of those distinctions start with the way the products are installed. Polycarbonate panels are typically a single extrusion that is inserted into an aluminum frame and secured in place in a manner similar to glass. FRP panels are commonly used in a built-up panel that includes an internal frame between two layers of FRP sheets. The thickness, properties, and materials used for the frame can be selected to produce differing performance capabilities. These include the strength of the resulting panel, which allows the creation of different sizes and span capabilities for the panels. It also includes the creation of a space between the FRP layers that can be filled with translucent insulation to boost the thermal performance of the panel. That means that the resulting U-factor of the glazing can be selectively varied with capabilities that far exceed typical glazing. Available thermal capabilities range from U-0.53 (R-1.88, comparable to double-paned glass) all the way up to U-0.05 (R-20, comparable to an exterior wall). Of course, the higher insulation levels tend to decrease the VLT levels, (except for some light-transmitting aerogel-type translucent insulation), so the project criteria need to be clear to select the most appropriate combination.

In addition to structural and thermal performance, there are some other notable differences between FRP and polycarbonate. Only FRP can demonstrate the full color light transmission that preserves the natural sunlight characteristics—all based on third-party verification of testing in this regard. FRP is also available in different colors for situations where altering the color of the light somewhat is preferred, including a range of clear, white, greens, blues, greys, and warm color variations. The FRP is also more easily manufactured to provide strong UV and fade protection to the interior of the building.

Photo courtesy of Kalwall® Corporation

FRP panels have been used for natural daylighting in a variety of commercial, residential,
and institutional buildings around the world.

Since both products are translucent, they can both claim glare control, but the range of options in FRP for light transmittance allows for a greater degree of control in that regard. Similarly, both products can claim resistance to impacts and offer durability and longevity as a feature. Note that the degree of such capabilities can vary considerably between manufactured products and should be looked at carefully for any glazing product under consideration. In terms of fire resistance, polycarbonate is a thermoplastic that is regulated by building codes. FRP has addressed this issue directly in many cases through the use of thermoset formulations that have been independently tested to demonstrate that they do not drip or melt when exposed to a fire. Thermoplastic polycarbonate, on the other hand, will in fact melt when fire is present and drip onto people and property below. When this condition is a consideration, be sure to be very clear on what the capabilities and/or limitations of the glazing products are in this regard.

While both polycarbonate and FRP can be effectively used for daylighting, reviewing the full depth and breadth of their other capabilities is clearly important for an overall successful building design. In particular, keep in mind that FRP panels on an integral frame creates a composite product with higher spans and strength compared to pure glazing products. This composite construction usually means the panels are self-supporting.

Total Product Performance

While the glazing is the first factor that everyone thinks of in daylighting, the reality is that the other components of a glazing panel are contributing factors to its performance as well. This point has been recognized by most fenestration professionals and has been the focus of the work of the National Fenestration Rating Council (NFRC). This not-for-profit trade organization is an independent source for data on all manner of fenestration products, including those used for daylighting. It offers standardized testing and documentation of the total performance of manufactured glazing products so that objective comparisons can be made between products. This applies to products that use all types of glazing, including glass, polycarbonate, and FRP, whether the glazing is transparent or translucent. It includes the total product performance taking into account the frame or other supports in a product, the glazing, and any accessory materials, such as thermal spacers, insulation, etc., that are used. The result of the NFRC testing is a label (or report in some cases) that allow all stakeholders to learn the following about a particular fenestration product:

• U-factor: The overall thermal performance of the unit is reported based on the total unit. In some cases, the U-factor of the glazing only may also be reported related to the center of the glazing or other locations.
• Visible light transmittance (VLT): This key element of daylighting is critical for predicting how well the daylighting strategy will work. Knowing what percentage of light is actually penetrating into the building helps to determine how much usable light is available. It can also help with an understanding of how much glare potential may exist if a high VLT is observed.
• Solar heat gain coefficient (SHGC): A fenestration unit that allows daylight to pass through will also induce solar heat gain. This is an important design consideration for the overall performance of the building, so being able to have reliable information on this point to compare products is invaluable.

Photos: © Scott Frances/OTTO

FRP panels can provide a quality of light suitable for museums that achieves good energy performance while controlling the amount of visible light and balancing the color spectrum, as shown here at Calder Foundation in New York City.

In addition to the above points, the American Architectural Manufacturers Association (AAMA) addresses testing fenestration products for some specific qualities. These include air and water resistance and impact resistance. The preferred level of resistance to impacts (referred to as “missiles”) is missile D, which is readily achieved by FRP products and less so by some others.