Arbab and Finley Glass in Architecture IJAGS 1 118 2010
Arbab and Finley Glass in Architecture IJAGS 1 118 2010
Arbab and Finley Glass in Architecture IJAGS 1 118 2010
DOI:10.1111/j.2041-1294.2010.00004.x
Glass in Architecture
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.
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 optical properties of glass, the construction of The solar is represented by the spectral irradiance at the
the insulated glass unit (IGU), and the development of earth’s surface, Et (BTU/h ft2 nm), ranging from about 280
materials used in the window and sash are important to 2150 nm4; the thermal is represented by a blackbody
parameters in defining the thermal performance of a emission spectrum at room temperature (721F, 221C),
window. In spite of its visual and psychological indis- ranging from about 3 to 50 mm with peak energy at 10 mm.
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
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.
(Tsol) heating and less natural daylighting. Except for This has been the basis of several HP blue13 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 ox-
ide, 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 spec-
trally 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
oxide (weight percent, as Fe2O3) and glass redox, calculated as the glass furnaces, heat is delivered to the batch and the melt
ratio of the weight percents of FeO over total iron oxides. Color was by overhead combustion of natural gas or oil.14 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
industry and some of its suppliers have developed along the float line in the glass-forming chamber.12,22
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
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
slightly lower U-factors with SHGCo0.40 and fall into anion, O 2 radicals (Arbab et al.12 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 absorp-
tion 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). The esthetics are appealing, but limited
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
spectrum (387 nm, or shorter wavelengths). In the pres- device using thin-film WO3 as a coloring electrode.19
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.
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