Daylighting Options in Facades

When it comes to incorporating daylighting into building facades, the discussion necessarily needs to focus on materials and systems that can do that. There are two fundamental types of material-based systems discussed as follows.

Glass-Based Systems

Glass is a well-known material that is used pervasively in buildings. Typically, it is incorporated into manufactured products that use wood, aluminum, or other materials to support, encircle, and frame one or more panes of glass. The glass itself can be clear, translucent, or opaque depending on the needs of the building design and the type of light and view qualities being sought. When the glass is clear, then glare control becomes an important design consideration. Translucent glass or glass treated with coatings, tints, or frits can address glare, but to varying degrees. Variations on sheet glass include glass block or thick channel glass sections (i.e., long U-shaped sections) that are primarily translucent in nature.

Depending on the building type and design, glass-based products typically become part of the facade in one of three ways. The first is to use sized openings in insulated walls to receive a manufactured window unit of a specified type and size. This is most common in residential and light commercial buildings and was the norm for most buildings prior to the 20^th century. The second typical system is a storefront glazing system, getting its name from its most common use: retail display windows. This is typically a field-assembled set of aluminum frames that receive full-height panes of glass to maximize view (in this case, typically into the store rather than out) and provide an economical facade. Storefront systems are intended to fall within a building structure and are considered best for first- and second-floor exterior applications or interior installations. A combination of these first two approaches is sometimes called a “window wall” where a large structural opening is filled in with multiple window units. The third approach to daylighting with glass is to use a curtain wall system. These systems can be field assembled or factory fabricated into “unitized” sections that are custom made to fit on the outside of a building structure. In so doing, the glazed sections are supported on the exterior side of floors, columns, beams, etc. creating a continuous facade appearance. Places where vision glass is not needed or desirable are filled in with opaque spandrel panels or can use translucent glass. While all of these glass-based systems are well-known, they can also be one of the more costly parts of a building facade. They also bring considerations of weight and the possibility of breakage.

Nonglass-Based Systems

The limitations of glass have prompted a number of glazing systems to be developed as viable and attractive alternatives. Fiberglass-reinforced panels (FRPs) are one type of glazing that has been commonly used for industrial or institutional settings. These panels are lighter weight than glass but have some inherent structural limitations typically requiring an internal grid of supports between two layers of FRP sheet. They are commonly translucent, offering diffuse light, but over time, the FRP glazing has been known to discolor affecting both appearance and light quality. There is also the option of placing translucent insulation between the two layers with improved thermal performance, albeit less light transmittance.

Another alternative glazing material is found in high-quality polycarbonate. This material was first used in single-panel products that were extruded into a single panel with large, (approximately 1½ inches) wide internal cells. This produced a single polycarbonate panel that some manufacturers described as “double glazed” since the inner and outer surfaces were separated by the internal cells. Most use tongue-and-groove methods to connect adjacent panels or set them in aluminum frames like storefront systems. While these worked well in some locations, many did not meet the performance requirements of evolving codes and rating systems or the expectations of some design professionals.

Fortunately, the development of polycarbonate products did not stop there. Newer products include extruded polycarbonate glazing panels with smaller, tighter cells (on the order of 18 hundredths of an inch) that provide superior light diffusion and material durability. They still have an inner and outer face separated by the inner cells, but because these glazing panels are comparatively thinner than their predecessors, they are often used in systems that place two polycarbonate panels into one unitized panel assembly. Compared to insulated glass, this type of “two-panel system” (referring to the two polycarbonate panels in the assembly) increases the thermal performance, reduces the weight of the glazing, and eliminates the breakage concern—all at less cost than glass.

There is a third type of alternative glazing, namely acrylic products. However, this is usually found in bubble style unit skylights rather than in building facades. It also carries some notable limitations due to fire code compliance (i.e., flammability), durability, span length, and discoloration.

High-Performing Translucent Daylighting Systems

All of the information just covered should make it clear that there are multiple options available and certain considerations to take into account when designing a daylighting system into a building. It is important to recognize, however, that all of the components need to be selected to work together as a system to create a high-performing facade that can produce both advanced daylighting and good thermal control. By understanding some of the advances in alternative glazing systems, a great daylighting design can be accomplished while still meeting high-performance goals for energy efficiency.

Perhaps the best example of these advances is found in currently available manufactured two-panel products using tight-cell polycarbonate glazing. These products incorporate two independent polycarbonate panels that are spaced apart with internal aluminum support members. This two-panel, translucent system provides architects with a unitized, integrated daylighting solution that can be readily worked into many different building types. As such, it is extremely versatile in that it can be used in configurations based on window, storefront, or curtain wall type of assemblies to suit different building structures or project conditions. We will take a closer look at these systems in the following sections.

Design Attributes

Translucent daylighting panel systems have sometimes been thought of as only a utility product. However, the latest high-performance, tight-cell, two-panel polycarbonate systems have become a feature that architects can use as a signature aspect of virtually any building exterior design. Perhaps the most striking example of the successful use of a two-panel system is the creation not just of daylit openings as part of a wall (i.e., punched window appearance), but rather, of an entire wall that becomes a fully daylit facade (i.e., luminous walls). The glare-reducing aspect of the translucent glazing provides a continuous facade with consistent diffuse light along the entire wall. This diffuse light creates the preferred light quality in most commercial and institutional building settings comparable to the desirable northern sunlight mentioned earlier. An important point to recognize is that this lighting quality is achieved not by introducing more daylight, but less. Too much light creates the stark contrasts that cause glare. Controlled diffuse light provides even illumination, thus eliminating glare and allowing for ideal lighting conditions. Therefore, more daylight is not necessarily better light—in fact, it may be worse.

© Derek Lepper

Project: Richmond Olympic Oval
Location: Vancouver, British Columbia, Canada
Architect: Cannon Design
Translucent glazing systems allow natural light to penetrate during the day but transform buildings at night by allowing internal electric lighting to illuminate the facade from within.

As an added design attribute, using translucent glazing allows the building to be transformed at night when the light source changes from the outside (the sun) to the inside of the building (electric lights). In this case, the same glazing now allows the building to take on a stunning glowing effect that gives the building a truly transformative quality. These facades can even be used at night to illuminate exterior spaces next to the building, reducing the need for additional exterior light fixtures and providing a more consistent exterior light quality too.

Looking more closely at the panel design, there are several aspects that can be used as an advantage to the appearance of the building. The first is the uniform, clean, and smooth appearance that can be achieved. Systems that employ concealed support members with glazing that covers and hides the aluminum only display the continuous glazing on the exterior. Hence, the resulting minimal vertical seams between panels give them the aesthetic quality of channel glass panels but with less weight and less cost. Horizontal seams can be reduced or eliminated as well since the internal supports and a unitized panel design can allow for full height spans. In these cases, the panels can be custom manufactured in lengths up to 40 feet and installed outside of the structure, just the way curtain wall systems are installed. Their lighter weight and internal structural strength make them easier to handle and less expensive than their glass-based counterparts.

The color of the glazing panels is another way to influence both the exterior and interior appearance of the building. Clear polycarbonate with small, tight cells will diffuse light effectively, but some prefer a white color that can diffuse the light and appearance even more. Other standard colors such as greens, blues, or bronze are also available that can enhance the light quality or be used only in selected panels to create a particular pattern or reinforce an identity for a building. Custom colors and color matching are also available from some manufacturers. Unique to the two-panel system is the ability to have different colors on the exterior and interior of the facade. This bicolor capability can create a desired appearance on the outside while allowing for a rather different one on the inside. It is also possible to add patterns or graphics over or within the glazing panels to fully customize a look. Whatever the final design configuration, high-quality two-panel systems can be selected that will not only look good when first installed but have been shown to retain their appearance and coloring over time.

Performance Attributes

From a performance perspective, different options in two-panel translucent systems can achieve different levels of daylighting and energy efficiency. These levels are based on the standard designations and testing protocols used for other glazing systems, including the industry standard National Fenestration Rating Council (NFRC). As such, any glazing materials or products being considered should be evaluated using this data, which is based on independent testing and certification by the NFRC. Such data and reports are available both from the manufacturer and the NFRC website (www.nfrc.org).

Using the NFRC test standards, both the glazing itself and the system (glazing panel and frame) can be tested with the appropriate values determined. The standard configuration is to provide an inner and outer layer of tight cell polycarbonate glazing, each approximately 10 millimeters (just over ³/₈ inch) separated by approximately 3 or 4 inches of air in between. Looking first at thermal performance, the insulating nature of the glazing compared to the conductive nature of glass yields good results. Tested values for the center of a standard two-panel glazing configuration yield a U-factor on the order of 0.23 (R-4.35), which is notably better than most double pane glass. NFRC also tests for the entire assembly, not just the glazing, so when the tested values for the complete system (panel and frame) are looked at, they are still fairly impressive, ranging from 0.28–0.26 U-factors (R-3.57 to R-3.85) depending on the size of the panel. To improve on these values even more, manufacturers offer various types of translucent insulation that can be placed between the glazing, resulting in glazing only U-factors ranging from 0.13 to 0.07 (R-7.7 to R-14.3). Testing the whole assembly yields results ranging from U-0.17 to U-0.09 (R-5.88 to R-11.11). These levels put the translucent insulated panels on par with values for some opaque, insulated walls.

Images courtesy of CPI Daylighting

Two-panel polycarbonate systems with an internal aluminum support frame can be glazed on the interior and exterior with different colored glazing that provides different performance characteristics. Translucent insulation can be added in the space between glazings for greater thermal performance as well.

NFRC also tests glazing and panel units for other important factors, such as visible light transmittance (VT) and solar heat gain coefficient (SHGC). These numbers are directly influenced by choices in the color and reflectiveness of the polycarbonate glazing since different ones are available. Two layers of clear glazing, for example, can yield values (for the glazing only) of 42 percent VT and 0.44 SHGC—all very favorable compared to glass. For projects where less light is desired to further control glare or reduce heat gain, different combinations of color and reflectiveness can be specified for the interior and exterior glazing. Depending on the choices made, VT can range from 13 to 30 percent and SHGC can range from 0.25 to 0.37. Both values can be reduced a bit further when translucent insulation is added. Understanding these values, colors can be customized to fit the performance needs of a particular project.