SSuummmmaarryyThere are few decisions that designers make that have moreimpact on the appearance and utility of a building than the glaz-ing selection. Glazing affects the look of a building from theinside as well as the outside, and it affects the comfort and pro-ductivity of the people who occupy the building. Furthermore,it has a large influence on the size and form of other buildingsystems, including lighting and HVAC. In recent years, the palette of glazing options available todesigners has expanded greatly. Designers now have muchmore control over how much light glazing lets into a buildingand how much heat, cold, and noise it blocks out. They canalter the characteristics of glazing performance by varying tints,coatings, films, number of panes, and other glazing features. Awhole-building lifecycle analysis using energy performance sim-ulation software can help lead designers to the best solutionamong the many choicesdesign briefenergydesignresourcesWith so many factors influencingglazing selections, whole-buildinglifecycle analysis is the best toolfor determining the most cost-effective solution.contentsIntroduction 2Why Clear Glass Is Vanishing 3Glazing Options 6Codes and Standards 11Which Glazing Option Is Best? 13Glazing Systems In the Future 15For More Information 17glazingpage 2 glazingIInnttrroodduuccttiioonnFew building systems affect the overall value of a building asmuch as glazing does. The building attributes that glazing canaffect include:■ Exterior aesthetics,■ Indoor views,■ Occupant visual comfort,■ Occupant thermal comfort,■ Annual energy costs, and■ The size and form of HVAC systems.In short, good glazing selections can make buildings moreattractive, more comfortable, more productive, and less expen-sive to own and operate. Over the course of the last fewdecades glazing technology has improved significantly, greatlyexpanding designers’ options to offer more value to their clients.For example, glazing is now better able to:■ Let in visible portions of the sun’s energy while reflecting thenonvisible energy that can add unwanted heat to a building.■ Block out unwanted elements of the outdoor environment,including heat, cold, noise, and glare.Yet, these new technologies seem underutilized in the market-place. Why? For one thing, selecting from the options now avail-able is complicated. The best glazing selection for any givenapplication often depends on local climate, orientation, shading,and interior space usage. Secondly, these advanced glazingtechnologies cost more at first than the old standbys. To have acomplete picture of their true lifetime cost, one must account forhow they reduce both annual energy costs as well as the firstcosts of HVAC systems.Good glazing selections can make build-ings more attractive, more comfortable,more productive, and less expensive toown and operate.page 3glazingFor example, when Apple Computers was designing a newresearch and development campus in Cupertino, California, theglazing was given a great deal of scrutiny (Figure 1). Accordingto Ted Davalos, the project manager, “No one wanted reflectiveglass for aesthetic reasons; we just didn’t like the mirror effect.But, of course, we needed to meet the state’s energy efficiencyrequirements.”In the end, Apple selected Libbey Owens Ford’s EverGreen glaz-ing, which combines a light green tint with the capability toblock out the sun’s heat. The relatively high visible transmit-tance of the glazing affords the researchers in the building goodvisibility while providing plenty of natural light. “The tint colorpleased everyone.” said Davalos. “The first costs were a bit high-er, but we saved enough in projected lifetime energy costs toraise the R-value of the roof from R-18 to over R-30.” WWhhyy CClleeaarr GGllaassss IIss VVaanniisshhiinnggFor many decades, single-pane, clear float glass dominated thecommercial buildings market. Although clear glass still has manyuses, it is being supplanted by other glazing technologies. Clearglass provides excellent views, but it also allows a substantialportion of radiation from the solar spectrum to pass through(Figure 2, next page). Although it is often desirable to get visi-ble light into a commercial building, during the air-conditioningseason nonvisible light adds unwanted heat that must be ulti-mately removed by the HVAC system at considerable cost. For designers to effectively specify glazing, they need to have agood understanding of not only how a given glazing systemtransmits visible light, but also what portion of the sun’s totalspectrum provides heat to the interior (Figure 3, next page).These characteristics of glazing systems are typically expressedusing the following technical terms:■ Visual transmittance (Tv) is a measure of the proportion ofvisible light that passes through a glazing system. GlazingSource: Pilkington LOFApple R&D Campus inCupertino, CaliforniaFigure 1:Careful consideration of systeminteractions during the design processresulted in this attractive building thatcombines natural illumination and goodoverall energy performance.page 4 glazingSource: Kreith and KreiderThe solar spectrumFigure 2:The human eye can see light between 380 and 720 nanometers in wavelength,less than half of the solar energy that strikes the earth. About 3 percent of thespectrum is in the ultraviolet region; the remaining nonvisible portion is in thenear-infrared region.UV3%Visible(45%)Near-infrared(52%)Relative intensityWavelength (nm)Spectral distribution of solar radiationEye sensitivity curve300 500 700 900 1,100 1,300 1,500 1,700 1,900SunIR transmittedVisible transmittedUV transmittedIRVisibleUVAbsorbed energyGlass paneHeat and light reflectance and transmissionFigure 3:When the sun’s rays strike a pane of glass, a portion is reflected, a portion isabsorbed, and a portion is transmitted.page 5glazingsystems with high values of Tv (0.7 to 0.9) provide lots ofnatural light and good vision, but they can also be a sourceof unwanted glare if not properly controlled. Systems withlower values of Tv (less than 0.4) can be visually distortingand quite gloomy on cloudy days. ■ Solar heat gain coefficient (SHGC) and shading coefficient(SC) are both measures of a glazing system’s net solar gain.SHGC, which is the more modern index, is the sum of thesolar radiation transmitted through the glazing and the por-tion of absorbed energy that ends up supplying heat inside.Glazing