Amazing Glass: Part One

Source: ARCHITECTURAL LIGHTING Magazine
Publication date: 2007-07-01

By James R. Benya

<i>This is the first in a three-part series about the most important light-managing medium, glass. Part one examines conventional building glass, particularly as it affects daylight and daylighting; part two will look into glass as a medium for lighting and lighting effects; and part three will explore the artistic opportunities of some new and exciting glass types.</i>

Energy gurus and passive solar geeks aside, most of us have historically paid little attention to glazing other than how it looks. If anything, most want it to be as transparent as possible. However, since the Leadership in Energy and Environmental Design (LEED) program and energy efficiency demand more efficient use of natural light, many architects, lighting designers, and engineers are now scrambling to master the technical side of this material as well.

Most practical uses of glass in buildings are for windows, clerestories, and skylights. <i>Windows</i> are defined as glass that is mounted more or less in the vertical plane with the intent to be looked at or through. <i>Clerestories</i> are defined as windows mounted above eye level whose primary purpose is to introduce daylight into a space. <i>Skylights</i> are generally defined as glass installed in a more or less horizontal plane. <i>Glazing</i> is a generic term that can include glass, plastic, or other light-transmitting materials as well as combinations of them. Within the last two decades, <i>coatings</i> have been developed to change the optical properties of glass; today, they are essential in achieving desirable performance values. <i>Films</i> can be applied to glazing to change properties as well.

While single-pane glazing is sometimes used in very benign climates, in order to provide insulation and protect special coatings most buildings use multilayer assemblies of glazing to create windows and skylights. For practical purposes, the following principal characteristics are used to evaluate alternatives.

The percentage of visible light that is transmitted through the glazing assembly. This is the essential characteristic for daylighting calculations. A perfectly clear window would have a VLT of 100 percent. Most practical assemblies for architectural use are between 35 and 80 percent.

The percentage of total solar radiant energy that is transmitted through the assembly. This is the essential characteristic for solar gain calculations. For ordinary windows without special coatings, the SHGC and the VLT are the same and sometimes called the shading coefficient (SC). However, with modern coated windows, the SHGC is almost always lower than the VLT. Such window systems are generically referred to as <i>low-emissivity</i> or <i>lowE</i> and are used in most commercial construction.

U-factor is the rate of heat loss through the window and its frame. This is the essential characteristic for heat gain and loss other than solar. The lowest practical U-factor is preferred when the building is exposed to extreme cold or heat.

While these characteristics are usually applied to clear glazing, they can also be applied to glazing that is diffuse or translucent (like white acrylic) or refracting (like obscure glass or prismatic acrylic), or that has any combination of refractive and diffusing effects.

In choosing a glazing system, a primary objective is usually to employ a window glass system that allows the transmittance of the most possible visible light (maximum VLT) while minimizing the solar gain (minimum SHGC). The ratio of VLT to SHGC is sometimes called the light-to-solar-gain ratio or LSG. Most typical window glass systems have an LSG between 1.2 and 1.4. In other words, to have high visible light transmission, it is necessary to allow in quite a bit of solar gain. However, a “dream glass” is now available using a special combination of low-lead glass and state-of-the-art coatings that achieve an LSG as high as about 2.4. When designing ultra-efficient buildings, having relatively clear windows (VLT .7 or more) with an SHGC less than .30 can be the magic combination, especially in cooling-dominated climates.

While the big numbers above (VLT, SHGC, and U) are principal factors in building design, there are several other issues to consider. These include:

By definition, the color of daylight is essentially perfect, with a color rendering index (CRI) of 100. However, almost every glazing system has a color characteristic, from very subtle hints of tint to strong variations of green, gray, bronze, aqua, and blue. Remember, just like any filter, the transmitted color is the opposite of the color that is most absorbed. Blue glass, for example, absorbs yellow light. While the color impact is often slight, a heavily tinted glass will cause the perceived color of daylight to change considerably, reducing the CRI of daylight considerably. For example, some green-glass systems have a CRI of 60, about as poor as old-fashioned “cool white” fluorescent.

An important, but far more subtle, color effect occurs in glazing with spectrally selective coatings, which is how low-E glazing is made. The purpose of these coatings is to cause the glazing to reflect infrared energy away from the building while permitting visible light into the building. If a glazing system has LSG>1.0, then it has a low-E coating. The challenge of low-E coatings is to pass visible red without passing invisible infrared energy. The wavelengths are nearly the same; the better the infrared heat rejection (LSG>1.0), the more likely that the low-E coating will also reject some visible red light. In other words, in order for a glazing system to have very high performance, it must, by nature, appear to be at least slightly bluish. When comparing glazing assemblies, do not be surprised that the highest performing products, even if sold as “clear,” lack the warmth of glazing systems with LSG<1.2.

The lack of coloration combined with a high VLT results in windows that appear especially clear. Premium, low-lead glass is used, usually as part of an assembly that includes coatings to reduce reflectance and solar heat gain. The key trait of low-lead glass is that it lacks the greenish tint of typical window glass, with results that are often considered spectacular and worth the extra cost. The key benefit is apparent sharpness and color dynamics; with minimum tendency to impart its own coloration, the color rendering of light is almost unaltered.

The larger question of whether a window appears adequately clear is often debated. In addition to low-lead glass, for clarity a high VLT is preferred. From my 30-plus years of work experience, I find that most architects feel that a minimum VLT of 50 percent is required for a window to appear adequately clear; personally, I believe the threshold is lower, with an absolute minimum of 35 percent before the window's sense of clarity is lost. So among the challenges of choosing glazing, one is typically faced with choosing the highest possible VLT for clarity while keeping the lowest possible VLT for energy efficiency. A related consideration is brightness; even the brightest computer screen is less bright than the darkest window by day, and there is a genuine concern that windows that are too clear will affect office work, especially on sunny days.

Risking both significant solar gain and brightness problems, the new <i>New York Times</i> headquarters in New York City is clad in glass with VLT over 70 percent in order to achieve “transparency,” a key design statement of the building. The project relies heavily on interior shading systems to help control both heat and glare problems. Given the amount of publicity on the design of this critically important project, I hope that post-occupancy evaluations will test both energy and brightness aspects of the glazing system to help better understand the functional implications of high-clarity glazing.

All glass reflects at least some amount of light. With typical window glass, the reflectance is often about 5 to 10 percent. Especially if the space behind the glazing is relatively dark, such reflectance appears as a mirror, defeating the sense of transparency. If the glass itself is also dark (VLT<.50), the effect is increased.

Glass can be coated to increase or decrease its reflectivity. Most architects and lighting designers are familiar with highly reflective glass used to create dramatic curtain walls; the reflectivity can also take on a tint to give the building a distinctive color. However, highly reflective coatings dramatically reduce the sense of transparency. Anti-reflective coatings, on the other hand, can be employed to decrease reflectivity to about 2 percent. Under almost any conditions, this glass tends to appear clearest.

Iridescence is a quality associated with glass coatings, anodizing, and other special materials in which slight “rainbowing” or other three-dimensional color effects are seen, usually as a function of the viewing angle. In glazing, iridescence is caused by low-E coatings. Low-E is actually a dichroic coating, designed to reflect both long-wave (infrared) and short-wave (ultraviolet) light while passing the visible spectrum. As with any dichroic coating, there is a range of optimal angles, beyond which incident light can be reflected with secondary dichroic effects. When combined with reduced VLT, iridescence can cause strange three-dimensional reflectance, resulting in a glazing situation that feels more like a fishbowl than a window.

For use as a solar gain control element, frits are a pattern of white reflective shapes embedded into the interior surface of the exterior pane of a glazing system. Typically made of ceramic, their purpose is to reduce the transmission of the glazing by reflecting a desired percentage of total light away from the building. Frit patterns can be as simple as dots, or they can be elaborate works of art. Because frits reflect light, they can serve as an important solar gain control method. Note that frits are not diffusing, like frosting or acid-etching, which are generally used on interior glass to make the panes translucent. Rather, frits simply reduce the effective size of the glazing system, allowing larger actual panes of glass that optically behave like smaller panes.

A variation of fritting is to employ photovoltaic (PV) cells. In addition to reducing the amount of transmitted light, the PV cells generate electricity. For south-facing windows and clerestories, PV frits are a good way to get significant double benefit from a single investment. The Lillis Business Complex at the University of Oregon in Eugene is one such example of a project where photovoltaic cells were used as a fritting device. (See “Daylighting Gets to Work,” Jan/Feb 2005.)

Since the 1970s, Lawrence Berkeley National Laboratories (LBNL) Windows and Daylighting group has been leading the field with innovations in research, product concepts, design assistance, and software. Those interested in working at a detailed level with glazing systems should consider becoming familiar with lab's numerous publications and software provisions. Start by downloading Window 5.2 (current version) and Optic 5.1 (current version) from the LBNL website, www.windows.lbl.gov. To support these important software systems, LBNL also maintains a database for a wide range of products, including windows, assemblies, coatings, and films, allowing complete analysis of glazing assemblies for their thermal as well as optical qualities.

As we start designing more daylighting into buildings, many of the 20th century's bad habits, such as overglazing and careless glazing orientation, will creep into designs. The tendency to favor style and view over solar concerns will continue to challenge architects and lighting designers, but going forward we must have better solutions and creative alternatives. Mastering the ability to address the many principles and side effects of glazing is new to most of us, but it can't be for long.

<i>In part two, (Sept/Oct 2007), the many alternatives to regular windows and skylights will be explored, including a wide range of concepts from distorting and diffusing panels to refracting and embedded optics.</i>

Daylight is the perfect-color light source. Even though the color temperature changes due to weather, time of day, and time of year, for all practical purposes the color rendering index (CRI) of daylight is 100, and color evaluations are often best made outdoors. With single-pane clear window glass, the CRI may drop a little (98+), but overall the color quality is close to perfect. However, when using multilayer glazing assemblies—and especially with tinted and lowE glass—daylight is altered. Here are a couple of interesting examples.

In an office building with dark-green tinted glass, electric lights in the core areas were described as “pink” even though they were actually modern 3500K T8 lamps. Upon inspection, workers in office areas near the windows had become adapted to green daylight with a CRI of about 55, worse than old-fashioned “cool white” fluorescent. When they left the area and entered the core of the building, they experienced transient color adaptation, which occurs when a viewer who is fully adapted to viewing a saturated color of light (green) will, upon entering a white light environment, see objects tinted to the color wheel opposite (magenta) until readapted to white light. Nothing could be done about it.

Housed in a modern medical office building with dark-bronze tinted glass, dentists and dental technicians complained about the color of daylight, which is important in the matching of crowns and cosmetic dental work. The approximate CRI of the filtered daylight was determined to be about 68 with a distinct greenish-brown tint. Although some energy efficiency was lost, the glass was replaced with clear glass, and the problem was solved.