Glass in Architecture ÃÃ Mehran Arbab* and James J

International Journal of Applied Glass Science 1 [1] 118–129 (2010) DOI:10.1111/j.2041-1294.2010.00004.x

Glass in Architecture ÃÃ Mehran Arbab* and James J. Finley Glass Research & Development, PPG Industries Inc., Cheswick, Pennsylvania 15024

Flat glass admits natural daylight, provides unique design options, and improves the quality of indoor life. For these reasons as well as the technical advances in its production and large area coating technologies, glass produced by the float process has become a distinct and pervasive building material in modern architecture. Esthetic choices reflect the unique design concept of the architect; on the other hand, the energy performance of the window glass, which is well understood and measurable, allows the builder to identify the optimum product for different regional climates. This article will review the state of art in flat glass and coated glass technology and will discuss the esthetic and optical characteristics of commercial glasses. It will also provide a phenomenological account of heat transfer across glazings. Finally, several recent trends in architectural glass technology will be presented.

Introduction The Glass House uses glass boldly as a building material to create a continuum of space between the In the introduction to Philip Johnson, The Glass outdoor and the living space. Several more examples in House,1 Kipnis quotes the late architect Le Corbusier: South Western Pennsylvania, where the present authors ‘‘the history of architecture was ‘the history of struggle live, can illustrate the varied use of glass in architecture. for the window’.’’ Natural lighting and physical con- Frank Lloyd Wright designed his masterpiece—Falling- nection to our environment are integral to the design of Water—with noticeably low ceilings, in part to guide functional residential and commercial buildings. How- the eye toward the large windows that frame the beau- ever, as Kipnis notes in the same article, a driving force tiful natural surroundings of the house. The ALCOA in architecture ‘‘is [for Philip Johnson] ‘first, foremost, headquarters in Pittsburgh is an example of open façade and finally a visual art’.’’ These dual demands of art and in commercial buildings, where the all-glass wall of the function have challenged the glass maker for centuries. six-story building is open to the Allegheny River and the surrounding city environment, creating visibility in both directions. The architect’s2 intent is to create an ‘‘open, casual, serendipitous, impromptu interaction; Ã[email protected] ÃÃ spontaneous communication, ubiquitous access for all Retired fellow, PPG Industries Inc. r 2010 PPG Industries, Inc. to all at all times’’ environment for collaborative, non- Journal compilation r 2010 The American Ceramic Society and Wiley Periodicals, Inc. hierarchical teamwork in the building. www.ceramics.org/IJAGS Glass in Architecture 119

The PPG Place in Pittsburgh—another building tioning. In colder climates, solar heat gain can be bene- that bears Philip Johnson’s signature, represents a mon- ficial as it complements the heating system, but indoor umental building, where the ‘‘visual’’ effect is dominant. heat can be lost through the window by radiative, con- Here, neogothic architecture and modern materials— ductive, and convective modes of heat transfer. Absorp- float glass and structural aluminum—combine to present tion and reemission of the indoor heat by the glass and a constant visual message to the public, which unlike its conduction along the edges of the IGU or through Johnson’s glass house cannot be pierced through by the the sash and window frame are the major loss processes. stare of the observer.1 As a result, the annual space-conditioning energy con- In the case of the FallingWater house, the glass sumption of 2006 residential and commercial window should be minimally encumbering as it is only a barrier stock in the United States alone was estimated to be to the elements and otherwise unnecessary to the archi- about 2.24 1015 and 1.39 1015 BTU, respectively.3 tect’s purpose. At the same time, if designed in more Meeting the requirements to simultaneously save recent years, the need for eco-friendly and energy-effi- energy, provide a comfortable and productive environ- cient construction would have almost certainly been on ment, while encompassing the esthetic desires of the ar- the architect’s mind. The ALCOA building was de- chitect is challenging, and selecting the right glazing signed at a time when modern technology could meet system will ultimately depend on a balance of energy both design and energy requirements, although the use savings, environmental impact, and esthetics. In this of intrusive shades reminds us that the glass in that paper, the principles behind the performance of window building is still in need of perfection. The PPG Place’s glass and the critical parameters for evaluating and com- monumental purpose on the other hand, requires glass paring glazing systems will be highlighted. Then, the that appears as a shimmering wall that focuses the eye technologies and the assortment of high-performance on the building and not into it. Here, while energy (HP) glasses and coatings that are in use today to meet efficiency remains important, color and gloss are pre- both the visual and energy efficiency expected of value- dominant. We wonder if Johnson would have chosen a added glazing products will be reviewed. Finally, there window glass that also provided more daylighting, if will be a discussion of future possibilities and trends. that were available to him at the time of his work. Contemporary tools available to the manufacturer of architectural glass enable product design to meet Heat Flow and Glazing Performance Parameters many of today’s needs of art and function. New modi- fications to the glass furnace and process development The basic requirement for an energy-efficient glazing have enabled a wide range of glass colors and perfor- system is to control the flow of energy that enters a space. mance attributes. Developments in glass coater and The sources of energy flow through a glazing system are coating technologies have resulted in exciting capabili- solar energy, reradiated thermal energy by matter that ties in the optical design of new glass products. absorbs solar energy, and conductive and convective heat Advances in the construction of durable insulated glass transfer due to the indoor–outdoor temperature difference. windows have made energy-efficient fenestrations The solar-thermal radiant energy spectral distribution, affordable and commonplace. shown in Fig. 1, illustrates these two distinct regions. The solar is represented by the spectral irradiance at the The optical properties of glass, the construction of 2 earth’s surface, Et (BTU/h ft nm), ranging from about 280 the insulated glass unit (IGU), and the development of 4 materials used in the window and sash are important to 2150 nm ; the thermal is represented by a blackbody 1 1 parameters in defining the thermal performance of a emission spectrum at room temperature (72 F, 22 C), m m window. In spite of its visual and psychological indis- ranging from about 3 to 50 mwithpeakenergyat10 m. pensability, and its direct energy saving and human The total flow of energy through a glazing system is productivity benefit as a source of daylighting, the win- conveniently expressed in terms of the individual solar dow contributes significantly to the thermal manage- and thermal components given by ment of buildings. Heat can be gained by transmission Q ¼ Qth þ Qsol ð1Þ of solar energy through glass. In warmer climates and seasons, this will result in unwanted heat gain that in where Q is the total flow in BTU/h, and Qth and Qsol are affluent societies is normally countered by air condi- the thermal and solar components, respectively. The 120 International Journal of Applied Glass Science—Arbab and Finley Vol. 1, No. 1, 2010

SHGC and Visible Light to Solar Gain (LSG) Ratio

SHGC is a measure of the ability of glass to block or transmit solar heat both directly transmitted or absorbed and subsequently released inward. SHGC differs from direct solar transmittance, which does not include the contribution of the reemitted absorbed heat. The SHGC is related to the heat ﬂow by Equation 3 and is given by

SHGC ¼ Tsol þðN AabsÞð4Þ

where Tsol the direct solar transmittance, Aabs the solar Fig. 1. Solar-thermal radiant energy spectral distribution. The 2 absorptance, and N the inward fraction of reemitted solar is represented by the spectral irradiance, Et, in BTU/h ft nm absorbed radiation, are all dimensionless parameters. As (W/m2 nm) at the earth’s surface (airmass 1.5),4 ranging from 280 to 2150 nm; the thermal is represented by a blackbody emission the transmittance and absorptance are wavelength de- spectrum at room temperature (721F, 221C), ranging from about 3 pendent, the SHGC is evaluated by integrating over the to 50 mm with peak energy at 10 mm. Note the log scale of the entire wavelength range of the solar spectrum such that wavelength axis. R E ðlÞ½TsolðlÞþN ðlÞAabsðlÞdl t R ð5Þ Et ðlÞdl amount of heat loss or gain by the glazing system is As an example, the solar portion of SHGC for these dictated by conductive, convective, and radiative trans- glazings can be visualized graphically by summing the fer processes. The solar and thermal components con- product of the spectral irradiance curve (E ), referred to tain coefficients or ‘‘performance indices’’ that have t 5 in Fig. 1, and the ‘‘ideal’’ cold or warm climate trans- been established to characterize these heat transfer mittance curve at each wavelength over the solar wave- properties. The coefficient of thermal heat transfer, or length range. The ideal cold climate curve transmits the U-factor, is related to the heat flow by the expression maximum solar energy and rejects 100% of the thermal Qth ¼ U A ðto tiÞð2Þ infrared energy. The ideal warm region curve transmits in the visible and rejects 100% of the solar and thermal 2 1 where U denotes the U-factor in units of BTU/h ft F infrared energy. The ultimate transmittance will depend 2 1 2 2 (W/m C), A is the window area in ft (m ), and to and on the region, with lower visible transmittance in 1 1 ti are the outdoor and indoor temperatures in F(C), warmer climates. The preferred method of rejecting en- respectively. ergy is by reflection as this reduces the inward fraction The solar heat gain coefficient (SHGC), is related to reemitted absorbed radiation. The significance of this the heat flow through the expression distinction will become clear in the description of Qsol ¼ SHGC A Et ð3Þ coated glazings. Spectral selectivity is defined in terms of the LSG as where SHGC is a dimensionless quantity, Et is the in- 2 cident spectral irradiance expressed in BTU/h ft . Both LSG ¼ VLT=SHGC ð6Þ SHGC and U-factor are calculated for standardized ambient conditions (winter and summer) using Window where VLT is the visible light transmittance. This is 5.2 to determine the overall performance of various shown in Fig. 1 as the spectral region between 380 and glazing products.6 780 nm. Glass with an LSG of 1.25 or more is consid- To simplify the discussion, the solar and thermal ered to be spectrally selective.7 High LSG is important properties will be described separately, as indicated where maximum daylighting along with minimum solar by Equations 2 and 3. As the focus is on the glazing, heat load is desirable. Reduction of energy costs due to only the center-of-glass, that is, ignoring the effects of air-conditioning and lighting can be realized by an the edge, will be considered. appropriate selection of the glazing system. www.ceramics.org/IJAGS Glass in Architecture 121

U-Factor Fortunately, emittance is a surface effect that can be changed by creating a ‘‘new’’ surface by depositing a Compared with opaque construction materials used low emissivity coating on glass. The coating is normally in the building’s façade, glass alone does not provide ad- located on an inside surface of the IGU and is protected equate insulation. As a result, there are increased thermal from indoor and outdoor exposure. losses via convective, conductive, and radiative transfer. To minimize thermal losses due to convective and Glass—Processes and Products conductive heat transfer, an insulated glazing unit, or IGU, is constructed by arranging two, or sometimes The float process was first introduced about 50 three panels of glass with their plane surfaces parallel years ago and has since become the predominant and separated by gaps. Today’s energy-efficient win- 9 method of flat glass production. The advent of this dows use a separate IGU consisting of a spacer to main- process has resulted in commercial availability of high tain the gap, and an adhesive to attach the glass to the quality and affordable glass as a primary building spacer. To reduce convective thermal transfer across the material. Large area glass is now routinely produced IGU, a low conductance gas is used to fill the gap. Ar- with excellent thickness uniformity and optical quality gon, having a larger molecular size than both nitrogen as well as consistent strength and residual stress. In more and oxygen (air) is preferred; krypton, an even heavier recent years, emphasis on esthetics and energy conser- but more expensive gas, is used mostly in triple glazing vation have resulted in the development of glasses with systems for optimum performance in cold climates. selective spectral properties, that is, various colors and The U-factor is related to heat flow by Equation 2 as absorptive properties in the nonvisible portion of the discussed in the last section. For an IGU, the coefficients solar spectrum. Figure 2 displays typical examples of that determine the center-of-glass U-factor are given by many of the colors available to the architect. 1 The tinted glass color pallet has evolved over time as a U ¼ ð7Þ 1=ho þ 1=hi þ 1=ht result of both the architectural community’s preferences and the glass manufacturers’ ability to respond to them. where h and h are the outside and inside heat transfer o i An examination of Fig. 2 suggests a preference for green, coefficients, respectively, and ht is the heat transfer coefficient of the glazing unit. The values of ho and hi are calculated for standard winter and summer conditions.5 The coefficient for the IGU, ht, incorporates the conductive properties of the glass, along with the conductive and convective transfer in the gas space, and radiative transfer across the gas space. Conductance through the glass is proportional to its thermal conductivity and inversely proportional to its thickness; across the gap, heat transfer depends on the properties of the gas, that is, thermal conductance, density, viscosity, and the width of the gap. Radiative heat transfer depends on the emissivity of theglasssurfacesenclosingthespaceandthetemperatures of the surfaces, which emit or radiate thermal radiation (see Fig. 1—Blackbody spectrum). A good emitter, that is, high emissivity surface, is a good absorber (according to 8 Fig. 2. Typical examples of commercially available transmitted Kirchhoff’s law and, as a consequence, absorbs and Ã Ã glass colors. Color is designated in the a and b coordinates.30 reradiates thermal radiation resulting in indoor to out- Along the aÃ axis, moving to the left or the right of the origin door heat loss in cold weather. Conversely, a low emit- indicates increasing green or red hues, respectively. Similarly, along tance surface reflects, rather than emits thermal radiation, the bÃ axis, respective hues above or below the origin are thus preventing heat loss. A float glass surface with an increasingly yellow or blue. A third dimension of color, LÃ (not emissivity of 0.84 is an efficient thermal radiator, resulting shown here)—describing intensity—is necessary for the complete in significant energy loss through radiant thermal transfer. definition of color. 122 International Journal of Applied Glass Science—Arbab and Finley Vol. 1, No. 1, 2010 blue, and neutral (grouped in the shaded region) colors, in oxides of iron, the other colorants used in architectural an increasing order. With one exception (the so-called glasses absorb essentially only in the visible range of the bronze in the upper right quarter of this chart), hues that solar spectrum, which constitutes nearly half of terres- are predominantly purple, red, or yellow have not created trial solar energy.11 Ferrous oxide (FeO) is unique in its the popularity that would encourage their commercial strong, relatively broad absorption in the solar infra-red production. It is useful to note that in the absence of a range with a tail extending into the visible. At the same coating, the transmitted color of glass, which is shown in time, ferric oxide (Fe2O3) absorbs the blue and ultravi- Figs. 2 and 3, essentially defines the reflected appearance olet wavelengths. As transmitted color is defined by the of glass color. On the other hand, optically thin film unabsorbed wavelengths, these two oxides function as coatings on either surface of glass can impart distinct col- blue and yellow colorants, respectively. In glass, both ors to glass with complementary transmitted and reflected oxidation states of iron exist in relative concentrations effects. The distinction is important, most visibly from the defined by the concentration of total iron oxide in glass so-called curb-side view of the building (we will return to and the fraction of the total that is present as FeO (a this subject below). measure of glass redox). Thus, depending on the relative Bamford10 has given a comprehensive account of concentrations of these two species, and in the absence glass colorants. Oxides of iron, cobalt, chromium, man- of any other colorant, a range of glass colors from yel- ganese as well as selenium and some of the rare-earth lowish green to bluish green is possible (Fig. 3). metal oxides have strong absorption in the visible (380– It is important to note that, for a given total iron 780 nm) range of the spectrum and, therefore impart concentration, higher glass redox, or higher FeO con- color to glass. The concentration of these colorants also centration, results in a more infrared-absorbing glass determines the percent transmittance of visible light that reduces the direct solar energy transmittance (Tsol) through the final product, that is, in addition to pro- through the window.12 At the same time, Fig. 3 dem- ducing more saturated colors, adding more colorants onstrates that the preferential increase of the FeO con- reduces the total amount of light that enters the indoors centration drives the glass color toward blue, spaces by absorption, thus resulting in less direct solar independent of the total concentration of iron oxide. 13 (Tsol) heating and less natural daylighting. Except for This has been the basis of several HP blue that are also displayed in the lower left quadrant of Fig. 2. This is an important aspect of spectral selectivity; controlling the FeO concentration, particularly by increasing the redox ratio of glass, can result in products that admit more visible light and relatively less solar infrared heat into the building. In addition to the choice of color, a carefully chosen combination of total concentration of iron oxide, redox ratio, and other colorants can result in opti- mizing the product attributes including color, SHGC, and visible transmittance; when all the above attributes are significant, it may become necessary to prioritize their importance. For example, minimizing SHGC, while maintaining a high visible transmittance, limit the range of possible colors. There are several limitations to the design of spectrally selective compositions. From a manufacturing point of view, the increased concentration of ferrous oxide also Fig. 3. Calculated transmitted color of soda–lime–silicate (SLS) reduced heat transfer through the glass melt. In most float window glass as a function of the total concentration of total iron glass furnaces, heat is deliveredtothebatchandthemelt oxide (weight percent, as Fe2O3) and glass redox, calculated as the 14 ratio of the weight percents of FeO over total iron oxides. Color was by overhead combustion of natural gas or oil. Increasing calculated for an SLS composition free from other colorants. Each the FeO concentration reduces the radiative heat transfer curve corresponds to the indicated redox for a range of total iron coefficient of the melt; this necessitates adjustments to the oxide concentration of 0.01 (near the origin) to 1.6% by weight. furnace operating conditions. www.ceramics.org/IJAGS Glass in Architecture 123

In addition to the manufacturing issues, there are heat with LSG values of 1.4 or greater. These products other compositional constraints from the final product are produced by applying coatings either on the glass perspective. Several major limitations include: ribbon (on-line) by chemical vapor deposition (1) The addition of iron oxide usually requires the (CVD)12, or off-line by sputter deposition16 of spe- addition of other colorants to produce the desired color. cialized coatings. In this section, three types of coat- As increasing the total concentration of colorants rap- ings—reflective dielectrics, transparent conductive idly darkens the glass, reducing the SHGC by increasing oxides (TCO), and multilayer-metal dielectric stacks its FeO concentration becomes increasing less feasible. are combined with clear and tinted glasses to illustrate In particular, for very high VLT requirements (e.g., the the broad range of possibilities, as well as the limita- ALCOA Building or the FallingWater house), adding tions of today’s glazing systems. The benefits in energy large concentrations of FeO is not an option. savings, when considering the U-factor and SHGC of a (2) The mechanism for reducing the solar heat glazing system for a particular climate region, are also gain via the composition route is absorption. In the ab- illustrated. sence of strong convective cooling (e.g., cool and windy The first type includes highly reflective products, outdoor air), the glass temperature increases and the hot which use either single high refractive index17 or multi- glass reradiates the heat, about 50% of which enters the layer coatings on clear or tinted glasses. Absorbing metal indoor space. Reflecting solar energy does not pose a oxide coatings, in particular, the spinel coatings18,19 similar problem. have unique esthetic properties, while providing a de- (3) Insulation in cold winter weather requires the gree of solar control. The Co–Cr–Fe–O spinel was the reflection of the thermal heat (721F, 221C) back to- first architectural coated product,20 which still enjoys wards the room. Infrared reflectivity is a surface phe- commercial success today. Having visible transmittance nomenon that occurs due to free-electrons in metal-like of 34% and reflectance of 37% on 3-mm clear glass, and surfaces.15 Oxide glasses, independent of their color are an LSG of 0.65, this coating provides shading in the highly insulating materials. visible region of the spectrum. Spinel coatings are also as (4) High visible gloss (e.g., the PPG Place) cannot durable as the glass surface, allowing ease of handling by be attained via composition alone. As the bulk com- glass fabricators. position of glass constitutes soda lime silicate (SLS), the The second type consists of passive solar ‘‘Low-E’’ real index of refraction remains essentially unchanged at products (defined to have a SHGC of 40.4021), mainly about 1.5 at 550 nm, with the relatively small con- comprising TCOs, having high visible transmittance centration of colorants (e.g., o2% of the total compo- around 82% on 3-mm glass, and emissivity around sition by weight). 0.20. Fluorine-doped tin oxide (SnO2:F), in particular, To counter these limitations, the architectural glass is deposited by CVD over the glass surface as it moves 12,22 industry and some of its suppliers have developed along the float line in the glass-forming chamber. advanced large area glass coating technologies. Thin op- As the index of refraction of tin oxide is much higher tical films can tailor the spectral reflectance, absorbance, than the glass substrate, the visible reflectivity and color and transmittance of glass with significantly more of the final product require optical layers below it to degrees of freedom. The next sections will include meet the requirements of highly transparent window examples of these possibilities and provide an overview applications, for example residential housing. Although of two major large area coating technologies deployed the final coating is thin ( 300 nm), SnO2:F, like the by glass manufacturers today. spinel coatings are as durable as the glass surface. The final type encompasses a new generation of ‘‘multilayer,’’ spectrally selective coatings, with high vis- Coated Glazing Products ible light transmittance (VLT), a range of SHGC’s, and LSG values 41.40. These coatings first appeared in the To fill the need of art and function as outlined in early 1980s when the sputter deposition process16 was the introduction, the visible properties of glazing prod- introduced as a manufacturing process. This enabled ucts range from low transmitting, and possibly highly large volume commercial production of multilayer reflecting, with LSG values o1.0, to highly transmit- coatings with silver layers. The first of this generation ting in the visible, while blocking the solar and thermal consisted of a single silver layer surrounded by metal 124 International Journal of Applied Glass Science—Arbab and Finley Vol. 1, No. 1, 2010 oxide layers on a glass substrate (glass/metal oxide— silver—metal oxide).23,24 The high refractive index metal oxide layers antireflect the low index silver layer in the visible spectrum.25 The silver layer is highly reflective in the solar and thermal infrared and has low absorbance in the visible region of the spectrum. The individual layers measure only tens of nanometers in thickness. The market for this product is primarily for use as passive Low-E glass in IGUs, providing a low U-value and a high light transmittance. Continuing with the progression of adding silver layers, it was with the introduction of the double silver Fig. 4. Percent transmittance (%T) versus wavelength (nm) in layer coating (glass/metal oxide—silver—metal oxide— the visible and solar infrared region of the spectrum of the low silver—metal oxide), first commercialized in the auto- emissivity coatings F:SnO2, and single (1), double (2), and triple 26,27 (3) layer silver coatings, and highly reflective spinel coating. The motive industry in the late 1980s that solar control coatings are on 3-mm clear glass. Low-E glazing (defined to have a SHGC of o0.407,21) with enhanced spectral selectivity was realized. The additional silver layers increase both the solar infrared and vidual layer), they can be formulated to impart specific thermal reflectivity with a corresponding reduction in reflected color (and the complementary transmitted SHGC and emissivity (and consequently U-factor), re- color) to the glass substrate. In most residential and spectively. Visible transmittance remains high due to the many commercial building applications, the esthetic low absorption of the silver layer, leading to LSG values preference is for neutrality of color—that is, no color approaching 1.8 or greater. Today, the drive to reduce at all, which is also often consistent with a high amount energy consumption in buildings and meet environ- of natural daylighting. The silver layer coatings show a mental standards has increased demand for double silver progressively sharper cutoff in the solar infrared with layer-coated glass in solar Low-E glazing products. less relative attenuation in the visible as the number of A nearly optimized coating for spectrally selectivity, silver layers increases, thus approaching the behavior of that is, approaching the ideal curve for the warm climate the ‘‘ideal’’ filter for warm climate curve, as shown in region—when considering a combination of perfor- Fig. 1. The triple silver layer and F:SnO2 coatings ap- mance, neutrality, and cost effective processing—is the proximate the warm and cold climate curves, respec- triple silver layer coating (glass/metal oxide—silver— tively, while the single and double silver layer coatings metal oxide—silver—metal oxide—silver—metal oxide) fall between these limits. This assures a range of coatings with an LSG value approaching 2.4. The triple silver that can suit most performance requirements for a glaz- layer coating has been shown to reduce capital equip- ing system. The spinel coating shows the opposite trend, ment costs for HVAC systems and annual energy sav- with low transmittance in the visible and a degree of ings returns, and dramatically reduced the level of CO2 solar infrared rejection. This coating provides visible emissions associated with the heating and cooling of light shading and a metallic reflectance, which conveys a commercial buildings.28 ‘‘shimmering’’ appearance. This coating is used in com- The physical properties that account for the behav- bination with HP tinted glasses to provide additional ior of these three types of coatings are illustrated in attenuation in the solar infrared, while producing a Fig. 4 in terms of percent transmittance (%T) versus tinted gloss esthetic. wavelength (nm) in the visible and solar infrared region Although numerous coating and glass combina- of the spectrum. The spectral selectivity is clearly indi- tions meet a wide range of functional and esthetic con- cated for F:SnO2, and the single (1), double (2), and ditions in glazing systems, there are limits to their triple (3) silver layer coatings. These coatings are essen- properties. Figure 5 depicts a landscape of the physical tially spectral filters that reflect, rather than absorb the properties for glazing systems in terms of VLT and LSG solar infrared, while transmitting the visible. Therefore, as a function of SHGC.7,29 The ‘‘physical limit’’ curve within certain design and materials constraints (e.g., the marks the boundary outside of which no physical glaz- complex index of refraction and thickness of the indi- ing system can be realized, and can be understood in www.ceramics.org/IJAGS Glass in Architecture 125

as the wavelength approaches the UV (below 380 nm) at lower values of SHGC. As the bandwidth of the cold climate curve is decreased further, the color becomes greener. If instead, the bandwidth in the visible (380 nm–780 nm) is kept constant and the magnitude of the transmittance is decreased, the neutral limit curve is formed, as indicated by the neutral color limit line in Fig. 5. Points to the left of the line will have color; points to the right can be essentially neutral or have color, depending on the detailed shape of the transmittance or reflectance curve of the coating or Fig. 5. Optical properties landscape for double-glazed IGUs in glass. As discussed earlier, the shapes of these curves terms of VLT and LSG as a function of SHGC. The IGUs have are designed by the glass maker for a certain perfor- outer lights with 6 mm clear, and tinted and (high-performance [HP]) tinted glasses; and one-, two-, and three-layer silver and mance and esthetic. The data shown in Fig. 5 are for double-glazed IGUs F:SnO2 coatings on clear; and three-layer silver coating on tinted and (HP) tinted glasses. The coating, when present, is on the #2 with 6 mm clear and tinted glasses; one, two, and three- surface, the gap is 6 mm and the gas is 90% Ar–10% air. The layer silver and F:SnO2 coatings on clear; and three-layer inner light is 6 mm ultraclear. The ‘‘physical limit’’ curve marks the silver coating on tinted glasses. The coating, when present, boundary outside of which no physical glazing system can be is on the #2 surface, the gap is 6 mm and the gas is 90% realized.7 The portion of the curve in the solar infrared Ar 10% air. The inner light is 6 mm ultraclear. Of par- (780–2150 nm) is formed by reducing the bandwidth of the cold ticular note are the relatively high LSG values of the silver climate curve (see Fig. 1), while keeping the transmittance constant. multilayer coatings, increasing as a function of the num- The portion of the curve in the visible is optimized for LSG ber of silver layers. Considering Equation 6, with SHGC (maximum photopic response wavelength)30 by further decreasing the bandwidth of the curve in the visible centered around 550 nm, varying over a wide range (0.56–0.28), while VLT varies while keeping the magnitude of the visible transmittance constant over a narrow range (78–64%), LSG ranges from about (see Finley and Heithoff29). Ultraclear glass (6 mm) is used in a 1.4 to 2.3. The figure clearly indicates that the highest double-glazed IGU configuration to model the ‘‘Physical Limit’’ LSG and lowest SHGC are achieved by the triple silver curve. The neutral limit line defines the boundary for neutral color. layer glazing. These coatings are particularly useful for The region to the left of the line has color; the region to the right can conditions where solar heat load reduction and daylight- be neutral or have color, depending on the shape of the ing are important. transmittance or reflectance curve of the coating or glass. The line is Several noteworthy aspects of architectural glass formed by reducing the magnitude of the visible transmittance shown in Fig. 2 are indicated in this figure. Data points (380–780 nm), while keeping the bandwidth in the visible with SHGC40.35 correspond to uncoated products. In wavelength constant. The LSG lines are the slope of the VLT versus SHGC. Glazing with LSV values of 1.25 or greater are spectrally spite of the scatter in data, we can point to two groups of selective.7 compositions, as suggested by the arbitrary trapezoids drawn about each group. While similar values of SHGC can be identified in the two groupings, there is a distinct terms of the ideal curves in Fig. 1. By narrowing the difference in ranges of VLT between them. For the group bandwidth of the cold climate curve, starting at the long on the lower right of the chart, this ratio is about one or wavelength limit of the solar infrared (2150 nm) and less, while for the higher FeO containing group on the moving towards the visible (780 nm), while keeping the upper right, the ratio is about 1.4, or higher, indicating a magnitude of the transmittance constant, there is a cor- selectively higher relative transmission of the visible light. responding decrease in the SHGC, with VLT remaining Admitting more visible light—at a given color—allows constant. This is indicated by the flat line section of the more daylighting while there is less need for air-condi- curve. Then, by further narrowing the cold climate tioning compared with the higher SHGC glasses of similar curve in the visible spectrum around the center point VLT and color. The performance data for the two groups wavelength at 550 nm to optimize LSG (maximum of product at the lower left of the chart show the reduced photopic response wavelength),30 VLT decreases along SHGC of their uncoated glass counterparts as a highly with SHGC, as indicated by the steep falloff of the curve heat reflecting coating is applied to one surface of the 126 International Journal of Applied Glass Science—Arbab and Finley Vol. 1, No. 1, 2010 substrate. While the coating strongly reduces the SHGC, there is less impact on the VLT through the glass pane due to the design of this coating. Also, the tightening of the range of the performance of coated substrates relative to that of their uncoated counterparts, demonstrates the dominating effect of the high performance coating used for this graph. Where additional shading and flexibility in esthetic are desirable features, combinations of tinted glasses with the reflective and silver layer coatings offer a wider variety of choices and additional benefit in solar heat load reduction. The neutral color limit shows the esthetic boundary related to glazing system color. Also noted is the gap be- Fig. 6. Center-of-glass U-factor and SHGC for passive21 and tween the physical limit and what is currently practiced. solar control7,21 Low-E glazing in double and triple glazed IGUs The physical limit is based on the cold climate curve, using 6 mm clear and high-performance tints. The coating is on the shown in Fig. 1, which assumes no absorption and re- #2 surface in the double glazed IGU, and the #2 and #5 surfaces in flectance of solar energy outside of the spectral bandwidth. the triple glazed IGU (note: outside surface is the #1 surface). Each Without using costly methods to fabricate multilayer point on the curve represents either a coated or uncoated substrate 1 m bandwidth filters,31 which better approximate a square designated as follows: uncoated ( ), F:SnO2 (), single ( 1Ag (A), (B)), double (~ 2 Ag), triple ( & 3 Ag) silver layer coatings. bandpass filter, approaching the characteristics of the ideal Each curve represents an IGU configuration designated as Double is beyond the current cost-effective processing methods 16 Glaze Clear Glass and Triple Glaze Clear Glass in the figure. The described earlier. Silver,havingmaterialproperties double glaze IGU has 90% argon-air fill with a 13-mm gap. The including the lowest absorptance and highest infrared triple glazed IGUs—with either 95% krypton-air fill and 6 mm reflectance, along with a low index of refraction, outper- gaps, or 90% argon-air fill and 13-mm gaps—are equivalent in forms any single thin film material in a multilayer coating performance and shown as one curve. Energy Star regulations for stack. The deposition process using these materials residential window and door32 for climate zones in the North, (including low absorbing high index metal oxide layers) North/Central, South/Central, and South climate zones of the United are indicated by the four boxes. Data are calculated based allows large area, high volume manufacturing of this type 6 of coating. As a result, silver-based coatings define the on NFRC methodology, using LBL’s Window 5.2 software. Winter nighttime U-factor is given expressed in BTU/h ft2 1F. current state-of-the-art technology.

Product Applications—IGU Configurations for Fig. 5) are shown in two groupings indicated in this Climate Zones figure by the ellipses. The curves represent different IGU configurations, and each data point on the curve As shown by Equations 2 and 3, the U-factor and represents an IGU glazed with either coated (# 2 surface SHGC fully describe the total energy flow across the IG for double glazed; # 2 and #5 surface for triple glazed) or unit. By adjusting these performance indices, glazing uncoated glass. The curve labeled ‘‘Double Glazed IG systems can be tuned using different combinations of Unit—clear glass’’ shows a significant decrease in both coatings and glass to suit conditions in different climate U-factor and SHGC from the uncoated to the F:SnO2 zones. This is illustrated in Fig. 6, where the center- and single silver layer (1 Ag) configurations. Of note are of-glass U-factor and SHGC are plotted for passive21 the data points labeled ‘‘1 Ag Layer (B),’’ which has and solar control7,21 Low-E glazing systems using clear higher SHGC relatively to ‘‘1 Ag Layer (A),’’ while and high performance tints. Included on the chart are maintaining low U-factor. This coating is specifically Energy Star requirements for residential window and designed to optimize the solar heat gain, while preserv- door32 for climate zones in the United States. The data ing thermal insulation for northern climate regions. In for the Low-E glazing on clear glass fall along two curves general, these IGUs have SHGC40.40 and are classified as indicated in the figure, and the data for coated and as passive Low-E, which are preferred in the northern cli- uncoated HP tints described in the previous section (see mate zones. Double (2 Ag) and triple (3 Ag) coatings have www.ceramics.org/IJAGS Glass in Architecture 127

12 slightly lower U-factors with SHGCo0.40 and fall into anion, O2 radicals (Arbab et al. and cited work the solar Low-E category. These configurations are pre- therein) that in turn result in the catalyzed oxidation ferred in the north/central, south/central, and southern and decomposition of organic species adsorbed on the climate zones, respectively. Additional silver layers have a surface of glass coated with anatase. The removal of greater effect on reducing SHGC than U-factor, account- hydrophobic organic species from the surface and the ing for the flattening out of the curve at higher SHGC. inherent photo-induced hydrophilicity of anatase films The underlying cause of this behavior is the dramatic de- result in the sheeting of rainwater on the surface of win- crease in solar infrared transmittance (Fig. 4) with a cor- dow glass. This, in turn, helps remove the particles of dust responding increase in reflectance, as opposed to the and other loosely adhering inorganic dirt from the surface fractional increase in thermal infrared reflectance, which thus rendering the glass cleaner for long periods of time. is already 490% for two and three silver layer coatings. Increasing the photoactivity of the coating enhances the The curve labeled ‘‘Triple Glazed IG Unit—clear glass’’ self-cleaning activity, and has been demonstrated. shows another significant decrease in U-factor with changes in SHGC primarily due to increases in absorption from the additional coating and glass. The IGUs glazed with a high-performance tint (6 mm) are within Dynamic Windows specifications for all climate zones with a three-layer silver coating. The SHGC is lower than three-layer silver on A dynamic window can respond to the changing clear glass with a similar U-factor. ambient conditions to adjust the amount of light and total These examples illustrate an inherent capability solar energy that enters the living space. In spite of their of both multilayer coatings and the manufacturing pro- potential as a critical element in zero-energy3 buildings, cess discussed earlier to design and produce glazing dynamic windows, and most notably various electrochro- systems to meet performance criteria. They also dem- mic (EC) technologies, it has not yet achieved broad onstrate how far the state of technology in glass coatings commercial success. The current state of commercialized has advanced the product performance for IGUs, but systems, do not meet all three market requirements of they also show the limitations that remain in realizing cost, functionality, and esthetic appeal. The cost of the further improvements in thermal insulation (U-factor) dynamic window to the builder or owner remains very and solar control (SHGC). Technologies, including high, ranging from a factor of five to ten times over vacuum glazing where the gap is evacuated, and dy- existing glazing systems primarily due to complex, low namic glazing (discussed below) are some of the answers volume manufacturing combined with the additional cost that pose future challenges. of system installation of power and control hardware. The most successful large area EC system commercialized for windows in buildings include inorganic coloring elec- Self-Cleaning Glass trodes, the most common being thin-film tungsten triox- ide (WO3).Theestheticsareappealing,butlimited This paper has focused on esthetics and energy effi- usually to a deep blue transmitted color in the darkened ciency of glass, that is, the two key attributes of archi- state. For low transmittance, low shading conditions this tectural glass that continue to drive the development of system has an exceptional performance and has applica- new technology. tions where it is desirable to reduce direct sunlight during In recent years, based on new insight into the prop- the day, such as skylights. Functionality is good but re- erties of photoactive materials, particularly anatase, a crys- stricted, as both visible and solar infrared regions of the talline phase of TiO2, several self-cleaning glass products spectrum change in transmittance simultaneously, and have been introduced to the market. The valence-con- high LSG cannot be realized with this system alone.33 duction band gap energy of anatase (3.2 eV) corresponds Figure 7 shows the transmittance as a function of to the photon energies within the UV range of solar wavelength in the clear and darkened state for an EC 19 spectrum (387 nm, or shorter wavelengths). In the pres- device using thin-film WO3 as a coloring electrode. ence of moist air, the photoelectric formation of electron- The chart illustrates the simultaneous change in trans- hole pairs near the surface of TiO2, results in the forma- mittance in the visible and solar infrared portions of the tion of highly reactive hydroxyl, OH, and superoxide spectrum from the lightened to darkened state. 128 International Journal of Applied Glass Science—Arbab and Finley Vol. 1, No. 1, 2010

the manufacturing method of choice for architectural glass. The past 20 years have witnessed significant advances in three areas of: heat-absorbing glass manufacturing, low-emissivity CVD coatings, and multilayer physical vapor deposited low-emissivity solar heat rejection glass coatings. These products have in turn allowed the designation of different climate zones relative to the thermal performance of window glass, resulting in more energy-efficient building facades. When current trends in the development of dynamic window and building integrated sources of renewable energy technologies translate into new cost-effective products, the concept Fig. 7. Transmittance as a function of wavelength in the clear and of zero-energy buildings will become possible; until darkened state for an electrochromic device using thin-film WO3 as then, a significant investment in research and develop- the coloring electrode.19 The chart illustrates the simultaneous change ment of these products remains necessary. in transmittance in the visible and solar infrared portions of the spectrum from the lightened to darkened state. Acknowledgements Some EC systems have been successfully commercialized for nonbuilding window applications, such as One of us (M. A.) would like to thank K. Arbab for dye-based systems for dimmable aircraft cabin windows bringing Kipnis1 to his attention. and automotive mirrors. This system is simple in construction, requiring only an EC dye between transparent conductive electrodes, but technical hurdles remain for References large-scale applications. Organic EC’s, which claim to 1. D. Whitney, and J. Kipnis eds. Philip Johnson, The Glass House. Pantheon have the potential to become the lowest priced system Books, New York, 1993, xi–xxxiii. with low cost materials and processing, have been 2. Rusli Associates. Available at http://www.rusli.com/alcoa.html (accessed Jan- uary 14, 2010). explored in recent years. However, as with all organic 3. D. Arasteh, S. Selkowitz, J. Apte, and M. LaFrance, ‘‘Zero Energy Windows,’’ systems, there are issues not only with stability upon ex- ACEEE Summer Study on Energy Efficiency in Buildings. ACEEE, Pacific Grove, CA, 2006. posure to sunlight, but with cycling between light and 4. ASTM G173-03e1. Standard Tables for Reference Solar Spectral Irradiances: 33 dark states during exposure at elevated temperature. Direct Normal and Hemispherical on 371 Tilted Surace. ASTM International, New materials and technologies, combining EC and pho- West Conshohocken, PA, 1999, doi: 10.1520/G0173-03E01. 5. ASHRAE Handbook (I-P Edition). Fundamentals. American Society of Heat- tovoltaic functions to create windows that supply their ing, Refrigerating and Air-Conditioning Inc., Atlanta, 2009. own power to control transmittance, are now being ex- 6. Energy and Environmental Division, Lawrence Berkeley National Labora- 34 tory. Energy and Environmental Division, Lawrence Berkeley National Labo- plored in R&D labs, but they are at the concept stage. ratory. Window 5.2. Energy and Environmental Division, Lawrence Berkeley Finally, liquid crystal-based or suspended particle National Laboratory, Berkley, CA, 2003. 7. Department of Energy (DOE). Spectrally Selectively Glazings, DOE/ device (SPD) technologies, which effectively scatter vis- EE-0173, 1998. Available at http://www1.eere.energy.gov/femp/pdfs/fta_ ible light in the off-state and are clear in the energized glazings.pdf (accessed January 14, 2010). 8. R. Siegel and J. R. Howell, Thermal Radiation Heat Transfer. Hemisphere state act as a visible light shade and opacity for privacy. Publishing Corporation, Washington, DC, 1992. SPD has been on the market for several decades, but has 9. F. V. Tooley, ed. The Handbook of Glass Manufacture, Vol. II. Ashlee Pub- only seen limited commercialization. lishing Co. Inc., New York, 1984. 10. C. R. Bamford, Colour Generation and Control in Glass. Elsevier Publishing Co., New York, 1977. 11. American Society of Testing Materials ASTM/E 891 - 87, reapproved. Amer- ican Society of Testing Materials, Philadelphia, PA, 1992. Conclusion 12. M. Arbab, L. J. Shelestak, and C. S. Harris, ‘‘Value-Added Flat-Glass Prod- ucts for the Building, Transportation Markets, Part 2,’’ Am. Ceram. Soc. Bull., 84, 34–37 (2005). Although the range of choice is limited by glass 13. L. J. Shelestak and G. A. Pecoraro, ‘‘Transparent Infrared Absorbing Glass chemistry and the physics of thin films, different com- and Method of Making,’’ 4,792,536 US, December 20, 1988. 14. M. Arbab, L. J. Shelestak, and C. S. Harris, ‘‘Value-Added Flat-Glass Prod- binations of various glass tints and coatings offer a wide- ucts for the Building, Transportation Markets, Part I,’’ Am. Ceram. Soc. Bull., range of architectural possibilities. The float process is 84, 30–35 (2005). www.ceramics.org/IJAGS Glass in Architecture 129

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