Glass in Architecture ÃÃ Mehran Arbab* and James J
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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 fac¸ade 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 Â 1015 and 1.39 Â 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 Â 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 Â A ðto À tiÞð2Þ infrared energy.