glaz1-bg

V I T A L S I G N S C U R R I C U L U M M A T E R I A L S P R O J E C T

GLAZING PERFORMANCE

Republic Newspaper Plant, Columbus, Indiana. Myron Goldsmith, SOM, 1971.

James H. Wasley & Michael Utzinger

Johnson Controls Institutefor Environmental Quality in Architecture,School of Architecture and Urban Planning,University of Wisconsin- Milwaukee


GLAZING PERFORMANCE

A Vital Signs resource package providingbackground information and experimentalprotocols for the study of architecturalglazing

© 1996 James H. Wasley & Michael Utzinger

Johnson Controls Institutefor Environmental Quality in Architecture,School of Architecture and Urban Planning,University of Wisconsin- Milwaukee

We thank the American Society of Heating, Refrigerating andAir-Conditioning Engineers; National Fenestration RatingCouncil; Alliance to Save Energy; Model Energy Code andCardinal IG for their permission to reprint their material inthis package. In the spirit of this Vital Signs project, anyother material from this Glazing Performance package maybe reprinted freely for educational use with credit to thesource. Anyone wishing to use all or part of this document ina commercial project must contact the authors forpermission. Please print and/or copy this document onchlorine free recycled content paper whenever possible.Chlorine related paper processing waste is a major pollutantof the Great Lakes, our backyard. February 19, 1996.


i GLAZING PERFORMANCE

CONTENTS

GLAZING PERFORMANCETable of Contents

TABLE OF CONTENTS

PREFACE

ACKNOWLEDGEMENTS

INTRODUCTION 1

INTRODUCTION TO FIRST-ORDER PRINCIPLES

Basic Physical Determinants of Incoming Radiation 3Solar and Terrestrial Radiation 3Earth/ Sun Geometry 4

"Overlay One:" Earth/ Sun Geometry"Overlay Two:" The Atmosphere"Overlay Three:" The Site"Overlay Four:" External Solar Control Devices"Overlay Five:" The Glazing

Solar Radiation Transfer through Glazing Materials 7Quantifying Solar Radiation Transfer 7Performance for a Given Wavelength 7

MaterialThicknessAngle of Incidence

Performance Across the Solar Spectrum 10The Concept of Spectral SelectivityManipulating Spectral Response

Solar Radiation Transmissivity Metrics 13The Shading CoefficientThe Solar Heat Gain Factor and CoefficientThe Coolness Index

Thermal Transfer Across Glazing Assemblies 16Thermal Comfort 16Center of Glass and Overall U Values 17

Center of GlassEdge EffectOverall U Value

Multiple Layer Assemblies 19Gap Spacing and FillAdditional Glazing LayersEdge Spacer and Frame Issues

Part I: Texts

i

iii

iv


CONTENTS

GLAZING PERFORMANCEii

GLAZING AND ARCHITECTURAL DESIGN 23

The Glazing Industry in the Twentieth Century 24A Time Line of Glazing Technology Developments 27

CODES, STANDARDS AND INCENTIVES 29Building Codes 29Energy Codes 30EPAct 1992 32State Codes 34NFRC 35Other Government Policies Relating to Glazing Selection 36Utility Incentive Programs 37

FUTURE DIRECTIONS: Ideas for Further Development 38

ANNOTATED BIBLIOGRAPHY 40

SUMMARY OF PROTOCOLS

1A: Codes and Incentives Worksheet

1B: Preliminary Field Observation Workbook

1C: Qualitative Drawing Studies

1D: Testing for Low-Emissivity Coatings

2A: Experimental Method for Determining the Transmittance of Glazing Samples

2B: Relating Glazing Area, Position and Orientation to the Plan

2C: Experimental Method for Determining Glazing Transmittance in the Field

3A: Experimental Method for Determining Center of Glass U Values in the Field

APPENDIX A: RESOURCES FOR INFORMATION ON GLAZING A.1

Primary Research Information Sources A.1Glass Manufacturers Survey of Available Information A.1Glass Product Manufacturers Available Information Listings A.2Other Manufacturers A.2

APPENDIX B: SOFTWARE RESOURCES A.5

WINDOW 4.1Microsoft Excel Workbooks

(included with this Vital Signs Package)

Part II: Protocols

Level 3: Advancedexperiments...

Level 2: Simple, single visitexperiments....

Level 1: Initialobservations...

Part III: Appendices

Protocols are page numberedindependently to allow for ad hocselection.


TOPIC HEADING 1

HEADING 2

1

GLAZING PERFORMANCEPreface

The goals of this Vital Signs Glazing Performance package are to teach students 1) to understand theinterrelationships of the design issues pertaining to the use of various glazings, particularly those issuesrelating to the thermal and luminous environments of buildings, 2) to identify the general types of glazingsfound in use and investigate their thermal and luminous performance characteristics, and 3) to be able tospecify glazing with an eye towards optimizing a structure’s thermal and luminous performance. As acompliment to these immediate pedagogical goals, this package provides a standardized format forcollecting case study information; information that plays a direct role in other Vital Signs packages.

The experimental protocols of the Vital Signs Project are divided into three levels, based on their relativecomplexity. Level 1 involves preliminary observations that can be made by an informed observer in a singlevisit and without special instrumentation. Level 2 involves short duration experiments that can be madewith standard Vital Signs instruments on a single site visit, and the Level 3 experiments are moresophisticated and require longer term data gathering. Also included in this package are two computermodeling exercises: one providing a general overview of the effects of glazing on energy flows in a buildingand the other models energy flows through specific glazing assemblies. These are meant to complimentand extend the field protocols.

The intention of these exercises is to promote both an intuitive feel for the issues involved in the architec-tural use of glazing materials, and to promote the scientific skills necessary to quantify some of thoseintuitions. Some of the exercises are graphic and others rely on words. All describe material issues ofarchitectural design that reach far beyond the choice of a glazing material to the design of the building as awhole. All are intended to deepen your competence as an architect, but they will only do so if you approachthem with the inquisitiveness and skepticism of a designer. This means not being passive but seeing everyscrap of information as a potential tool for the creation of great works.

The more observant you are the better a designer you are likely to be. Here are a few of the tools necessaryto observe and hence to design elegant, appropriate thermal and luminous environments. Good Luck!

Figure 1: The Republic NewspaperPlant, Columbus, Indiana. MyronGoldsmith, SOM, 1971.

These cover images exemplify theissues of concern in this GlazingPerformance package. The building,like this text, suppresses everyarchitectural feature that mightmoderate the impact of theenvironment on the structure exceptfor the glass itself. Within this narrowinvestigation the nature of thematerial is revealed; it is bothtransparent and reflective, dense andephemeral...

The Glazing Performance Packagedoes not stand alone, but requires thecompany of the other VITAL SIGNStopics to capture the complexinteraction of architecture andenvironment. Do the glass walls of theRepublic stand alone without severeenvironmental costs? This packageprovides a few of the tools to answerthat question....


GLAZING PERFORMANCE

PREFACE

The work represented in this VITAL SIGNS package was produced under the auspices of the VITAL SIGNSProject at the University of California- Berkeley. Funding for the project was provided by the EnergyFoundation and Pacific Gas & Electric. Additional support for equipment and staff for this project wasprovided by the University of Wisconsin- Milwaukee through both the School of Architecture and UrbanPlanning and the Johnson Controls Institute for Environmental Quality in Architecture. Students enrolled inthe Fall 1994 Architecture 520 course tested a number of the experimental protocols presented in thispackage. Mika Imai and Shan Sivakumaran assisted in the production of this package.

We would like to thank the reviewers for their helpful comments and criticisms. The draft package wasreviewed by Jeff Cook, Arizona State University; Walter Grondzik, Florida A&M University; John Klingman,Tulane University; Jack Kremers, Kent State University; and Alison Kwok, University of California-Berkeley.Bob Koester, Ball State University, beta tested and reviewed the package. Many of these reviewers'suggestions have been included in this package.

Cris Benton, Bill Burke and Alison Kwok deserve thanks for their excellent coordination of the Vital Signsproject.

The authors don't believe our work stops with the printing of this package. We hope faculty and studentsusing this work will tell us of their experiences and improvements. In addition to connecting with the VitalSigns project on the internet through the UC-Berkeley site, you may contact us on our Vital Signs homepage at the Johnson Controls Institute for Environmental Quality in Architecture web site. Both addressesare provided below. We will maintain the Excel templates described in this package on our web page. Anyquestions or comments that arise in the application of these protocols may be directed to us at our website, via e-mail or phone.

World Wide Web sites:

VITAL SIGNS Project http://www.ced.berkeley.edu/cedr/vs/Glazing Performance http://www.sarup.uwm.edu/jci/vs/

e-mail: [email protected]@alpha2.uwm.edu

phone: (414) 229-4045

James Wasley and Michael Utzinger

January 1996

A special thanks to Fetch, the binary wonder dog, for moving our files from Milwaukee to Berkeley.

GLAZING PERFORMANCEAcknowledgements

Figure 2: The Jacobs II or solarhemicycle house designed in 1948 byFrank Lloyd Wright and located inMiddleton, WI.


GLAZING PERFORMANCE

INTRODUCTION

1

The unexamined life is not worth living.

-Socrates

This package deals with the thermal and luminous performance of glazing. The ability to determine theseperformance characteristics is increasingly important to any study of a building’s environmental perfor-mance for several reasons. First, the glazed area of a building's envelope generally represents the mostdynamic connections that a building has to its environment. By default if not by design, glazing allows heatand light to flow through the skin of a building. Second, glazing technology is changing rapidly and thosechanges are being implemented in the field faster than any other aspect of environmentally responsivedesign. Where blanket assumptions about transmittance and heat flow were once accurate enough forsimple analysis, today they are not. Currently available products are many times more insulative than theirrecent predecessors. They manipulate radiant energy in ways that border on the fantastic. And finally, bythe nature of these technological advances, classifying glazing by simple inspection is an increasinglydifficult task. Radically different performance characteristics are available in visually indistinguishableassemblies. It is common now to specify glazings tailored not only to the type of construction and climatebut specific orientation and application in a building. Life complicates itself; it evolves from one level ofcomplexity to the next. These changes in the nature of glazing are bringing a new complexity to the life thata building has as a microclimate in relation to the macroclimate. They literally are creating the conditionsfor a new species of architecture.

As stated in the introduction, the package provides a provisional structure for the organizing of fieldstudies; the 'patient search' for wisdom about the way that buildings actually work. It is our hope that onesignificant result of the Vital Signs project as a whole will be the creation of a universally availablecollection of case studies of architecture from around the world. Architecture is an illusive art; even itspurely sculptural existence requires both patient and engaged observation over time to appreciate. Its lifein relation to the dancing flows of energy that animate the earth requires an even greater commitment ofimagination.

Every power we have gained to build more and faster has been accompanied by a loss of basic observa-tional wisdom. Every degree of professional specialization that we have achieved has removed us from thetrial or joy of living with our work. The design and construction industries are notoriously conservative, andyet we do a poor job of actually conserving and transmitting from one person to the next the wisdom ofsuccessful craft. The VITAL SIGNS project on one level is a way to create and conserve the knowledge thatone solution has actually worked while another hasn't.

As architects in particular, the very question of critically examining the performance of our work seemsalmost rude. We know only of the grossest failures; the subtleties of both good and poor performanceelude us. These are issues that seem through some unstated rules of the game to be either unimportant orout of our control, dependent on the type of design work that clients rarely pay for, contractors rarely honorand we as architects inevitably sacrifice for the sake of more immediately tangible results.

Ironically, the more we understand about how our buildings actually work the less we will have to beashamed of. Designing structures that endure because they fit easily and well within their environmentswill become part of what is recognized as excellence in design.

Figure 3: The Lucile HalsellConservatory, San Antonio BotanicalGardens. Emilio Ambasz and Assoc.,and JonesKell, Architects.

GLAZING PERFORMANCEIntroduction


INTRODUCTION

GLAZING PERFORMANCE2

As implied by the rhetoric of 'VITAL SIGNS,' the overriding goal is to document and to understand the livingrelationship between a work of architecture and its physical environment. Each resource package describesonly a small piece of that puzzle and every piece is not only connected to but overlapping with every otherpiece. The Glazing Performance package specifically overlaps with the Dynamics of Solar ShadingDevices and Interior Illuminance and Daylight Control packages. In general terms it overlaps with anyenergy analysis package that requires a characterization of the building envelope.

As a consequence of this division of labor, the Glazing Performance package is one of the more narrowlydefined packages of the Vital Signs project. The discussion, exercises and experiments focus on thecharacteristics of transparent materials in themselves; on the nature of glass primarily. This draws a verysmall circle that in theory ends where the glass and frame end and the room or the outside depth of thewall begins. Any larger increment of study would be more properly labeled 'fenestration' and not 'glazing.'

This focus is justifiable only in that the actual material science of glazing has changed so dramatically inrecent history. Exercises in the package constantly cross the line that distinguishes the glazing from itssetting, especially in the introductory exercises designed to sharpen general awareness. The issues crossthose lines. All that the present focus on glazing alone allows is a depth of coverage of issues andresources that at one time were simple but are now complicated.


3 GLAZING PERFORMANCE

INTRODUCTION TO FIRST-ORDERPHYSICAL PRINCIPLES

GLAZING PERFORMANCE:Introduction to First Order Principles

BASIC PHYSICALDETERMINANTS OF

INCOMMING RADIATION

Figure 4 Electromagnetic radiationspectrum, after Duffie and Beckman(1991). The light grey band is usedthroughout this Vital Signs package toidentify the visible radiation spectrum.

0.1 1.0 10.0 100.0

Wavelength in microns

Sun ~ 6000°K (15,926,000 watt/m2/ µm max.)

Incandescent lamp ~ 3000°K (497,700 watt/m2/µ m max.)

Molten glass ~ 1500°K (15,600 watt/m2/ µm max.)

Fire ~ 750°K (486 watt/m2/µ m max.)

Earth ~ 293°K (4 watt/m2/ µm max.)

Rela

tive

spec

tral

ene

rgy

Figure 5 Relative spectral energydistribution for various surfacesmodeled as black body emitters.Adapted from Button and Pye (1993).

In order to understand the energy flows that we are describing when we talk about theperformance of various glazings, we first need to understand the nature of the solar energyspectrum and the effects that determine how much of the sun’s energy will shine on ourwindow to begin with. These effects can be conceptualized as a series of overlays or filters,from the scale of the solar system to the scale of the building. Each overlay describes adifferent modification of the incoming radiation.

SOLAR AND TERRESTRIAL RADIATION

The sun radiates electromagnetic energy in wavelengths from approximately 0.2 microns (µm) to over20.0µm. The profile of this solar radiation as it reaches the earth’s atmosphere is a spike with its maximumintensity at approximately 0.5µm, the center of the range that the human eye has evolved to respond to. Forour purposes and with the phenomenon of vision in mind, this spike can be broken into three regimes;ultraviolet radiation, visible radiation or light, and shortwave or near-infrared radiation. A fourth regime,that of longwave or far-infrared radiation, represents the radiant energy of earthbound materials that havebeen warmed by the sun and are reradiating that heat at a reduced energy level. This bandwidth, alsoreferred to as ‘terrestrial radiation’ spans from 3µm to 50µm.

10 0 10 2 10 4 10 6 10 8 10 1010 -8 10 -6 10 -4 10 -2

ThermalRadiation

Wavelength, µm 25,400 µm = 1”

UltraViolet

Near Far

Infrared

Visible

Solar

Radar, Radio & TV

Cosmic, Gamma & X-rays




Ultraviolet (UV) radiation makes up a very small part of the total energy content of solar radiation,roughly 8%- 9%. UV radiation fades fabric and degrades other finish materials. UV exposure is linked toskin cancer. At the same time, UV exposure is linked to the bodily production of certain vitamins, to thehealthy growth of plants and so on. The visible range of 0.35µm to 0.78µm represents only 46%-47% ofthe total energy received from the sun. This means that when you feel the warmth of the sun on your face,less than half of that sensation is due to radiation that you can see. Within this segment of the spectrum,of course, are all of the wavelengths that constitute the colors of the rainbow, from blue at the shortwavelengths to red at the long wavelengths. The final 45% of the sun’s total energy is in the near-infrared range of 0.78µm to 5µm. This is the remainder of the warmth that you feel in the sunshine. Inaddition to the spectrum of solar radiation there is a spectrum of terrestrial radiation that fills out the far-infrared range spanning from 3 to 75µm. These long, low energy wavelengths are the wavelengths thatheat radiates away from materials that have been warmed by the sun. Biological metabolism alsogenerates heat which radiates for the skin surface in these wavelengths.

EARTH/ SUN GEOMETRY

If we were designing a solar panel that would orbit the sun independently of the earth’s elliptical orbit,away from earth’s atmosphere and always facing the sun directly, the power received from the solarradiation described above would be a constant equal to 127 watts/ft2. Evaluating the performance ofwindows down on the home planet requires that we consider all of the ways that this constant solar outputis modified before it reaches our earthbound fenestration. These modifications can be conceptualized as aseries of transparent 'overlays' that describe the stages of the journey of the solar radiation from sun tosun room.

"Overlay" One: Earth- Sun GeometryA first set of effects to account for in modeling available solar radiation is the geometry of the sun’sposition in relation to our spot on earth. These relationships are (1) the elliptical orbit of the earth, (2) theroughly spherical shape of the earth, (3) the earth’s daily rotation around its own axis, (4) the fact that theearth’s axis is not perpendicular to the plane described by its orbit but tilted 23.5° away from perpendicular,and (5) the fact that this axial tilt remains constant as we orbit the sun, which has the effect of first tiltingthe north pole towards the sun and then away from the sun over the course of a year. Thus we have polesand equators, days and nights, seasons of increasing daylight hours, seasons of decreasing daylight hours,solstices, and equinoxes.

To model this relation between sun and site from our earthbound point of view we need to account forthree variables: the latitude, the time of day and the day of the year. The latitude affects the relativealtitude of the sun above the horizon, the time of day locates the sun in the sky relative to due south, andthe time of year locates the sun in relation to its changing paths across the sky, marking either shortdaylight hours and a low sun path in the winter or long daylight hours and a high path in the summer.

The first small effect of earth ~ sun geometry that we might note is the variation of solar radiation receivedbecause of the elliptical path of our orbit. The solar constant of 127 watts/s.f. (1367 watts/m2) is thepower density of solar radiation arriving at the earth’s atmosphere. The constant does not differentiate bywavelength but lumps all incoming solar radiation together, and is represented by the area under theextraterrestrial distribution curve in figure 13. The solar constant is defined as the power density of solarradiation arriving at earth at the mean earth/sun distance. The solar constant, 127 watts/s.f., actuallyvaries by ± 3% through a yearly cycle due to the elliptical orbit of the earth. Earth is closest to the sunduring northern winters, which makes them slightly milder than they otherwise would be. In a geologicaltime frame the ellipse of earth's orbit shifts to alternately favor the southern hemisphere; one factor intheories of global climate cycles.

UV AND HEALTH

The health debate surrounding UV iscomplicated and for now beyond thescope of this VITAL SIGNS project. Asan aside, all glass inherently blocksmost UV radiation. Glazings thateither selectively admit more orcompletely block all UV areincreasingly available, so whether UVis desirable or undesirable in aparticular setting, the control of thatsetting is within the realm of design.One place to look for a possible case-study of glass with special UVfiltering might be in a recentlyconstructed or renovated art museum,where special glazings areincreasingly used to protect artworksfrom degradation.

Figure 6: 'Overlays' as a metaphor forthe categories of modifications thatdetermine the profile of solarradiation reaching any given glazing.

External Shading

Site Effects

Atmospheric Effects

Earth/ Sun Geometry




The earth's axis is tilted 23.5° relative to its orbital plane. The direction of tilt is constant, with the axisthrough the north pole pointing towards the pole star Polaris. As earth orbits the sun, its poles alternatelypoint towards and away from the sun, changing day length and relative solar altitude. This variation ismeasured by declination, defined as the angle between a line connecting the earth’s and sun’s centersand the plane described by the earth’s equator. This 23.5° tilt creates the framework for the seasons bybringing a given spot on the northern hemisphere either 23.5° closer to perpendicular with the sun’s raysat noon on the summer solstice or 23.5° further from perpendicular at noon on the winter solstice. At theequinoxes, the line connecting earth and sun lies in the equatorial plane and the declination is 0°. Daylength is 12 hours over all latitudes and the zenith angle of the sun at noon is equal to the latitude.

The hour angle of the sun is defined as the degree of rotation of a location away from solar noon. Theearth rotates 360° every 24 hours, which means that it rotates 15° per hour. Solar noon is the point in theday when the sun is directly over the local meridian. In relating clock time to rotation, morning hours arerepresented as (-) and afternoon hours are (+), so that 9am. is represented as -45° or 3hrs. x 15°/hourbefore noon.

The solar zenith and the solar altitude are two complimentary angle measures that locate the sunvertically. The zenith is the angle between the sun’s rays and the sky’s zenith. It is the angle of incidence ofthe sun on the ground surface. The altitude describes the angle between the sun's rays and the horizon.

The solar azimuth is the angle of the sun’s position in relation to due south; where south has an azimuthof 0°, north an azimuth of 180°, east azimuth is negative and west is positive.

"Overlay" Two: The AtmosphereThe earth’s atmosphere modifies the spectrum and intensity of incoming solar radiation in several ways.The carbon dioxide and water vapor in the atmosphere selectively absorb certain wavelengths, primarily inthe shortwave infrared range. Ozone in the upper atmosphere absorbs ultraviolet radiation and effectivelyterminates the upper range of radiation received at 0.30µm, which explains why ozone depletion is linkedto skin cancer and other effects of increased exposure to ultraviolet radiation. The molecules and dustparticles in the upper atmosphere scatter some of the incoming radiation in the visible and UV spectrum.This scattering is identified as Rayleigh attenuation.

Figure 9: Azimuth Angle

0.2 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

500

10 00

1500

20 00

2500

0

Wavelength - µm

Dire

ct N

orm

al S

pect

ral I

rradi

ance

- w

att /

m2

/ µm

Visible

Atmospheric Scattering(Rayliegh Attenuation)

Molecular Absorption(O3, H 2O, CO2)

Direct (Beam) Solar Radiation Reaching Earth’s Surface

Extraterrestrial Solar Radiation

The area in grey represents the solarspectrum reaching the earth as beamradiation. The area in black representsthe portion of the spectrum absorbedby various molecules present in theatmosphere. Specific moleculesabsorb specific wavelengths ofradiation. The area in white representsthe portion of the visible and UVspectrum that is scattered in the upperatmosphere. Some of this scatteredradiation arrives on earth as diffuseradiation, giving the sky its blue color.

Figure 10: Effects of Rayleighscattering and atmosphericabsorption on beam solar irradiancearriving at earth's surface. Adaptedfrom Duffie and Beckman (1991).Light grey band identifying visiblespectrum.

Figure 7: Earth/ Sun Geometryshowing Declination Angle.

Earth’s Axis

DeclinationAngle

Equatorial Plane

ZenithAngle

AltitudeAngle

Figure 8: Zenith and Altitude Angles

EW

N

S

Azimuth Angle (+) (-)




Since atmospheric effects are dependent on the distance that incoming solar radiation travels through theatmosphere, the atmosphere works in conjunction with the cyclical patterns of earth- sun geometry tocreate a second set of overlays describing the total possible solar radiation available at a given location.Two forms of solar radiation data are important: clear day global irradiance and average horizontal andvertical insolation.

Clear day global irradiance is the maximum solar radiation that can be expected in a given place on agiven day of the year. Models to estimate atmospheric transmittance as a function of atmospheric pathlength, altitude and visibility are included as functions in the Sun.xlm Excel macro which is part of thispackage. Clear day global irradiance is used to evaluate limiting conditions, such as heat gains fordetermining peak cooling loads. Average insolation, on the other hand, is monthly average solarradiation for a location usually provided in the form of Btu/s.f./day and based on long term measurementstaken at a particular location. This information is much more reflective of the typical conditions of a placeand is useful in total energy consumption analyses. Sun.xlm includes average solar radiation data for 70cities from around the globe and the ability to add new city data.

The solar flux (watts/s.f.) is the amount of solar radiation falling on a surface at a given time. This can bedetermined for a surface of any orientation. For direct radiation, the solar flux is directly proportional to thecosine of the angle of incidence of the solar radiation to the surface, so that as the sun moves away frombeing perpendicular to the surface the solar gain falls off at first slowly and then rapidly. As illustrated inFigure 11, a 30° angle of incidence will only decrease the possible total by 14% but a 60° angle willdecrease it by 50%. Once atmospheric effects have determined the maximum available radiation on site,this angle of incidence effect explains the common knowledge that orienting a solar house up to 15° toeither side of south hardly affects its performance, while beyond 15° solar performance drops off rapidly.

"Overlay" Three: The SiteA third group of effects to overlay on our picture of the flux of solar radiation is location specific andaccounts for the significant influences of regional differences of altitude, relative humidity and seasonalweather patterns. Specific regional and local weather patterns, for example, can create seasonal patternsof relatively cloudy or overcast weather, affecting the available solar radiation at the ground in predictableways. This information is available from local meteorological services. The orientation of the glazing(azimuth and tilt) will affect the amount of sky and ground reflected radiation reaching the glazing.

This category could also include the local effects of site obstructions and ground reflectance that anarchitect or researcher would model as part of a site analysis, but that would fall out of the range of thedesigner's ability to change.

"Overlay" Four: External Solar Control DevicesOverhanging eaves and shady porches, deep set windows, external shutters, attached shading devices...All of the elements of architecture that are the means by which the designer can modify the incoming solarradiation are included here and discussed in detail in the VITAL SIGNS package Dynamics of Solar HeatGains through Windows.

"Overlay" Five: The GlazingAll of the above categories of 'overlays', from the scale of the solar system to the scale of the shadingdevice hanging over a window, determine the quantity and spectral properties of the solar radiation strikingthe surface of the glazing. This package accounts for the final, crucial layer of transparent material thatshapes the profile of the incoming solar energy before it arrives and disperses within the building.

60˚30˚

100%86%

50%

0%

0˚

Figure 11: Variations in Solar Fluxdue to Variations in Angle ofIncidence with respect to any surface.

Figure 12: Effective incidence angleon Sloped Surfaces. Adapted fromDuffie and Beckman (1991).

For a vertical surface (slope=90) bothdiffuse radiation from the sky andground-reflected radiation have aneffective angle of incidence of 58degrees.This mean angle of incidence onvertical glazings is a usefulassumption when measuring thetransmittance of radiation through aglazing in the field under grey skyconditions rather than the clearconditions called for in the level 2protocols.

0° 15° 30° 45° 60° 75° 90°Surface Slope from horizontal in degrees

Effe

ctiv

e an

gle

of in

cide

nce

of d

iffus

e so

lar r

adia

tion

90°

85°

80°

75°

70°

65°

60°

55°

Ground reflected solar radiation

Diffuse solar radiation from sky




0.00 0.50 1.00 1.50 2.00 2.50 3.000

50

100

150

200

250

Wavelength in Microns (µm)

Sola

r Irra

dian

ce ~

Wat

t / s

.f. /

Mic

ron

Figure 13: Visible and Total SolarRadiation Reaching Earth'sAtmosphere.

The total area under the curve is thesolar constant of 127 watts/s.f.. Thelight grey area is the visible radiation,and represents roughly 46% of thetotal, equal to 58 watts/s.f..

QUANTIFYING SOLAR RADIATION TRANSFER

The solar radiation that arrives at a given glazing at a certain time and date can still be described by thediagrammatic spectral distribution curve described earlier, with reductions in the power of certainwavelengths and perhaps even slight increases in the power of other wavelengths (if, for example, a highlyreflective landscape boosts the intensity of reflected solar radiation on the glazing...). This is the incidentsolar radiation on the glazing surface.

The performance characteristics of glazing that are important architecturally can be roughly described asconcerning the flow of solar energy (heat and light) in and the flow of terrestrial energy (heat) both in andout across the boundary represented by the glazing. Considering the flow of solar energy in through theglazing, two measurements are used to give a simplified representation of the spectrum of incident solarradiation and its modification by the glazing. These two measures, which are the percent of incident visibleradiation transmitted by the glazing (visible transmittance or τvis), and the percent of incident total solarradiation transmitted by the glazing, (solar transmittance or τs). If solar radiation is assumed to be halflight and half near-infrared radiation, near-infrared transmittance (τ

ir) is approximated by doubling τ

sand subtracting τ

vis. These measures are the object of several of the protocols of this glazing package.

These transmittances provide enough information for most architectural purposes. It is important to notethat they represent averages across large spans of the solar spectrum. At the end of this section we willreturn to these measures and note their limitations in trying to describe the actual behavior of advancedglazing systems interacting with the spectrum in complicated ways.

PERFORMANCE FOR A GIVEN WAVELENGTH

The degree to which radiation of a given wavelength is either transmitted, absorbed or reflected isdependent on four variables, three of which are properties of the glass: the material composition of theglass, the thickness of the glass, and the refractive effect of the glass on incident radiation. The fourthvariable involves the application of coatings to modify the performance of the glass.

MaterialGlass in a material sense is not a solid but a supercooled liquid. Unlike most solids but like many liquids,the electrons in glass molecules are confined to particular energy levels. Since the molecules cannot skipback and forth between different states of excitement in the bandwidths of visible and near infraredradiation, they do not absorb and dissipate those forms of radiant energy. Instead, the energy passesthrough the molecules making up the glass as if they weren’t there. Typical glass does absorb wavelengthsoutside of the visible and near infrared bandwidths, absorbing a large percentage of UV radiation and alsoabsorbing all longwave infrared radiation. (See ‘the Greenhouse effect’ below.)

The profiles of radiation absorbed by different types of glass are different, however, because unavoidableimpurities in the soda-lime-silica mix absorb radiation that might otherwise pass through. ‘Heat absorbing’glass is one example of this fact put to use to change the performance characteristics of the glass. Heatabsorbing glass is made by increasing the trace levels of iron oxide in the sand, which gives the glass itsgreen cast. Iron oxide absorbs primarily in the near-infrared wavelengths, thus cutting down on the amountof solar radiation transmitted. Advances in tinting additives have also led to more spectrally selective heatabsorbing glass, which admits more daylight for the amount of heat absorbed and thus appears clearer.These tints appear slightly bluish rather than green.

SOLAR RADIATIONTRANSFER THROUGHGLAZING MATERIALS




Visible

Heat Absorbing0.5% Fe2O3

Standard Glass0.1% Fe2O3

Water White0.02% Fe2O3

1.0 2.0 3.00.2 4.0

100%

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The rate at which glass absorbs solar radiation is defined by the extinction coefficient. Low absorption'water white' glass has an extinction coefficient of less than 0.1/ inch. Heat absorbing high iron oxidecontent glass can have an extinction coefficient of 2.0/ inch.

The material also affects the incoming radiation by bending or refraction. Refraction is an effect thatoccurs between transparent materials of different densities, such as air and glass. The bending evident inrefraction is a physical representation of the longer time it takes light to move through the denser of thetwo materials, and it is dependent on the angle that the light strikes the boundary. The index of refractionis a constant for a given glazing material, and since different wavelengths are refracted different amountsin a given material, the index of refraction has different values for different wavelengths of light. Theseprinciples are seen in the behavior of prisms and lenses, which both take advantage of the bending of lightby refraction. Where the two surfaces of the glass are parallel, the divergence caused by the initialrefraction at the first boundary between air and glass is countered by an equal and opposite refraction atthe boundary between glass and air. Slight variations in the thickness can be detected in very old sheetglass or glass with a coating of imprecise thickness by the telltale rainbow iridescence of the surfaceunder certain light conditions. The refractive index of window glass is 1.526.

ThicknessThe thickness of the glass is significant for the intuitive reason that if impurities in the glass absorbradiation, the longer the path through the material the more radiation will be absorbed. For clear glass,doubling the thickness from 1/8" to 1/4" increases the absorption by 6% more of the total solar radiationand by 2% more of the visible radiation.

Angle of IncidenceThe path length of light through glass is a function of both the glass thickness and the angle of inci-dence of the incoming radiation. When light is coming in perpendicular to uncoated glass very little of it isreflected. This reflection grows in proportion as the angle of incidence increases; at first slowly and thendramatically, until a point at approximately an 87° angle of incidence where all incident light is reflected.This angle is identified as the cut off angle.

Figure 14: Spectral transmittance of6 mm thick glass with various ironoxide contents for incident radiationat normal incidence. Data from Dietz(1954). Adapted from Duffie andBeckman (1991).

GREEN GLASS

Green glass got its name from thegreen color of the glass due to highiron oxide content. This heatabsorbing glass was developed to cutdown both on glare and airconditioning loads in glass skin officebuildings, but fell out of favorbecause the absorbed heat causesthe glass itself to become quite hot,producing uncomfortable radianteffects in a space even while cuttingdown on the overall heat gain. Withthe development of low-e films andmultilayer glazing systems, an outerpane of heat absorbing glass can nowbe effectively isolated from the insidespace, and its reradiated heat energymore effectively directed to theoutside.

Angle of Incidence

Normal Vector to Window

Figure 15 : Angle of incidence withrespect to a vertical glazing surface.Radiation striking the surfaceperpendicularly has an angle ofincidence of 0°. Radiation movingparallel to the surface has an angle ofincidence of 90°.




Even at a 0° angle of incidence there is a certain percentage of radiation reflected at each surface of theglass. For typical glass this reflectance amounts to approximately 4% of the total at each surface. Asillustrated in figure 16 , reflectance and transmittance are strongly related, with reflectance rising atangles of incidence greater than 45° as transmittance falls off. Since each sheet of glass has two reflectingsurfaces, a front and a back, a minimum of 8% of the available light will be reflected. This transparency canbe increased either by the relatively expensive refining of the raw materials to eliminate impurities, or morerecently by the layering of anti-reflective selective surface coatings on the glass. "Water White" glass is anexample of a highly refined glass. Because of its cost it is used in situations where transparency is at apremium, such as in the construction of solar collectors.

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Figure 16: Transmissivity as aFunction of Angle of Incidence forstandard 1/8" glass. Generated in theExcel Workbook VSglass.xla includedin this package.

Notice that the glass remainsapproximately 85% transparent toincoming radiation until between 40°to 50° and then drops off rapidly.

Figure 17: "Daylight Transmittancefor Various Types of Glass." FromASHRAE (1993) p.27.32. Table 24.Reprinted with permission.

Note that this table characterizes τvisonly.




PERFORMANCE ACROSS THE SOLAR SPECTRUM

The variables of the glazing material, its thickness, and the angle of incidence of the incoming radiationdetermine whether energy of a given wavelength will be transmitted, absorbed or reflected. But solar andterrestrial radiation is spread across a large spectrum of wavelengths, and one of the most powerful factsabout transparent glazing materials is that they affect different wavelengths of energy differently.

The Concept of Spectral SelectivityAs mentioned above, clear glass admits most visible and near-infrared radiation but is opaque to and henceabsorbs far-infrared radiation. This helps to produce what is known as the 'greenhouse effect,' which hastraditionally been seen as the primary example of spectral selectivity of glass.

Incoming solar radiation that is transmitted through the glazing is absorbed by the surfaces of a space andthen reradiated at a lower energy state as longwave or far-infrared radiation, only to be absorbed againrather than transmitted back out by the glass. As this radiant energy is absorbed, the temperature of theglass is raised and it in turn radiates heat energy both towards the inside and the outside. In this way someof the longwave energy striking the glass from the inside is returned back to the room, depending on suchthings as the difference in temperature between the inside and out and the amount of air motion on eitherside carrying heat away from the surface of the glass by convection.

This effect turns out to be not as significant as once assumed (see sidebar discussion). It is also a responseto terrestrial radiation, rather than solar, which is measured by the coefficient of heat transfer (U value)discussed later, rather than by total solar transmittance. Heat absorbing glass is an example of selectivitythat operates within the solar spectrum, absorbing increasingly more near-infrared than visible radiation asthe iron-oxide content is increased as illustrated by the different transmittances of standard and heatabsorbing glass illustrated in Figure 14.

This potential of a transparent material to be made selectively transparent to different wavelengths ofelectromagnetic radiation is extremely important. Conceptually, this allows the components of the solarand terrestrial spectrum to be teased apart and filtered out, depending on the specifics of an application.The two primary applications that this potential is being developed to respond to are solar heating anddaylighting, though there are also developments at the opposite end of the spectrum dealing with UVradiation.

In a building where heating is the primary need, such as a house in a cold climate, the ideal spectrallyselective glazing would admit all of the visible and near-infrared radiation of the sun, and reflect back intothe room all of the far-infrared terrestrial radiation. In a building where cooling is the primary concern, suchas an office building being baked by the waste heat of its electric lighting, the ideal glazing would admitonly the visible light, while screening out all of the near- infrared heat that accompanies it. This wouldallow daylight to be substituted for electric lighting, lowering the waste heat load in the space as well assaving electricity directly through the reduced use of electric lights.

The power of this concept is clear when you consider that the visible portion of the spectrum representsroughly 50% of the total solar irradiance, so that such a selective filter would admit all of the 'light' ofsunlight, but only half of the solar heat. This concept is carried one step further by theorizing an ideal 'cooldaylight' filter that would narrow the bandwidth of visible light admitted from the range of 370-770 nm tothe narrower 430-690 nm (see figure 19) This cuts out only the wavelengths on the edges of human visionwhere the eye's response is less than 1% of its peak sensitivity. This cool daylight bandwidth represents99.4% of radiation visible to the human eye, but only 34% of the total solar heat load.

THE GREENHOUSE EFFECTREVISITED

The fact that glass is "opaque" tolongwave or terrestrial radiation isoften cited to explain why a glassroom such as a greenhouse heats upmore than the garden next to it. If thisexplanation is correct, why are low-ecoatings that reflect longwaveradiation of so much interest?

Where typical glass absorbslongwave radiation, warms up, andreradiates it in all directions, a low-ecoating prevents the glass fromreradiating that heat energy off of thecoated surface. Rather than the heatbeing absorbed and reradiated bothinwards and out, it is all reflectedback in the direction from which itcame. In this way, low-e glazing trapsmore heat than conventional glazing.

Despite this difference betweenconventional and low-e glazing,however, the obvious explanation ofthe ‘greenhouse effect’ is overlookedin making the point about glass beingopaque to longwave radiation: Anygreenhouse traps the air that iswarmed by its sun-warmed surfaces,cutting down dramatically on heattransfer by convection. This reductionof convection losses is the secret ofthe greenhouse. Open the window,whether clear or low-e, and the solargreenhouse releases its heat.

Figure 18: The Lucile HalsellConservatory, San Antonio BotanicalGardens. Emilio Ambasz and Assoc.,and JonesKell, Architects.




Figure 19: "Comparison ofExtraterrestrial Spectrum and HumanVisibility Function with Ideal FilterTransmittance Spectrum." FromASHRAE (1993) p27.14 fig. 13.Reprinted with permission. Light greyband indentifying visible spectrumadded.

In this theoretical ideal, thebandwidth identified by the solid darkline is slightly narrower than therange of human vision, identified bythe dotted line. In reality, the behaviorof light does not allow for such neatdivisions.

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Human Light Sensitivity

Manipulating Spectral ResponseThese theoretical limits of spectral selectivity are being approached by current developments in glazingtechnology.

There are two categories of approach that have been developed. The first is to tint the glass, altering itsmaterial composition so that it absorbs radiation. Glass can be tinted to appear grey, bronze, blue, andgreen depending on the additives. Tints do absorb solar radiation, but only the green and blue colorsselectively absorb more infrared radiation than visible radiation. Some dark tints admit more heat thanlight, a testament to the fact that they were developed to address glare problems at a time when theenergy costs of cooling were not considered important. Even the new generation of blue and green tintsthat have been 'tuned' to select the highest ratio of near-infrared to visible solar radiation possible arelimited by the fact that they absorb rather than reflect the radiation that they are screening out. There doesnot yet appear to be a selective tint that absorbs near-infrared radiation without imparting some recogniz-able color to the glazing.

The second approach has been to add coatings of different materials to the surface of the glazing. Thisincludes both mirrored glass and clear low-e coated glass, as the two products are closely related.

"Mirrored "or evaporated monolithic metallic coatings represent an early technical/manufacturingstage of sophistication of low emissivity or "low-e" coatings. The basic principle in both is that a layer of ahighly conductive, low emittance metal applied to the glazing acts to suppress radiation from the coatedsurface. Emittance and reflectance are inversely related, and so the coating reflects radiation. This makesthe behavior of coatings fundamentally different than the behavior of tints.

Mirror coatings such as gold and silver are thin enough to be transparent, but thick enough that they reflectwavelengths throughout the solar spectrum more or less equally. They do reflect light in a consistent color.Advanced low-e coatings use various metals and thicknesses to arrive at a layer that interferes with certainportions of the spectrum while being transparent to others.

THE IRON AND THE STOVE

The property of emittance isdemonstrated in the objects of a castiron stove and a cloths iron. Both arehot. The stove has a high emissivity; itradiates energy freely, as you can feelat a distance. The polished metalsurface of the iron has an extremelylow emissivity. Radiation is inhibited.You find out if the iron is hot only bytouching it, allowing conduction tooccur. The silver surface of a low-ecoating works in this way.




'Thin-film' or 'softcoat' low-e coatings use the same silver coating as a silver mirrored glazing, applied in anultra-thin layer and sandwiched between layers of anti-reflective coating with a spiked surface that acts toreflect light through the glazing by presenting oblique surfaces to the incoming radiation. Similar to theanti-reflective coating on a pair of glasses, the anti-reflective coating is then given its own protectivecoating.

There are two general categories of low-e coating technology that have been competing in the market-place. The 'softcoat' mentioned above is called soft because it is fragile, and is used only in protectedsituations, such as sealed within an insulating glass unit. Metal is 'sputtered' onto the surface of eitherglass or plastic film by a process that gives precise control of the thickness of the coating, making it thehighest performing and most versatile coating technology. The ability to vary the thickness of theseextremely thin layers of metals are what allows sputtered coatings to be tuned to selectively screen eitherthe far-infrared or both the near and far-infrared. The first variation is used to capture solar heat, thesecond to screen out solar heat while admitting daylight. This process must occur in a vacuum indepen-dently from the glass manufacturing process, making it less economical than its hardcoat rival.

The lowest emissivity currently available, according to Johnson (1991), is 0.03, or 3% of the emissivity ofan ideal or black body radiator at the same temperature. Standard sputtered low emissivity coatings run inthe range of 0.10. These coatings reflect up to two thirds of the UV radiation admitted by standard doubleglazing. Johnson notes that these coatings produce slightly pastel colored reflections, but that technologi-cal advances are expected to eliminate this.

'Hardcoat' processes are integral with the manufacture of glass. There are two such technologies, withPryolytics being commercially available. Pyrolytic coatings are thicker than sputtered coatings and workbest at reflecting longwave radiation for heating applications. Emissivities run in the range between 0.17-0.20, which is down from the 0.40 emissivity that they were known for in the 1980's. The coatings arecolorless but do produce a slight surface haze as seen from the outside under bright light conditions. Again,Johnson predicts that this effect will be eliminated.

Figure 20: Comparison oftransmittances of standard glass withsoftcoat and hardcoat low-e coatingson glass. Adapted from Johnson(1991). The Pyrolytic coat is the 'hard'coat, and vacuum deposition is the'soft' coat. Light grey band indicatesthe visible spectrum.

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SOLAR RADIATION TRANSMISSIVITY METRICS

Beyond the basic measurements of τvis and τsol that can be experimentally measured in the field, there areseveral metrics either in use or under discussion that attempt to better characterize transmissivity forarchitectural purposes as the glazing itself becomes increasingly sophisticated. These terms are controver-sial and are not dealt with here in the depth required to render final judgement. They are included as partof the street vocabulary of glazing, with the suggestion that advanced study might include entering into thedebate.

The Shading CoefficientThe shading coefficient is a dimensionless ratio defined broadly as the solar gain through a givenfenestration compared to what the solar gain would have been through an unshaded opening of the samesize and orientation glazed with a standard single pane of 1/8" double strength clear window glass. It wasdeveloped to provide a simple reference to a known product.

The shading coefficient has been used to characterize the impact of various shading devices and glasstypes designed to screen out incoming solar radiation. The measure is used differently in differentsituations and by different groups, however, and the underlying simplification on which it is based isbecoming increasingly unworkable as glass technology progresses. For these reasons many would like tosee its use discontinued. On the other hand, it is a metric that is thoroughly entrenched in use. Many of thenewer terms discussed here incorporate the shading coefficient's baseline reference to a single sheet ofglass even though the comparison is anachronistic.

The shading coefficient is used to characterize the reduction of solar radiation caused by external shadingdevices of different designs, various types of glazing materials and coatings, interior shading devices suchas venetian blinds and so on. It is also used by manufacturers of glazing products to specifically character-ize the reduced solar gain through their products, taking them out of the context of a specific location,orientation and design. This is the measurement that you are likely to run across in research on glazingperformance, as seen in figure 21.

Figure 21: "Shading Coefficients forSingle Glass and Insulating Glass. "From ASHRAE (1993) p.27.19.Table 11. Reprinted with permission

ho represents a heat transfer

coefficient for the outside surface,including both convection andradiation. h

o=4.0 assumes a 7.5 m.p.h.wind speed. ho=3.0 assumes still air,relevant in evaluating worst casesummer cooling conditions. Noticethat the difference in shadingcoefficients between the twoassumptions is larger for heatabsorbing glass than it is for clear.The less air movement, the more thatheat absorbed by the heat absorbingglass will be transferred to theinterior rather than back out to theexterior. This decrease in performanceis reflected by the higher shadingcoefficient.




The shading coefficient provides a way to quickly quantify the reduction of solar gain expected from a givenshading strategy in order to calculate the total solar load on a building. The danger is that by relying on theshading coefficient to make decisions about glazing we risk missing all of the subtleties that make modernglazing performance so novel, as well as missing the relation of the rest of the shading and daylighting andheating strategies available to compliment the performance of the glazing.

• The shading coefficient only considers total solar radiation which includes the ultraviolet, visible andnear infrared spectrum. The most important advance in glazing in the last twenty years has been thedevelopment of selectively reflective surfaces that can be tuned to block only certain portions of thespectrum. By lumping the values together the rating does not reflect the distinctiveness of differently tunedproducts.

• The shading coefficient is only calculated for normal angles of incidence, while coated glazings alsochange in their response to increasing angles of incidence.

• The shading coefficient does not consider the use of daylighting to offset internal gains from electriclighting, since it is only concerned with external gains.

• The published shading coefficients that model the performance of shading devices necessarily averageall of the effects of location, time of day and time of year, which are precisely the subtly shifting effectsthat good external shading device design strives to exploit.

• The ratio is based on the performance of a single pane window, which is increasingly anachronistic.

The complexity of modern glazing systems cannot be adequately evaluated by the single shading coeffi-cient.

The Solar Heat Gain CoefficientThe solar heat gain coefficient (SHGC) is equal to the solar transmittance plus the fraction of the solarradiation absorbed by the glazing which enters the building as heat. This metric provides an accuratemeasure of the solar heat gain potential of a glazing unit. The SHGC is described in ASHRAE FundamentalsHandbook, (1993) Chapter 27: Fenestration. It is also computed for glazing in Window 4.1 and will beincluded in upcoming NFRC ratings. For the purposes of the NFRC rating the SHGC includes the solarradiation absorbed through the frame, which describes the solar heat gain of the overall unit just as theoverall U value describes the thermal performance of the overall unit. ASHRAE and Window 4.1 providemethods for calculating the frame performance independently.

The Coolness IndexThe coolness index is a term under discussion that is meant to describe the ability of a given glazing toadmit daylight while screening out solar heat gain. Johnson (1991) defines this as the ratio of the shadingcoefficient to the visible transmission, Dx = SC/τvis. In this definition, the lower the number, the cooler thelight. The lowest possible coolness index value is 0.36, reflecting the amount of heat energy containedwithin the visible portion of the solar spectrum.

Schuman, writing for Lawrence Berkeley Laboratories, ("Cool Daylight" Progressive Architecture. April,1992) defines the coolness index as the Glazing Luminous Efficacy (K

e) by the inverse relationship; K

e= τ

vis/

SC. A low Ke signifies a glazing that does not distinguish between visible and total solar radiation well anda high Ke signifies a glass that does.




The concept that 'cool glazings' can admit daylight into a space at a lower heat penalty than electriclighting is crucial to the future of energy conserving design. Since the authors of this package feel that theshading coefficient ads an unnecessary reference to single glazing, we would propose a third variation onthe same concept, defining efficacy in conventional lumens/watt units by simply taking the ratio of visibleto the solar heat gain coefficient, τ

vis/SHGC. In conventional terms, the luminous efficacy of incoming

solar radiation is roughly 93 lumens/watt. As previously discussed, the theoretical limit for 'cool daylight'admits only 34% of the total heat energy of the spectrum, or 280 lumens/watt. An insulated unit ofAzurlite™ over a clear glass with a low-e coating admits 59% of the visible with a SHGC of 30% (asmodeled in Window 4.1). This gives it a luminous efficacy of twice that of the incoming solar radiation, or186 lumens/ watt.

The power of this statement can be seen by comparing this with the efficacy of conventional electriclighting sources: An incandescent lamp produces between 8 and 20 lumens per watt of energy, a fluores-cent lamp between 32 and 102 lumens/watt, and a metal-halide lamp between 70 and 100 lumens/watt.(Stein and Reynolds, (1992) p.967.) At 100 lumens/watt, even the most efficient metal halide lampgenerates as much waste heat for a given amount of light as is admitted by standard clear glass; threetimes the amount admitted by 'cool glazing' at its theoretical limit. Granted, daylight is not constant andboth the fenestration and the space must be designed to take appropriate advantage of it when it isavailable. If these issues are addressed by design the potential for 'cool daylighting' is obvious.

SOLAR EFFICACY

The efficacy of a light source is theratio of light output to power input.Efficacy has a value of lumens perwatt (or lux per watt). The solarconstant has already been given as127 watt/s.f. Robbins (1986) gives anextraterrestrial solar illuminationconstant of 11,800 fc (one fc equalsone lumen/s.f.). This is the lightarriving from the sun outside earth'satmosphere at the mean earth sundistance. The efficacy of solarradiation reaching earth is then 93lumens per watt.

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LEGENDLow-E on GreenPlain 6mm ClearSoft Coat: maximum SolarHard Coat: maximum SolarPlain TintsPlain ReflectiveLow-E and Tints1/8” Standard Glass

Figure 22: Relationship betweentotal solar transmittance and visibletransmittance for various glasses andtreatments. Adapted from Johnson(1991).

• The grey area defines thetheoretical limits of glassperformance. The lower line is thelimit for transmission of visible lightonly, with both ultraviolet and infraredblocked. Visible light is roughly 46%of the incident solar radiation, andthis limit moves from 0 at the lowerleft to 46% total solar at 100% visibletransmittance. The upper limiting linecorresponds to 100% transmittance ofinfrared and ultraviolet with visibleincreasing from 0% on the left to100% at the upper right corner. Thediagonal dashed line represents thecondition of equal transmittances ofvisible and total solar radiation.• Solar heating glazings should fallnear the top right corner.• Cool daylight glazings should lienear the bottom line.• The student might plot glazingtransmittances measured in the fieldon this figure.

Note: The scale of the Shading Coefficient is included along the leftside of the graph for reference along with the plot of a 1/8" standardglass pane, the reference for the shading coefficient.




Characterizing the solar radiation transmitted into a building tells only half of the story of the environmen-tal performance of the glazing. Once the sun goes down the fact that glass has traditionally been poor atretaining heat comes back into prominence. Actually, the room has been exchanging energy with theenvironment all day. Heat has been flowing in as solar energy and either in or out as thermal energy. Thenet flow of heat across the glazing is the real issue. This flow is governed by the difference in temperaturebetween environments and by the resistance to heat flow through the glazing assembly in question.

Heat is transferred through the envelope of a building by several paths; conduction, convection, radiationand infiltration. Conduction refers to energy moving through a material by direct contact, one moleculecolliding against the next. Convection refers to the process of that heat energy warming the air or otherfluid in contact with the surface and then being dissipated into the environment as the fluid moves over thesurface and away. Radiation refers to the direct propagation of energy through space as discussed in theprevious section. All objects radiate energy; the radiant flows that we are concerned with in a building areboth the influx of solar energy and the absorption and reradiation of that energy in the terrestrial orlongwave infrared wavelengths. Finally, Infiltration accounts for the direct movement of air betweenbuilding and environment, such as through the cracks around a poorly fitted window. Quantifying infiltrationfalls outside of the scope of this VITAL SIGNS package, though it is closely associated with discontinuitiesin the fabric of the envelope such as windows. Discussions of infiltration effects can be found in ASHRAEFundamentals (1993) Chapter 27 and in Stein and Reynolds (1992).

The rate that heat flows across a building assembly into or out of a building is called the heat transmissioncoefficient or U value. The U value of a glazing assembly includes convection, conduction and radiationheat transfer paths. It is given in Btu/hr ft2 °F (W/m2 C). The U value is the inverse of the total R value orresistance of the assembly and is defined as U= 1/ R.

Because R values measure the resistance to heat flow, the larger the number the greater the resistance.Conversely, the larger the U value the greater the rate of heat flow. Both terms will be encountered inmanufacturer’s literature and in the VITAL SIGNS experiments. Each measure is useful in different contexts.R values are additive in series for determining the resistance to heat flow through sequential layers ofmaterials, layers of air, selective surface effects etc... U values are additive in parallel, when averaging thearea weighted effects of parallel heat flow paths such as in calculating the overall thermal performance ofa wall with a window in it. Since the overall R value is the inverse of the U value and vica-versa, a typicalsingle pane of window glass listed as having a U value of 1.11 has an R value of 1/1.11 or 0.90.

THERMAL COMFORT

Thermal comfort is dependent on more than the net balance of heat energy, just as visual comfort isdependent on more than an adequate quantity of light. The sensation of comfort is based on ambient airtemperature, on relative humidity, on air motion such as drafts caused by infiltration or convection, andon the temperature and emittance of the surfaces of the space.

Just as other objects radiate and absorb energy, our bodies radiate heat to surrounding objects that arecooler than body temperature and receive radiant heat from objects in our surroundings that are warmer.The closer you are to a surface, the larger it appears to your body and the more that it dominates theresulting flows of energy. Sitting next to a cold window means that your body will radiate to that coldsurface. Moving away from the window lessens the effect. Your experience of comfort is based both on theaverage resulting energy flow and on its spatial uniformity. In other words, the cold window feels uncom-fortable in part because that side of your body is experiencing a radiant surface very different from the

THERMAL TRANSFERACROSS GLAZING

ASSEMBLIES




other side. Sitting around a campfire this same sensation feels good, in part because you continually adjustyour relation to the fire. Radiant transfer does require a clear path between the hot and cold objects.

While improvements in the thermal performance between a single pane of glass (R=0.9) and a double pane(R=1.9) might not seem significant in comparison with other resistance values (R=15 for a typical insulatedwood frame wall), the difference raises the temperature of the inside surface of the glass enough to cutdown noticeably on direct radiative transfer. The inside pane stays closer in temperature to the room, theoutside pane stays closer to the temperature outside, and the drop in temperature happens in the spacebetween. The window 'feels’ warmer. The same holds true in a hot climate; lowering the inside surfacetemperature of the glass by disconnecting it from the heat outside helps a space feel cooler than itotherwise would, regardless of the air temperature.

Low-e coatings increase this disconnection dramatically, which is one of their primary advantages. Amaterial's emissivity, its ability to radiate energy, modifies the effect of the material's surface temperature.Low-e coatings lower the emissivity of the glass, which means that the glass is prevented from eitherabsorbing or radiating energy; instead radiant energy is reflected. The temperature of the window might below, but that cold sink is not what your body 'sees.' Rather, you experience the radiant energy of the roomand your own body reflected back at you. 'Heat Mirror' is a fitting trade-name.

CENTER OF GLASS AND OVERALL U VALUES

A window unit can be characterized as having one U value at the center of the glass, a second U value atthe edge of the glass, a third U value for the frame and an average or overall U value as an assembly. Forsingle pane windows, the U values of the glass and frame were close enough that these distinctions didn’tmatter, but as the technology of glazing has advanced, the edge and frame conditions have become theweak link in the chain. Understanding the relationship between the frame and the center of glass U valuespoints out the challenge ahead in the design of frames and spacers.

Center of GlassThe U value measured at the center of a glazing assembly is the U value of the glazing itself, unaffected byits edge condition. Energy is flowing in only one dimension- through the glazing perpendicularly. If theglazing is at all sophisticated the center of glass U value will be significantly lower than the overall unit'sperformance in the field. For this reason it is useful in comparing the performance characteristics of variousglazing options in theory only. As used in product literature it can be misleading.

Edge EffectAt the edge of a multi-pane assembly the insulating gap between conductive materials is collapsed by thespacer connecting the layers together. Clasping this spacer, the frame extends the connection of inside toout with solid, often highly conductive, materials. This continuity provides a 'thermal bridge;' a directconductive path for heat to flow through. This bridging action holds the performance of the edge far belowthe performance of the center of glass, lowering the overall U value for the assembly dramatically. Thebetter the glass becomes, the more the edge and frame drag it down.

There are several issues at work here. The frame’s structural role in securing the glass to the fabric of thebuilding makes it difficult to overcome the thermal bridging that creates these poor efficiencies. 'Thermallybroken' frames are available, consisting typically of two separate extrusions that are joined by a 'key' ofless conductive material, but the conductivity of typical key materials is still much higher than theconductivity across the glazing gap. Such thermally broken frames do reduce conductivity enough to cutdown on condensation problems.




More importantly, the resulting thermal bridge created by the spacer and frame also lowers the perfor-mance of the glazing itself at the edge of the frame. This is the 'edge effect.' Around the edges of theglazing heat flows in two directions, not just one. As with the center of the glazing, it flows perpendicularlythrough the glazing towards the colder side. Due to the thermal bridge at the frame heat also flowslaterally through the glazing towards the edge.

This is intuitively obvious if you imagine a longwave infrared image of the window that would allow you tosee heat flow by highlighting the differences in the radiant temperature of the glass and the frame. Seenfrom the outside on a cold day, the center of a ‘superwindow’ would be a uniform cool color and the framewould be hot. From the inside the image would be reversed and the frame would be cold blue, a fact thatyou can verify by touch. The edge effect is the transitional zone between those two conditions. From thisdescription it also follows that the edge effect is not a set dimension but one that varies depending on theassembly. Stein and Reynolds (1992) cite an empirically derived average edge effect dimension of 2.5".

Overall U ValueIn the end, the overall U value is dependent on a number of variables. It depends on the size of the glazing,where the smaller the window unit the higher the ratio of perimeter to area and the more the edge effectsdominate the performance of the whole. It is also dependent on the number of panes, the gap spacingbetween panes, the content of the gap, the orientation of the glazing with respect to vertical, the emittanceat each surface, the indoor and outdoor boundary air conditions, and the operability of the window. All ofthese effects together are represented by the overall U value, a number that can be calculated for a givenknown glazing assembly using data available in ASHRAE Fundamentals Chapter 27 or derived experimen-tally using the level 3 protocol. The National Fenestration Rating Council provides overall U value ratingsfor the products that it certifies (see the NFRC sidebar in the Codes section of this package).

Figure 24 illustrates the range of performance possible from various combinations of frame types, spacertypes, numbers of panes and low-e coatings. The chart also highlights the difference between center ofglass performance and overall performance.

0

1

2

3

4

5

6

7

8

9

10

Double PaneSingle Pane Triple Pane Quad Pane

Glaz

ing

R va

lue

~ Hr

• S

F •

°F /

BTU

0 0 1 0 1 2 2

Figure 24: Center of Glass andOverall Thermal Resistance for aRange of Fenestration Products in aVertical Orientation. Adapted fromdata given in ASHRAE Handbook ofFundamentals (1993) Table 27.5.

Thin black lines represent center ofglass values. The low end of each lineis characterized by 0.25" gap spacingsand air filled gaps. The high end ischaracterized by 0.5" gap spacingsand argon filled gaps.

Thick grey lines represent overallunit values. The low end is of eachline is characterized by aluminumframes without thermal breaks andstandard metal edge spacers. Thehigh end is characterized by insulatedfiberglass/vinyl frames and insulatedspacers.

Bold numbers below each set oflines state the number of low-ecoatings for each assembly.

Frame

Center of Glass

Edge of Glass

Figure 23: Edge Effect. The frameand spacer in a glazing assemblyprovide a bridge for heat flow,reducing the total performance of theassembly. The thermal bridge coolsthe glass near the edge, creatinggreater heat transfer rates across theedge of the glazing.




The thin black lines represent a range of center of glass performance and the thick grey lines represent thecorresponding range of overall performance. Notice how with the higher performance windows the centerof glass values climb much faster than the overall values.

By looking at the center of glass values for each fenestration type, you can see the difference that low-ecoatings make. Look at the double or triple pane examples. Notice that the rate of increase at the top ofthe range is much greater than at the bottom: The low-e coatings have a far greater effect when used withlarge gap spacings and argon fills than they do with small gaps and air.

By looking at the relationship between a center of glass range and an overall performance range, you cansee the impact of the edge and frame on performance. Look at the quad pane, which has two low-ecoatings. Notice that an uninsulated frame and spacer brings the performance down to the range of anuncoated double pane unit.

The ranges illustrated are for vertical operable and fixed windows and vertical sliding glass doors. Slopedskylights are not included. The overall thermal resistance for sloped skylights is significantly less than forvertical windows of the same design.

MULTIPLE LAYER ASSEMBLIES

Increasing the number of layers of glazing decreases the U value of the glazing unit and also decreases itstransmissivity. This makes the design of multiple layer units a question of both finding appropriatecompromises between insulation and light transmission. Increasingly it has also meant finding ingeniousways of improving the performance of the individual components to avoid having to make these trade-offs.

Gap Spacing and FillGlass is a highly conductive material, while air isn't. The gap between the panes of an insulating glass unitincreases the U value by separating the layers of glass with an insulating layer of air or other gas. As thegap between glass layers increases, the U value decreases, until convection begins in the air (or gas). Inaddition to air, both argon and krypton are used as gas fillers in multilayer glazing assemblies. Air is the

Figure 25: "Center of Glass U- Factorfor Vertical Double- and Triple- PaneGlazing Units." From ASHRAE (1993)p. 27.2, Figure 1. Reprinted withpermission. Triple pane chart notshown.




least expensive choice, but argon is readily available as an industrial by-product and it has a lower thermalconductivity than air. Both argon and krypton are 'noble' gasses, meaning that they are inert and non-reactive.

Heat does flow through each of these gasses quite effectively through convection, however, which dictatesthat the space between panes be small enough to prevent convection currents from forming within thecavity. From a thermal standpoint, the optimal gap distance is the maximum that still inhibits convection,which for air and argon is approximately half an inch. Krypton, a rare and therefore expensive gas, has alower conductivity than either air or argon and it achieves this within a narrower gap. As seen in figure 24,the optimal gap spacing ranges between 0.25" for krypton in an assembly with a low-e coating to the morecommon 0.5" for air in either a plain or low-e coated assembly. Because of its expense, krypton is used onlyin 'superwindow' construction, where three gaps are created by the suspension of two separate filmswithin an insulated glass unit. The 0.25" optimal gap spacing of Krypton allows the overall dimension ofsuch an assembly to remain around an inch. A four layer unit filled with air would be approximately 1.75"thick overall.

Besides the losses that take place when convection currents become established within the gap, larger gapdimensions mean larger edge spacers with more surface area expanding the edge effect through contactwith the fill gas. More importantly from a practical point of view, thicker glazing units are difficult to use inconventional residential frames or in retrofit work. In practice, the maximum marketable thickness ofresidential glazing is between 3/4" to 1" outside face to outside face. Large glass units in commercialconstruction can be much thicker, depending primarily on the necessary thickness of the sheets of glassthemselves for structural and safety reasons.

As an aside, the optimal acoustical separation is achieved with gap spaces approaching six inches, whichsuggests two separate glazing assemblies rather than one sealed edge unit.

Additional Glazing LayersEvery layer of insulating air space that is created increases the thermal resistance of the assembly. For thisreason triple and even quadruple layer assemblies have been manufactured. The added weight makes suchproducts difficult to use in operable windows, and the added transmission loss does produce a noticeabledimming of the view, which is accentuated by the perceived thickness of the glass. For these reasons aswell as others, multiple gap glazing is now created by suspending one or more layers of low-e coatedtransparent film rather than adding additional layers of glass. The films have the same or slightly highertransmissivity as glass, producing a unit that is the same weight as a standard insulating unit and thesame thickness or only slightly thicker, depending on the characteristics desired.

These multilayer units are an ideal application of the 'softcoat' low-e coatings, which were first developedon films rather than glass for economy of manufacture. The films are difficult to see sealed within aninsulated glass unit, but the edge spacers have an obvious seam down the center where the film is held.Also look for a 1/16" circular hole through the film at one of the corners of the glass. This allows forpressure equalization, where otherwise the warmer side would expand and bow the film, causingdistortion.




Edge Spacer and Frame IssuesJohnson (1991) provides an in-depth description of both current edge spacer technology and potentialfuture directions. The field is changing rapidly, and the number of variations of techniques and materialcombinations is beyond this introduction.

The spacer must perform several functions; it must mechanically join the glass sheets and expand andcontract at the same rate as the glass so as not to break its bonds; it must contain the gas sealed withinthe unit and prevent water vapor from entering and fogging the unit. Ideally, it must also act as a thermalbreak.

For reference, the metal edge spacer assembly found in standard insulated glass does all of these thingsexcept provide the crucial thermal break. The metal portion of the spacer is a rectangular tube filled withdesiccants that insure that the gap stays dry. The perforations that are visible in an installed unit are thereto connect the gap with the desiccants. The tube is adhered to the glass with a primary sealant ofPolyisobutylene that seals in the gas and keeps out water vapor. In the deconstructed example of figure 27below, this seal can be seen as an inner black band. The fact that it is smeared on the glass that has beenremoved is an indication that it doesn't completely set but remains tacky to insure that the seal remainsintact. The end is then capped with a second seal of another variety of adhesive such as polysulfide orpolyurethane, which acts as the primary structural connection.

Figure 26: Low Conductance EdgeSpacer Design from Cardinal IGbrochure 8-93 10M. XL EDGE® is atrademark of Cardinal IG. Reprintedwith permission.

Figure 27: Dissected Insulating GlassSample Resting on the Sill of aSimilar Unit Installed in a Building.

The grains in the photograph are thedesiccant. These can often be heardrattling inside an intact sample.Notice the perforations of the spacer,indicating the presence of thedesiccant. Notice the two distinctseals; the inner seal is tacky and hasdesiccant pellets stuck to it. The outerseal is dry and of a lighter color.

Such a demonstration should not beundertaken without first verifying thatthe desiccant used in a given sampleis nontoxic. If this information is notspecified on the sample a MaterialSafety Data Sheet describing thedesiccant should be obtained from themanufacturer.




An interesting set of problems to note in closing is that as the thermal performance of the glazing getsbetter the stresses that are set up within the glazing unit are increased to the point where large higherperformance windows, such as found in commercial applications, must be constructed with temperedglass. This does not affect the VITAL SIGNS of the units per se. To prevent thermal stresses from buildingup locally within the glass, however, both the edge spacer and the frame should cover as little of the glasssurface as possible so the entire sheet is exposed to the same thermal forces. One intuitive response to thethermal bridging caused by the spacer is to bring the frame up around the glass a few inches. In practice,the differential heating and cooling of the center and the edge would cause the glass to fail.

This same logic holds for the handling of perimeter heating and air conditioning at the fenestration. Withhigher performance glazings care must be taken not to direct either hot or cold air at the glass in a way thatcould set up differential stresses. This translates in practice to holding the air supply away from the glassseveral inches and insuring that such things as curtains do not act to funnel the hot or cold air directlyagainst the glass. The more dramatic change that high performance windows have begun to allow is theelimination of perimeter heating loops altogether. The West Bend Mutual Life Insurance Building ( WestBend, Wisconsin. 1993) is an example of an office building where the elimination of the perimeter heatingloop paid for the extra expense of high performance glazing. For all of the reasons discussed, the glassremains comfortably warm through the Wisconsin winter.

Figure 28: "RepresentativeFenestration Frame U-Factors in Btu/(h ft2 F) - Vertical Orientation." FromASHRAE (1993) p. 27.4, Table 2.Reprinted with permission.

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