2002-03 - high performance

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Glass Magazine

® •

March 2002 1

ver the past 50 years, there has been a constantevolution of glass products and technology. Wehave seen the development of tinted glass, insulat-ing glass, heat-treated and safety glass, reflectiveand low-emissivity coated glasses, and spectrally-selective glazings.

This wide array of products has provided de-sign professionals with countless aesthetic possi-bilities and significant opportunities to achieveincreased energy efficiency and occupant comfort.

This trend is not unique to North America. Inother parts of the world, glass components are uti-

lized, and often showcased, as part of the buildingenvelope. European glass and window systems fre-quently involve double-wall glazing. Operatingwindows, which allow natural ventilation to re-duce or eliminate mechanical ventilation, are

commonly used. The German energy code evenlimits the area in the core of a building to ensurethat offices have access to natural light comingthrough glazing systems.

Regardless of the specific climatic condition,today’s glazing products afford architects and engi-neers more design freedom and greater opportu-nity to achieve reduced building costs and ongoingoperating savings through energy conservation.

High Performance ProductsGlass products are a complex part of a building’s

envelope and play a key role in the energy effi-ciency of the building. While current glazing sys-tems cannot achieve the same insulating perfor-mance or direct solar heat gain reduction asmaterials like masonry or metal panels, neither can

these opaque components allow thefree use of natural daylighting likeglass products. Modern design,therefore, is premised on a recogni-tion of the trade-offs among alterna-tives and the integration of these toachieve desired aesthetics, and, moreimportantly, optimum first costs

and long-term energy conservation.A high performance glass prod-

uct has three principal performancecriteria: insulating performance orU-factor, solar control or solar heat

Design opportunities with

glass productshigh performance

There’s more freedom and opportunity to achieve reduced buildingcosts and ongoing operating savings through energy conservation

By Susan Reil ly

o

High performance glass products allow

structures to include large expanses of glass

so that they can take advantage of daylight-

ing opportunities while keeping energy

costs at a minimum.



2Glass Magazine® • March 2002

gain coefficient (SHGC), and visiblelight transmittance (Tv). The key is toassess the interaction of all three and theirtotal impact on a building’s energy con-sumption and to select products thatmeet the most efficient performancespecification. Table 1 lists several glassproducts currently available from differ-

ent manufacturers and illustrates therange of performance achievable. It mustbe emphasized that this is a small sam-pling of the hundreds of products avail-able today.

Glazing functions as an enclosure,and has a key role in the heat gain/lossbalance of a building. Higher perfor-mance products are better insulators thantheir predecessors and typically achieve aU-factor of 0.35 Btu/hr-ft2-F or less. Incold climates, where wintertime spaceheating is significant and reduction of

heat loss is important, a lower U-factorresults in less heat loss, and improvesinterior comfort. In hot climates, the U-factor is less important; solar heat gaincontrol and reduced air conditioning arekey elements of design, making a lowersolar heat gain coefficient the mostimportant criteria.

In office buildings, artificial lightingaccounts for 40 to 50 percent of theenergy used, and a good daylightingdesign can minimize lighting energy use.

Because of the potential benefits of day-lighting, high performance glazing isoften defined as having a visible lighttransmittance greater than the solar heatgain coefficient. However, high perfor-mance glazing is not limited to this defin-ition. In buildings without actively con-trolled daylighting, managing solar loads

and heat loss may be the most importantselection criteria. Another important con-sideration is minimizing glare, which mayrequire a low visible light transmittance.

As design professionals establish spe-cific glass product performance criteriafor a project, typical considerationsinclude aesthetics, relative first costs,impact on HVAC, and interior lighting.As a base line, they also must considerapplicable local building codes. In thefew areas without codes, it is common torefer to ASHRAE/IES Energy Code for

Commercial and High-Rise ResidentialBuildings (ASHRAE/IES 90.1). It is themost widely used standard and containsprescriptive requirements for glazing U-factors and solar heat gain coefficients forlocations throughout North America.

Energy Use and the Building CodesGlazing systems are specifically addressedin the energy codes because of the signifi-cant impact they can have on energy use.The most widely recognized commercial

building code for energy efficiency isASHRAE/IES 90.1. This standard wasdeveloped by the American Society of Heating, Refrigeration and Air Condi-tioning Engineers (ASHRAE) and theIlluminating Engineering Society of North America (IES). Currently, there aretwo versions of the ASHRAE/IES 90.1

standard in use: the 1989 version and the1999 version. As of June 2001, 26 stateshad adopted ASHRAE/IES 90.1-1989and six states had adopted the most recentversion of the standard, ASHRAE/IES90.1-1999, or its equivalent. Many of thestates that adopted ASHRAE/IES 90.1-1989 automatically did so through adop-tion of the 2000 International EnergyConservation Code (IECC).

Th e glazing system performancerequirements in ASHRAE 90.1 are basedon a detailed cost analysis. The cost analy-

sis uses actual products and real costs toestablish the cost effectiveness of glazingoptions in different climates. The analysisshows that high performance products arecost effective in almost all climates.

California updated Title 24, its energycode, in 2001, and it is very similar toASHRAE/IES 90.1-1999. In comparingthe current Title 24 with the old version,the new SHGC requirements are lowerthan the old. The lower SHGC require-ments are the result of applying the same

Table 1

Sampling of High Performance Insulating Glass Units

Product* Manufacturer Solar Heat Gain Visible Light U-FactorCoefficient Transmittance Btu/hr-ft2-F

Sun-Guard LE-40 Guardian 0.31 40% 0.33on Clear

NP-61 Guardian 0.29 51% 0.31on Green

EverGreen Pilkington 0.39 59% 0.49

EverGreen/Energy Pilkington 0.34 55% 0.33Advantage Low-E

Solarban 60 on PPG 0.37 69% 0.29Clear

Solarban 80 on PPG 0.23 47% 0.29Clear

* All of these are 1-inch insulating glass units comprised of 6 mm glass and a 1/2- inch air gap between the glass.

Center of glass values given for solar heat gain coefficient, visible light transmittance, and U-factor.



Glass Magazine® • March 2002 3

High performance glass products have allowed designers to expand glass in innovative applications for

sports facilities, such as the glass curtainwall on Heinz Field in Pittsburgh, Pa.

cost effectiveness analysis that ASHRAEused in developing the latest version of ASHRAE/IES 90.1.

An im portant prerequisi te of ASHRAE 90.1-1999, IECC 2000, andCalifornia Title 24 is that glazing musthave Nation al Fenestration Ratin gCouncil (NFRC) ratings for U-factorand SHGC. N FRC, a nonprofit pub-lic/private organization created by thewindow, door, and skylight industry, hasrecently adopted a site-built procedure

for rating commercial windows. Theseratings verify that the fenestration prod-ucts installed in a building meet perfor-mance specifications.

Designing with the Energy CodesAs mentioned earlier, the energy codescan help define the most appropriateglazing for a project. BecauseASHRAE/IES 90.1 is based on an exten-sive cost-effectiveness analysis, the pre-scriptive requirements in the code, espe-cially the 1999 version, provide a solid

starting point. In addition to the pre-scriptive building envelope option, thecode includes two other compliancemethods, which are particularly instruc-tive when using an integrated designapproach.

The other compliance methods inASHRAE/IES 90.1 are the buildingenvelope trade-off option and the energycost budget method.

Basically, the energy code sets anenergy budget for a building. The pre-

scriptive method gives you one way tomeet the budget without any flexibility.The building envelope trade-off optionand energy cost budget method allow formany different designs to meet the energybudget.

The trade-off option allows trade-offsbetween envelope components, such asglazing systems and wall insulation. Theenergy cost budget method allows trade-offs between all the building componentsand systems. For example, glazing with a

lower SHGC than the prescriptive re-quirement can offset a less efficient light-ing design. Any building with more than

50 percent glazing area must use theenergy cost budget method to demon-strate compliance.

Two software tools have been devel-oped to help show compliance withASHRAE/IES 90.1 using the trade-off option, and could easily be applied dur-ing the design process to identify the best

glazing system for a project. They areENVSTD , which was developedthrough ASHRAE, and COMcheck-EZ,which was developed by the Departmentof Energy. Both programs can be down-loaded free from the Web. For COM-check-EZ, go to www. eren.doe.gov/ buildings/codes_standards/buildings/commercial_codes_ products.html. ForENVSTD 4, go to www.ashrae.org. (Goto ASHRAE Site Map, PublicationsUpdate.)

COMcheck-EZ is more flexible than

ENVSTD. It has a simple, two-screendisplay to input the envelope and deter-mine compliance for the envelope. Thereare just four steps to using COMcheck-EZ to show envelope compliance (Fig. 1).

1. Click on the Project tab and enterthe building location and type of build-ing.

2. Click on the Envelope tab andbegin to enter the envelope componentsby selecting the component from the listrunning across the screen.

• For walls, enter the gross area, which

includes opaque wall area and windowarea.

• For windows, enter the U-factor,

Fig. 1



solar heat gain coefficient (SHGC), andprojection factor (PF). The projectionfactor characterizes external shading,such as overhangs; use Help for a descrip-tion on how to calculate it.

• Daylighting also is handled by speci-fying visible light transmittance and thedaylighting control factors (DLCF). To

engage this feature, the user must selectDaylight Control Factor under Optionsand specify the orientation of each wall.Otherwise, the wall and window orienta-tion does not need to be specified.

• Hit Enter after the last entry, andreview whether or not the envelope com-plies. A positive percentage means theenvelope complies.

3. Save and print the report.Now, let’s take a look at how COM-

check-EZ could be used in the designprocess. Consider a 10-story, 100,000-

square-foot office building with 12-footfloor-to-floor heights located in Atlanta,Ga. (IECC climate zone 7A). Each floorhas an 80 by 125-foot floor plate with thelong axis oriented east-west, i.e. facingsouth, and there is 30 percent windowarea on each elevation. In Atlanta, theIECC 2000 prescriptive tables (Chapter8) require that the windows have a U-fac-tor of 0.70 and an SHGC of 0.40. Afterinputting the wall, roof, foundation, andwindow prescriptive requirements,COMcheck-EZ reports that the design is

14 percent better than the minimum coderequirements. Increasing the window areato 40 percent shows the design is still 2percent better than the IECC 2000 pre-scriptive requirements.

Windows with metal frames and insu-lating glazing (two lites with a 1/2-inchair space) meet the 0.70 U-factor, but thesame windows with clear, uncoated glasswill not achieve the 0.40 SHGC. For themost part, glazing systems that incorpo-rate tinted glass, low-emittance (low-E)coatings, reflective coatings, or spectrally-

selective tints will meet the 0.40 SHGCrequirement.

Consider a high performance windowwith a metal frame and low-E glazingwith a U-factor of 0.60 Btu/hr-ft2-F andan SHGC of 0.30 or less. The lower win-dow U-factor improves the energy effi-ciency of the building by 1 percent overthe example above, while the lower win-dow SHGC increases the energy effi-ciency by another 8 percent. Daylightingcontrols with the high performance glaz-

ing increase the efficiency by an addi-tional 5 percent. In fact, this buildingcould have 50 percent window area withthe high performance windows and stilloutperform the code by 5 percent.

As the glazing system area increases,the SHGC code requirement stays thesame or decreases. In Atlanta, a buildingwith no more than 25 percent windowarea needs an SHGC of 0.50; with 25.1percent to 40 percent window area, anSHGC of 0.40 is required; and with morethan 40 percent window area, an SHGC

of 0.30 is required to meet the prescrip-tive requirements in the code. There aremany glazing products that can meet the0.30 SHGC, although most of the tintedoptions alone will not get there. Most of the reflective coatings have lower SHGCvalues, and tints combined with low-Ecoated products also can achieve the lowSHGC value. The high performance glaz-ing available today has made it much eas-ier to comply with the energy codes,regardless of glazing area.

4Glass Magazine® • March 2002

Europe has always been a leader in the use of glass for innovative, attractive design. The Tours Belgacom

building in Brussels, Belgium, makes extensive use of the product.

In colder climates where heating isimportant, insulation plays a much moreimportant role than it does in hot cli-mates. This makes the building envelopetrade-off option even more attractivebecause trade-offs between the roof, walls,floors, and glazing systems are more sub-stantial. With the same building and theIECC prescriptive criteria for Chicago,Ill., consider substituting better glazingfor the code-compliant U=0.50 andSHGC =0.40 windows. Glazing systemswith a U-factor of 0.45 and an SHGC of

0.30 improve the energy performance of the building beyond code requirementsand, thereby, allow the designer to con-sider other design options that couldimprove the first costs of the project.

To identify the best glazing for a project, the energy cost budget method orthe use of an hourly, whole buildingenergy simulation program is the mosthelpful. Whole building simulations pre-dict the interactions between all buildingsystems in terms of peak and annual



Glass Magazine® • March 20025

energy use. The most common tools areDOE-2, Blast, and Energy10. Employ-ing such a tool is more time intensive andcostly, but if used by the design teamfrom the beginning has the potential topay for itself and more. Also recall thatASHRAE 90.1-1999 requires compli-ance via the energy cost budget method

when the glazing area is more than 50percent of the exterior wall area.

Many projects today have establisheda goal of complying with certificationprograms such as the LEED GreenBuilding Certification Program. It pro-motes a more sustainable and environ-mentally-benign approach to the designand construction of a building. A prereq-uisite of LEED is compliance withASHRAE/IES 90.1-1999, and addi-tional credits can be earned by achievinggreater energy efficiency than required by

the standard. The improved efficiencylevels must be demonstrated using theenergy cost budget method, meaningthat hourly, whole building energy simu-lations must be run. The buildings earn-ing these credits typically have high per-formance glazing systems, daylightingcontrols coupled with an efficient light-ing design, and high efficiency chillers.

Other BenefitsWhile only a small sampling of high per-formance products has been mentioned

in this article, it is important to remem-ber that there are many more energy-effi-cient products that can result in a muchmore comfortable space. The challenge istaking advantage of all the benefits highperformance glazing offers within thedesign of a project. This can be accom-

plished through an integrated designapproach that recognizes the synergisticrelationship between building systems.

For example, the default assumptionfor glazing is that it has a shading coeffi-cient of 0.50. This is equivalent to a solarheat gain coefficient of 0.43. In manylocations, the energy code requires that

the glazing system have a solar heat gaincoefficient of less than 0.35. If the SHGCof the glazing system is less than thedefault, the cooling loads will be signifi-cantly reduced. Reduced cooling loadslead to lower air flows, smaller ducts,smaller pumps, smaller chillers, etc. Thistranslates into substantial first cost savings.

Few buildings have active daylightingcontrols to dim electric lights or turn themoff when the natural light coming throughthe glazing is sufficient to light an area.The operating cost savings from such con-

trols are tremendous, both in energy sav-ings and lower replacement costs. Recentstudies have shown increased productivitylevels in daylighted spaces, particularly inschools (Heshong Mahone Group 1999).The controls typically have paybacks of two to five years. A few showcase build-ings have even taken into account that theelectric lights will be off during peak cool-ing periods with daylighting. This haslowered their cooling loads, and the costsavings on the cooling side have paid forthe daylighting controls.

On the heating side, high perfor-mance glazing systems significant lyimprove thermal comfort and can elimi-nate the need for supplemental base-board heating. An HVAC design hand-book states that baseboard heating isgenerally specified when the heat loss

DefinitionsLow-E Glazing. Glass with a nearly invisible, metallic coating. The coating has a low emit-tance, meaning it absorbs and emits a small percentage (typically less than 20 percent) of thethermal radiation incident on it. Uncoated glass absorbs and emits more than 80 percent of

the thermal radiation incident on it.

Solar Heat Gain Coefficient. The fraction of solar heat admitted through a glazing product,including direct and indirect transmission.

Spectrally-selective Glazing. Glazing designed to have a high visible light transmittance anda low solar heat gain coefficient.

U-factor. The coefficient of heat transmission (air to air) through a building envelope compo-nent or assembly, equal to the time rate of heat flow per unit area and unit temperature differ-ence between the warm side and cold side air films (Btu/h –ft2 °F).

through the exterior wall is greater than450 Btu/hr-lineal feet of wall. Therefore,if there is a 9-foot tall glazed wall withlow-E insulating glazing (U-factor of 0.40 Btu/hr-ft2-F), baseboard heating isonly needed when it’s colder than –55˚F.There are comfort issues that counter this,although in climates with winter design

temperatures in the 0˚F range, low-E in-sulating glazing can eliminate the needfor baseboard heating.

High performance glazing systemshave greatly expanded design opportuni-ties for a project. Advances in coatingtechnology and tinted glass have made abroader range of products available,many of which have a solar heat gaincoefficient of less than 0.40.

The current energy codes can be usedto identify the most suitable product andshow that glass-enclosed buildings can

meet the energy code with the use of highperformance glazing. High performanceglazing also improves comfort conditionsand has the potential to reduce first costsas well as operating costs.

Susan Reilly is president of Enermodal Engineer-

ing, Inc., based in Denver, Colo. With over a

decade of experience in sustainable design and

building energy efficiency, she works as a consul-

tant to the window industry, performing window

energy and daylighting simulations using a variety

of software programs. She conducted research for

this article in collaboration with the TechnicalCommittee of the Primary Glass Manufacturers

Counci l (PGMC). She can be reached at

sreil ly@ enermodal.com. More information about

PGMC is available at www.primaryglass.org.

g

2002-03 - high performance

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