This chapter provides information on some of the glazing system options currently available and new technologies being developed. Many of these systems can be combined to increase the performance of the overall unit.
Clear float glass is the typical type of glass used today. It is manufactured by placing molten glass on a tin bath. Because the glass is lighter than tin, it floats, creating a clear, completely smooth surface with two parallel sides. Clear glass has no significant thermal resistance (R-value) from the pane itself. However, it has a value of R-0.9 to R-1 due to the thin films of air on the interior and exterior surfaces of the glass. It also has a high ability to transmit visible and near-infrared light.
Tints are absorptive materials that are available in both glass and plastic glazings. Tints are typically added to the material while in the molten stage of manufacturing and are dispersed within the glass mixture. They can be used in a single or multiple pane system. Tints absorb a portion of the incident solar radiation. The absorbed radiation is then transformed into heat within the glass, and depending upon the interior and exterior climatic conditions, some of this unwanted heat may be transferred to the building interior. Absorptance levels depend on the absorbing material (tint) and the thickness of the glass. Thicker glass provides more absorptance and transmits less visible light.
The application of tints lowers the shading coefficient of clear glass because some of the light and solar heat is reflected and absorbed. Common tints available are gray, bronze, blue, green, and combinations of these shades. Characteristics of these tints are summarized below.
· Gray glass transmits approximately equal amounts of visible light and infrared.
· Bronze glass transmits less visible light and more infrared than gray glass.
· Blue and green glass transmit more visible light and less infrared than gray glass.
Tinted glass alone can only reduce a shading coefficient to about 0.65. Combining tints with other glazing technologies such as reflective surfaces and coatings can further reduce shading coefficients (SCc) or improve the coolness index (Ke). To be most effective (reduce interior heat gain), the tinted glass pane should be used as the exterior pane of a double-pane or insulated glass system.
Mirrored glazing is a form of tinted glazing consisting of metallic particles that reflect visible radiation. The SCc and the solar heat gain coefficient (SHGCc) of this type of glazing should be evaluated carefully before specification. It can reduce the indoor natural light considerably, creating dark, unpleasant interior spaces. Heat Mirror glass has also been reported to increase the SHGCc on adjacent buildings from reflected solar radiation.
Translucent coatings allow for the transmission of most of the visible light, but disperse or scatter the light and distort the view. Translucent coatings can be placed within the glass, embossed on the surface, or applied as a surface film. They are useful for applications where indirect daylight and privacy are desired. Translucent glass alone may have little, if any, effect on the SCc or Tvis of the glass. As with tinted glass, it can be added to a system to enhance properties or meet specific needs.
Spectrally selective tints are created to be naturally selective to visible light. These tints are more selective in the visible and near-infrared spectrum than traditional tints and maintain relatively low SCc and high Tvis. The addition of selective coatings (low-e) can increase the Ke, whereas reflective coatings can decrease the Ke. Coatings on suspended films between glass offer even more flexibility in Ke targets.
Low-emissivity coatings were developed in the early 1970s primarily by the Massachusetts Institute of Technology (MIT) and introduced into the market around 1980. Most low-e coatings have relatively high Tvis and reflect 40 to 70 percent of infrared. Put loosely, low-e lets in light but blocks the heat. Various low-e coatings allow different amounts of UV radiation, ranging from a 0.05- to 37-percent reduction.
A low-e coating is a microscopically thin, virtually invisible, metal or metallic oxide coating deposited on a glazed surface. The coating may be applied to one or more of the glazing surfaces facing an air space in a multiple-pane window, or to a thin plastic film inserted between panes. The coating limits radiative heat exchange between panes by reflecting heat back into the home during cold weather and back to the outdoors during warm weather. This effect increases the insulating value of the window. A low-e coating can be applied on glass or plastic films using one of two methods.
1. Pyrolytic (hard coat) is a single layer of metallic oxides applied while the glass is still in a semimolten state. This surface is strong and durable and can be used on single-pane units. It has the highest shading coefficient (SCc) of low-e coatings (compared to sputtered) and can sustain high temperatures (i.e. can be used in solar collectors). The drawback of this surfacing is that it may make large expanses of glass look slightly blotchy with color shifts away from truly "clear."
2. Sputtered (soft coat) is a multilayer application of a metallic, heat-reflecting layer sandwiched between two antireflective dielectric coatings to maintain light transparency. The layers are applied to finished glass in a vacuum chamber. This system is fragile and must be protected in an multipane unit. It has the lowest SCc of the low-e coatings. Sputtered coatings are currently the most common application, roughly 80 percent of the market.
Various methods are used to integrate the low-e coated surfaces into window assemblies
· Low-e coated glazing is used on one of the panes in an multipane unit. The low-e coating can either be located on the inside of the exterior pane or on the exterior of the interior pane, depending on desired performance.
· Suspended coated film (SCF) is a low-e system developed to insulate better than typical low-e and provide more flexibility in Ke values. Using the sputtering process, a wavelength-selective low-e coating is applied to thin plastic film that is then suspended between two plates of glass. This creates a unit that, as far as convection and conduction are concerned, is essentially a triple-pane unit. The two airspaces also provide better sound control than standard multipane units. What differentiates SCF units is the spectrally selective coating on the suspended film. The coating blocks over 99 percent of UV radiation without blocking significant portions of visible light. The result is a unit with a low shading coefficient and high visible light transmission. Several coating types are available so that the appropriate shading coefficient and amount of light transmission and insulating value can be selected for a given application. Combining different glazing options is an excellent way to achieve desired performance. For example, the combination of two suspended films, low-e glass, and inert gas filling can achieve R-values as high as 107.
· Retrofit plastic films consist of a laminated polypropylene film, with or without a low-e coating, that can be applied directly to an existing glass pane. The films are intended to reduce the shading coefficient and increase thermal resistance and are available on the open market. Depending on the film used, shading coefficients can be reduced 35 percent on single-pane units and 22 percent on double-pane*. Retrofit films generally decrease the visible transmittance of the original glazing.
Many solar control films are commercially available to retrofit existing windows. They may be low-e, tints, and/or reflective. Most retrofit films will improve the performance of the glazing but are typically not as effective as glass that is coated during manufacture. The SCc and the Tvis are usually the target characteristics to be altered by adhering these films. All films, even those sold as clear, will affect the Tvis to some extent. Correct installation and durability are two important issues when considering retrofit films.
Plastics glazings were originally used for safety glass. They are less brittle and lighter in weight than glass, and they block essentially all UV radiation. Plastics are common for skylights where lightweight, safety glazing, or molded shapes are desirable. They can also be formed in structured and textured sheets. Many of the same coatings and additives discussed in this chapter can be applied to plastics. However, even though low-e coatings can be applied to plastic suspended films, they currently cannot be applied to plastics used as the structural glazing itself.
Plastic glazing deficiencies include degradation of the plastic over time and exposure to weather, greater thermal expansion, flammability, and low melting temperatures. These are important issues when considering plastic.
Several types of plastic glazings are currently available:
· Acrylic glazing has good light transmittance and longevity. It tends to be soft and easily scratched. It can be frosted for translucence and privacy. Typically it is used in skylights. Acrylic is easily molded to a variety of shapes.
· Polycarbonate is similar to acrylic, harder, and less likely to scratch. It is a UV-stabilized material for use in vertical and overhead glazing. It has added properties to optimize its strength and thermal characteristics.
· Fiber-reinforced plastic consists of a double layer of plastic combined with fiberglass mesh or insulation. It is commonly seen as corrugated roofing panels that offer a translucent and flexible protection. Panels filled with fiberglass insulation are often used where a high thermal resistance combined with translucent glazing is preferred. Exposed surfaces of some products may be susceptible to erosion.
Historically, double- and triple-pane window voids were air-filled. Even though the air fill creates the insulating effect of multipane units, conduction and convective currents can develop. With the emergence of alternative gas fills, the heat transfer properties of multipaned glazings have decreased.
The most common alternative gas fills are inert gases. Many safe and naturally occurring gases have a significantly lower thermal conductivity than air. By hermetically sealing these gases between two layers of glass, the conductance of the window can be decreased. The most commonly used gas is argon, which is easily extracted from the atmosphere. Krypton is more effective, particularly in small spaces, but is more costly to obtain and use than argon.
When combined with special coatings, gas-filled units can achieve very high insulating values. For small thickness, the replacement of air with argon can effectively add about R-1 to the unit performance. An ideal air space should be 0.5 in. The wider the thickness of the fill space, the less advantageous the alternatives to air become. If the airspace is too wide, convective currents can develop within the multipane unit, minimizing its effectiveness.
An alternate to gas fill is creating a vacuum, or evacuated space, between the panes of glass. Although not fully developed, this system theoretically has no convective or conductive heat exchange between the panes of glass. The long-term integrity of the seals at the glass edges and the structural stability of the unit have not been perfected to make this a viable alternative. The seal must be able to keep air density within the unit to less than 1 millionth of normal atmospheric pressure. An air density of only 10 times this amount is sufficient to re-establish conduction to normal levels. The current technology consists of two panes of glass about a half millimeter apart with vacuum between the panes. Tiny invisible glass spheres or silica foam within the evacuated space keep the unit from collapsing. Note that even using a vacuum does not eliminate conduction through the window spacers, or prevent radiative exchange through the glazing, or decrease air infiltration.
Using transparent insulation has great potential for translucent and semitranslucent glazing with high R-values. Several types of transparent insulation are not currently available commercially; however, others have been used for many years. The three major types of transparent insulation (fiberglass, aerogel, and honeycomb) are described below.
· Translucent fiberglass insulation is sandwiched between two panels of reinforced fiberglass. The density and thickness can be changed to modify the properties of the product. This product is currently available.
· Aerogel is made from 4 percent silica foam and 96 percent air. Microscopic cells of foam entrap gas, preventing convection, but allowing light to pass. This material creates a haze due to the scattering of light from the air pockets. It has the potential of reaching R-20 per inch. Radiation and conduction through aerogel are also reduced. This product is available in Europe, and a similar product is being developed in the United States.
· Honeycomb or capillary structures absorb solar radiation and re-radiate or transmit it directly indoors. Inner walls function like low-temperature wall heaters, creating a passive heating system.
Directionally selective materials reject or redirect incident solar radiation based on a geometric relationship between incoming light and the material. These glazings can redirect light to a predetermined location. Examples of these glazings are glass block, silk-screened glazings, prismatic devices, enclosed louvers, holographic films, and imbedded structures.
Frit is the most common angle-selective coating. Frit consists of a ceramic coating screen (translucent or opaque) printed in small patterns on a glass surface. The pattern used on the glass controls the light based on its angle of incidence. The color of frit controls the reflection or absorption and the control of view or visual privacy. Visual transparency can also be controlled by applying frit to both sides of the glass in such a way that at some angles it appears transparent, while at other angles it appears opaque. Angle-selective materials can be thought of as a series of fins or overhangs within a piece of glass, which filter or block light.
Prismatic systems redirect light by the principles of refraction. Refraction is the "bending" of light as it passes through a material. A common example is the apparent "bending" of a pencil as it is immersed in water. A common prismatic device is the Fresnel lens, made of microscopic prismatic materials embedded within the glass to focus light. Depending on the application, Fresnel lenses can focus light inward or outward. Other than Fresnel lenses, prismatic systems have limited commercial availability.
Another type of directionally selective glazing in the research and development stage is holographic films. Holographic films consist of diffractive structures of photopolymers or embossed films that are applied to glass to direct light deep into a space. Light is redirected and remixed as desired, or as a function of the angle of incidence and wavelength of the light. If the film is applied to the upper portion of a pane of glass, light that falls on that area is redirected. Holographic devices work in a way similar to traditional light shelves but are able to redirect for desired light penetration. Holographic film offers reduced maintenance compared with light shelves, but does not provide shading of lower portions of glass. Glass will appear darker in areas where film is applied but will remain undistorted.
Current goals are to produce a product that will direct sunlight falling on the device toward the ceiling, deep into the room, which will reflect glare-free, diffused light into work spaces.
Switchable optical windows, or smart windows, have the ability to change their physical properties based on predetermined conditions. These chromogenic glazings (which have the ability to change states) can be altered either passively or actively. Where a change is desired, switchable materials can provide reduction of glare, privacy, daylight and solar control, and reduction of UV transmission. Most are still in the developmental stages and are not yet available for large-scale commercial projects.
In a hot climate these switchable glazings can modulate the intensity of incoming sunlight. When combined with continuous dimming controls, switchable materials may provide a significant benefit in reducing peak electrical demand for cooling and lighting energy in commercial buildings.
Switchable optical materials are of several types, each characterized by the means with which to control its properties, as summarized below.
· Photochromic materials change their properties as a function of light intensity. Sunglasses have used this technology for some time. The primary benefit is for visual comfort and glare control. Several skylight manufacturers now offer this option. Optical properties are changed as the metal halides in the glass are exposed to light. This creates a clouded appearance. As the absorptance increases, transmissivity decreases. The material reverts back to its original transparent state in the dark. Disadvantages with photochromics are that the threshold for change is fixed. Therefore, there is no seasonal selectivity to allow more solar gain in winter and less in summer. When activated, photochromics reduce only the visual transmittance, not the infrared, so much of the solar heat gain is unaffected.
· Thermochromic materials change properties as a function of temperature. Optical properties are changed as liquid- and gel-based materials or thin-film solid-state devices are exposed to heat. The material reverts to its original state when cooled. In its exposed state, the material has a clouded appearance. As with photochromics, a disadvantage with thermochromics is that the threshold for change is fixed.
· Electrochromic materials change properties as a function of applied voltage. Properties range from colored, to intermediate, to bleached. These systems are more complex than thermo- and photochromic systems. The threshold for change can be altered in an existing unit, allowing for occupant, daily, and seasonal adjustment. Controls can be operated manually or linked directly to building operating systems. Electrochromic coatings can be put on various layers of single- or double-pane units, or combined with other glazings (with or without other coatings).
There are two major types of active electrochromic coatings: liquid crystal devices and solid-state devices.
· Liquid crystal molecules (which are randomly distributed, and scatter and absorb light) are suspended between two transparent conductor layers. When electrically charged, the molecules align and allow light to pass. The clear position requires a continuous charge of electricity to maintain transparency, making this product either on or off. This system does not alter the shading coefficient and therefore provides little energy savings potential. It is used primarily for privacy and glare control on interior applications. Although this option is currently on the market, its high cost makes it applicable only for specific uses. Liquid suspension is similar to liquid crystal but dims through a range of tints as a result of applied voltage. This system requires a continuous voltage in all stages.
· Solid-state metal devices have a layer of metal deposited, similar to low-e coatings. By applying low voltage, properties can change between clear and tinted and a variety of states in between. Unlike liquid crystal, disruption of power will not change the state_ rather, a small applied voltage is necessary to make each phase change.