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Daylighting performance of buildings: 60 European case studies

The results of a three-year monitoringcampaign of buildings throughoutEurope are presented in this book. Thedaylighting behaviour of 60 buildingswas observed and measured. Varioustypes of buildings were involved, fromoffices to museums, libraries tochurches and more specific buildingssuch as airports or factories. Classicalbuildings (such as the Pantheon inRome built in 128 A.D.) are included,some of which are historicallandmarks (such as Ronchamp churchby Le Corbusier), others are morerecent (such as the Stansted AirTerminal by Foster & Associates). Thesizes range from one single room of 11metres (Anatomical Theatre,Göteborg, Sweden) to a large scaleoffice building (100,000 m2 floor areaof the Tractebel building, Brussels).The study shows the extraordinarypotential of daylighting techniques, toimprove amenity and energyperformances for the benefit ofbuilding occupants and managers.However, opportunities are oftenmissed, with performance ofdaylighting solutions sometimesoverestimated by designers, or withsignificant problems of overheatingand insufficient attenuation of glare.Above all, this monitoring campaignshows the broadscope of daylightingdesign, and the importance of carefulassessment of side effects which needto be analysed with the benefit ofexperience and knowledge of thephysical principles; and appropriatelymanaged.

A three-year monitoring pro-grammethroughout EuropeFor a period of three years betweenSeptember 1994 and August 1997, alarge scale monitoring programmewas conducted at European level,dealing with the daylightingperformance of 60 buildingsthroughout Europe. More than 10organizations were involved in the

task which required about 30 peopleto carry out measurements, processthe data and supply the results to thecoordinator. The task includedobservations, indoor luminousmeasurements for specific climaticconditions and calculation ofperformance indicies. For somebuildings, a specific Post OccupancyEvaluation study (POE) was

The understanding of the behaviour of a

building with respect to daylight requires themeasurement of specific parameters sometimesin locations with difficult access.

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conducted to assess the effects ofdaylighting strategies on theoccupants. In parallel, a simulationgroup conducted detailed energycalculations for six monitoredbuildings aimed at comparing theenergy saving potential of theproposed daylighting techniques witha reference case or other options.

Using reality as a source of infor-mationIn recent years, there has been agrowing concern about thedevelopment of tools to provideassistance in daylighting design. Theoldest and most used tool is still thescale model (light propagationfollows the same rules in a scalemodel and in full scale reality). Now,various computer programs havebeen proposed either to simulate thedaylighting behaviour of a buildingwith well characterized daylightingsources, or to assist designers in theirstrategic decisions. For any tool used,there is always some doubt about itsvalidity, and concern about its limitedfield of investigation.

It is clear that the performance ofdaylighting systems can be judgedobjectively and subjectively, and thatenergy aspects are partly visible andpartly invisible. However, only on-siteobservations and measurements candetect some aspects of daylightingwhich are difficult to predict withtools: exact final optical performanceof systems, rendering of the indoorspace, quality of views, dynamics ofdaylight, and above all, the globalimpression given to the visitor as wellas the occupants. Finally, it has beenfound that a database of buildings,some of them well known and most ofthem accessible to the public, wouldbe of great interest to buildingdesigners.

Selection of 60 buildingsThe 60 buildings were selected fortheir interesting daylighting features,either generally or at least regarding aspecific feature. Ease of access alsoplayed a part in their selection. Allparticipants made proposalsregarding the buildings they intendedto monitor and it was decided that 60buildings appeared to be themaximum achievable number withrespect to the allocated budget. Adecision was made to include

standard building configurations andnot to focus only on cases where thesolution was elaborate or unusual. Itwas decided to offer a large range ofbuildings and applications, withsolutions offering a large variety ofways to bring daylight into buildinginteriors.

A standard procedure for collectinginformationIn order to extract useful informationfrom the 60 buildings, a monitoringprocedure was established. It dealtwith a series of measurements andobservations, and with their analysis.The following is a summary of thetasks conducted:

Tasks

1 Geometric assessment (glazingarea/dimensions)

2 Daylight factor assessment, thoughthe simultaneous measurement ofindoor and outdoor illuminationunder overcast sky conditions.

3 Material characterization: measure-ment of reflections and transmissionsof opaque surfaces, glazing materialsand awnings.

4 Visual comfort assessment, throughthe determination of luminances inthe field of view of the occupants.

5 Homogeneity of daylightpenetration, through the comparisonof vertical illuminance measured inthe centres of rooms.

6 Assessment of luminous fluxpenetration through variousapertures, under standard overcastconditions (external horizontalilluminance equal to 10,000 lx)

7 Photography, with wide anglelenses and fish-eye lenses to displaythe patterns of daylight penetration inthe space, for diffuse light andsunlight.

8 Recording of occupants’ comments

9 Energy calculations for variousdaylighting options

The procedure developed for this taskrequired some training seminars tohelp teams to master the various

aspects of daylighting measurement.The sensors used by the teams werecalibrated together. However, errorscan still occur, particularly ifmeasurements are taken in non-standard climatic conditions. Therecording of material properties onsite also required some specificconditions of incident light on thesurfaces. It is clear that some errorsmay have still occured in themonitoring campaign, although agreat effort was made to minimisethese.

The importance of objectiveassessmentAny example of architecture can beconsidered as an optical elementallowing some daylight to penetrate itand then be reflected by the indoorsurfaces. In order to compare twobuildings, a reference light sourceneeds to be defined, regardless of itssignificance with regard to the localclimate. The best reference skycondition is a totally overcast sky, theluminance of which is larger near thezenith that near the horizon (typicallytwo to three times). Since we aremainly interested in relative values,such as ratios of indoor to outdoorvalues (daylight factors), or ratios ofindoor to indoor values (luminancecontrasts, homogeneity, etc.), theglobal brightness of such skies has noimportance, and measurements can beconducted any time during the year: asky twice as bright as the referenceone leads to indoor spaces two timesbrighter. This also means thatmeasurements under overcast skyconditions apply for Mediterraneanbuildings, even if such skies are rarein these climates.

Objective assessment helps us todiscover the difference between theperceived brightness of a space andthe exact amount of daylight used tolight it. This started useful discussionsabout the comparison of theperformance of daylighting systemsand about the role played by materialfinishes.

Finally, it was decided that objectiveassessment was probably the bestmethod for a large-scale monitoringprogramme, to reduce the differencesin assessment between participants,and to provide useful data to thelargest possible audience.

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A typical exercise which wasconducted for many case studies wasthe assessment of the luminous fluxprovided by daylighting systemsunder standard overcast conditions(representing an outdoor horizontalilluminance of 10,000 lx). This alloweda quick comparison of the amount oflight admitted by facade windowsand secondary lighting windows. Thevalue could also be compared withthe amount of light provided byluminaires to give an idea of thepotential of daylighting to replace forartificial lighting.

Simple systems often perform better.The amount of natural light entering abuilding is related to three majorfactors:1) the luminance of the section of thesky as “seen” from behind thewindow,2) the associated solid angle of thissection,3) the capacity of the window to bringdaylight inside (area andtransparency).

The final amount of light availableinside is related to the area of theabsorbing surfaces (by comparisonwith the window area) as well as theirreflectances, and particularly those ofthe surfaces directly hit by theincident daylight.

For this reason, most systems withadditional surfaces, even if reflective,tend to globally decrease daylightpenetration through the reduction ofthe solid angle of light collection (2) ,and adding additionnal lightabsorptions in process (3). This meansthat greater control of daylight oftenleads to a reduced overall luminousperformance. In this way, the bestperformance remains that of thehorizontal roof aperture collectingdaylight from a large section of thesky with very little obstruction. Butwe all know that protection againstsunlight and the provision of a viewto the external environment areneeded in most buildings.

For this reason, it was found that acombination of simple systems (roofand facade apertures for instance)performed better than advancedfacade systems attempting to deviatediffuse daylight deep into thebuilding through the addition of

Example of assessment of the origins of natural light fluxes entering a room per 10000 lm oftotal incoming light flux. This technique clearly displays the amount of light provided by thesecondary daylighting window by comparison with the facade window.

����� A combination ofsimple systems andbright indoor finisheswas found to performrather well.

����� Corridors of theReiterstrasse Officebuilding in Bern arelit with natural light,partly hidden fromthe field of vision,filtering betweenobstructions, leadingto a low luminance,glare free, butpleasant space.

340 lm

530 lm130 lm

13%53%34%

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�����In the bright Baroque church of the Neresheim Monastery, daylightfactors range between 1 to 1.5%, values higher than usually found incathedrals where values around 0.5% or below are common.

reflective surfaces.

Furthermore, complex systemsinvolving highly reflective surfaceshave performances which are verydependent on maintenance anddurability of the components. Dust,condensation or surface deteriorationquickly reduce optical efficiency,sometimes by more than 50%.

Some spaces perform well withrather low levels of lightIf we take away the cases for whichhigh illuminances are required (suchas about 500 lux on the a work planefor instance, or more for some factorywork), many spaces can appear bright

� �� �� �� �� � The Architect is while office in Athens is a good example of abuilding where facade window area has been reduced to a minimumsize because of the climate, still achieving a bright work space near eachwindow. The rest of the room benefits from ambient light admitted by a

central roof monitor equipped with shading devices.

at modest levels of illuminances. Thereason can be that the generalbrightness of the space is higher thanthat expected due to its function. Thisis the case in the church of theNeresheim Monastery, where indoorfinishes are particularly bright, andstained glass has been replaced withclear windows.

Circulation spaces frequently do notneed a high level of illuminance.Values of 10 to 50 lux may beacceptable as long as the eye of theoccupant is adaptated to theluminance of the indoor surfaces, andnot to the outdoor luminance, whichis usually much higher. This suggests

that apertures should be partlyhidden from the field of vision, withnatural light pouring in behindarchitectural elements.

Adding small apertures gives betterperformance than increasingwindow sizesIncreasing window sizes leads tomore daylight being admitted, but canalso cause more glare and moreconstraints regarding the shadingsystems. Beyond a certain level ofdaylight penetration, increasingwindow area may in fact generate farmore problems than the benefits ofletting some additional natural lightin.

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The translucent floors and ceilings of theWaucquez department Store in Brussels allowdaylight to penetrate deep into the building,because their area (comparable to the area ofthe glazed roof) compensates for the opticallosses associated with the triple absorption oflight coming from the outside. At groundlevel, daylight factors reach values above 1%.

����� These secondary daylighting windows are clearly undersized withregard to their potential to add extra daylight to the corridors from thedaylit offices.

In college “La Vanoise”, Modane, France, the most attractivedaylighting feature is the tilted secondary lighting system allowingdaylight to penetrate from the atrium roof to the interior. The tilt angleof the windows leads to a transmitted light equal to 3 times thattransmitted by vertical windows. Daylight distribution in classroomsbecomes balanced and the general impression is of a very bright space.

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�����In the Stansted Air Terminal, the rooflightelements associated with the supportingcolumns are the major aesthetic feature of thelarge square hall (120 m x 120m).

For instance, increasing daylightfactors from 3 to 4% (through anincrease in window area of 33%) willonly lead to an increase of thedaylighting period during the year ofless than 5%. The challenge thereforeis in bringing more daylight into areaswhere daylight factors are low, andlighting requirements high. Examplescan be found among the case studies,where roof apertures, secondarydaylighting windows or double sidelighting solutions are used instead ofsingle side facade solutions.

����� In the Trapholt Art Museum, Denmark,daylight is brought from the ceiling throughhanging canvas 3D forms, a unique andremarkable design.

Translucent FloorsSecondary daylighting is of greatinterest, as a well accepted way tobring daylight deep into a building.Internal walls or ceilings can betransparent or translucent. However,the light falling on these surfaces canbe 10 to 100 times lower than theilluminance on a vertical window onthe facade. This suggests thatsecondary daylighting can be appliedonly if the daylight factor on theconcerned surface is sufficient(typically more than 1%) and theglazing area should be as large orlarger than the original window areaon the roof or the facade. Ideally, asecondary lighting window shouldoccupy the entire surface of theindoor wall or partition.

Daylight, a large contributor to theamenity of the space.The daylighting system can also beconsidered as a piece of sculpture, inthe same way as chandelier behaveswith artificial lighting. It becomes anaesthetic object on its own. However,it needs to be assessed in terms of its

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�����Overglazed facades and poor shading maylead to indoor spaces with extreme glare oroverheating. Occupants may have no otheroption but to install their own protectivemeasures.

ability to fulfill visual criteria such asglare control (moderate luminances)and overall performance (productionof a luminous flux and distribution inthe space). Among the 60 buildingswhich have been studied, some ofthem have integrated aestheticelements, such as the rooflights ofStansted Air Terminal, UK, thehanging pyramids of TrapholtMuseum, Denmark, the occuli ofBibliothèque Nationale in Paris, etc.Sometimes they integrate artificiallighting so that artificial and naturallighting are associated in the finalrendering.

Sizing of daylighting systems needsto be based on visual specificationsfor the usersThe larger the window area in thefacade, the higher the risk thatoccupants will be exposed to glareand overheating in summer. Hence,shading devices, which must operatein glare situations and provide thedesired attenuation, have a crucialrole. If their luminous transmittance istoo high (above 10 % in general), the

risk of glare is significant, withluminance reaching more than 1,000cd/m2 for an illuminance on theawning of 40,000 lux.

Increasing use of computer screensmake luminance control more criticalVision on computer screens withtypical maximum luminance values inthe range of 80 to 120 cd/m2 issensitive to veiling reflections due toluminous elements around them

A daylight source generates highluminance. First, through direct visonof the sky (2,000 to 10,000 cd/m2)) orthe outdoor environment (similarvalues if lit by sunlight). Second,because it causes high levels ofilluminance on surfaces near theaperture (1,000 to 5,000 lux typically,and more if there is a sun patch). Ifthe receiving surface is bright(reflectance above 60 %), its corres-ponding luminance can reache valuesin the range of 200 to 1000 cd/m2.Even if the typical reflectance of acomputer screen is low (less than 5 %usually), the luminance of the

reflection on the screen may be 50 cd/m2 for a 1,000 cd/m2 light source, and100 cd/m2 for a 2,000 cd/m2 lightsource. This is significant bycomparison with the maximumbrightness (about 100 cd/m2) of thescreen. The resulting situation is aveiling luminance or reflection whichmakes reading the screen difficult.

Atria: buffer spaces which may alsowork as “light boxes”Various monitored buildings includedatria, designed mainly for thermalreasons, acting as buffer spaces withtemperatures warmer than outdoortemperatures in winter. While heatlosses can be reduced, overheatingneeds to be avoided through highventilation rates and proper shading.

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The addition of an atrium as arefurbishment of a courtyard maylead to a reduction of daylight to theadjacent windows by more than 50 %.

The main difficulty is that illuminancelevels on atria walls are much lowerthan on external facades (often a thirdto a fifth). Daylight falls on thewindows surrounding the atria at ahigh incident angle, leading to poorpenetration in the interior (daylightpenetrates well two metres typically).Also, shading on the atrium roof willaffect the amount of light available.Secondary daylighting windowsfacing into the atrium need to belarge, at least 50 % of the wall surface,to offer any significant contribution ofdaylight to lighting needs.

The role of surface finishes wasassessed. It was found that thereflectance of the floor of the atriumwas a significant factor in thedaylighting of the two lower floorssurrounding the atrium.

Specific problems at high latitudesAt high latitudes, the sun’s trajectoryis closer to the horizon. If one wantsto collect sunlight during the heatingseason, which is predominant acrossthe year, south-facing clerestories aremore appropriate than horizontal roofglazings.

On the contrary, sunlight is a serioussource of glare, and fixed overhangson south facades would need to belarge and would reduce significantlythe penetration of diffuse light fromthe sky.

However, the fact that the sun iscloser to the horizon leads to shadingfrom obstructions, which can besignificant throughout the year.

�����Multiple apertures tend to be preferable tocreate bright intriors. However, this may leadto more veiling reflections on computerscreens if the apertures are not hidden or ifthe surfaces receiving most of the light are toobright.

����������In the atrium of the Beresford Court officebuilding, daylight reflections on the brightfloor contribute substantially of the lightpenetrating the lower floor.

Thermal trade-off often affectsperception of global performance byoccupantsAlthough no specific monitoring wasperformed regarding the energyperformance of the 60 buildings, thethermal trade-offs have been part ofour concern: when it was possible,occupants and users wereinterviewed, and for six selectedbuildings detailed thermal analyseswere performed to assess the energyimpacts of the daylighting features.

The energy performance of thebuildings as they are today wascomputed using the energysimulation programme ESP-r (ESRU,1997) and comparisons were made forconfigurations without the daylight-ing features. The RADIANCEprogramme (Ward, 1993) was usedboth for the generation of opticalparameters for ESP-r and for specific

����� In Göteborg, Sweden, the south-facingclerestory above the atrium of a law courtbuilding collects low-level sunlight andreflects it downward.

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����� In Trondheim, Norway, the sun’s elevationabove the horizon is low. This leads tofrequent shadowing effects by neighboringbuildings, and severe risks of glare when thesun is visible.

assessment of glare issues.

For most cases, it was found that thesavings in annual lightingconsumption tended to be large, butthat the thermal impact is slightlypositive in the case of atria, butnegative with light redirecting facadesystems. When an increase in thermalloads was reported, they tended to besmaller than the benefits of savings inlighting electricity. However, glareand reduction of diffuse lightpenetration was found to be critical.For instance, glare was found to besignificant when atrium walls wouldreceive direct sunlight.

ConclusionWorking together within a group ofabout 30 people for three years hasled to the establishment of a commonknow-how in daylighting monitoringwhich will benefit all participants,their colleagues and institutes, while asignificant and valuable resource hasbeen created for building designersand daylighting specialists. No doubtthese references will also facilitate theadvancement of knowledge whenexperts and teachers will refer to themwhen explaining daylightingprinciples. It is expected that otherdaylighting monitoring campaignswill be launched in the future, and bemore ambitious in terms of theirassessment of performance.

AcknowledgementsThanks are due to: Vincent Berruttofrom the National Engineering Schoolof State Public Works (ENTPE,France) for his contribution in theselection of the case studies, and thecoordination of the monitoringprocedure; Pascale Avouac-Bastie(ENTPE, France) who had the tedioustask of collecting the data provided byall participants, checking the quality,and modifying the graphs whennecessary; and John Goulding (UCD,Dublin) who provided assistance inediting and reviewing text. The taskwas conducted with the financialcontribution of the EuropeanCommission, Directorate General XIIfor Science, Research andDevelopment.

ReferencesESRU, Energy Simulations ResearchUnit “ESP-r, A building and plantenergy simulation environment, userguide version 9 series”, ESRUpublication, University of Strathclyde,Glasgow, 1997.

Ward G., The Radiance 2.3. SyntheticImaging System, Lawrence BerkeleyLaboratory, 1993.

List of ParticipantsBuilding Research Establishment, WatfordUnited KingdomConphoebus, CataniaItalyFaculty of Architecture, CambridgeUnited KingdomThe Martin Centre, CambridgeUnited KingdomDanish Building Research Institute,CopenhagenDenmarkFraunhofer Institut für BauphysikGermanyBBRI, BrusselsBelgiumEDAS, Dept of Architecture, GlasgowScotlandEnergy Research Group University CollegeDublin, DublinIrelandRoyal Institute of Technology, GävleSwedenBuilding Services Engineering,SwedenDanish Technological Institute, CopenhagenDenmarkESBENSEN Consulting Engineers,CopenhagenDenmarkChalmers University of Technology,GöteborgSwedenLNEC, LisboaPortugalSimos Lighting ConsultantsSwitzerlandA.N. Tombazis & Assoc. Arch.GreeceNational Observatory of AthensGreeceESRU, University of StrathclydeScotlandFaculty of Architecture, TrondheimNorwayNorwegian Electrical Power ResearchInstitute, TrondheimNorwayLESO -PB, EPFL, LausanneSwitzerlandUPC - Architecture, BarcelonaSpainEcole Nationale des Travaux Publics del’Etat, LyonsFrance