Atrium Comfort Design

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64arq: Vol 1: spring 1996envi ronment

Climatic responsive atrium design in EuropeDennis Ho24 St. John's RoadLondonNW11 OPJUnited Kingdom

This paper considers the relationship between atrium design and different climatic conditions inEurope. Th e analyses may be used to inform design proposals for buildings which seek to optimisethe thermal buffering characteristics of atria. Particular attention is given to certain parameters witha potential to fo rm climatic responsive and energy efficient atrium buildings.

The potential energy savings of atria are now widelyackn owle dged . Apart f rom act ing as thermal buf fers toreduce winter heat loss through the internal atrium facades(the separating walls between the atrium and the mainbuilding), atrium spaces can offer spatial and visualamenit ies to otherwise monotonous deep plan bui ldings.The atria, however, can also increase the overall energyco nsu mp tion of the buildings if they are artif icially l it and air-condi t ioned to meet co mfort cr iteria such as those forcont inuously oc cupie d spac es (Baker, 1992).

It is, of course, incorrect to assume that an atriumbuilding which works well at one latitude wil l also work wellat others. Atr ia, for instance, should catch the sun an davoid the breeze in winter in a northern climate and avoidthe sun and catch the breeze in a southern climate.

The aim of this research was to gain an understandingof the relationship between atrium design and differentclimatic conditions. It examines the possibil i t ies of varyingcertain parameters in atrium de sign, such as bui ldinggeometry, orientation, envelope design and venti lationstrategies, thereby suggesting strategies for the design ofenergy efficient and climatic responsive atrium buildings.

It is impossible to give an analysis based on all thepossible com binat ions of atr ium form an d cl imate aroundthe wo r ld. As the single European market is providing moreopportunit ies for architects to practise in the EC incountries other than their own, it is obvious that anunderstanding of ho w bui ldings respond to the di fferentcl imates wi thin Europe is mu ch n eeded. Special emphasis

is, therefore, placed on a few generic types of atriumbuilding in different climatic zones in Europe.Methodology of the analysesThe parametric and compa rative analyses are divided intofour stages. First, a series of generic atrium types [Fig. 1] isexamined. Sec ond, the effect of variation in th eperformance of the internal atrium faca des such as glazingratio, shading strategies an d thermal performance isanalysed. Third, various m odes of venti lation strategies arestudied. Finally, recommendations on energy efficient atriumdesign for four climatic zones are given. Here, an attem pt ismade in the classification of the climatic zones based ontheir temperature, solar radiation and sky luminancecharacteristics. The analyses ado pt the classificationsystem use d for the LT me thod (Baker, 1991).1 The fourclimatic zones [Fig. 2] are represented by four locations,and their mos t relevant characteristics are as follows:

Zone A Temperate Climate - Mid European C oastal e.g. Kew(5128'.0019'W): m ild winters with low solar radiation , mildsummers.Zone B Northern Climate - North European Coastal e.g. Stockholm(5921 '.1757'E): cold winters with low solar radiation and shortdays, mild summers.Zone C Continental Climate e .g. Stuttgart (4850'.0912'E): co ldwinters with high radiation and longer days, hot summers.Zone D Southern Climate - Southern and Mediterranean e.g. Milan(4526'.0917'E): mild winters with high radiation and long days , hotsummers.
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6 5arq: Vol 1: spring 1996envi ronment

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The analyses are based on a stat ic compu ter m odel , theATRIUM model . The model responds to var iousparameters such as orientation and form in a simplif iedway. Although the ATRIUM model has its l imitations, it isspecif ic enough to inform early design decisions andgeneric enough to allow individuality in the design to bedeveloped (Baker, 1988).2 A list of the main parameterscan be foun d in note 3.

For the purpose of the study, criteria based on themonthly neutral temperatures of the atrium, thetemperature at which most people feel comfortable undersedentary conditions, are used." The aim is to achieve theneutral temperatures within the atria with minimum energyconsu mpt ion. Due to the vast amount of data, a selectionof the results which serve to demonstrate the principles ispresented in this paper.Building form - shape and proportionIn passive atrium b uilding de sign, the shape a nd proportionof the atria primarily influence the amount of heat transferbetwee n the atria and the amb ient environment. Forexample, a compact form with low surface area to volumeratio (SA/) has relatively small heat gain and loss. In general,results of the analyses confirm that there are seasonal shiftsin the atrium temperatures.

By covering the cou rts forming atria, the temperaturesof the atria are higher than the am bient temperaturesthroughout the year. This temperature rise is much morenoticeable during the winter thus confirming the thermalbenefit of atria in winter. It has also been found that atria ina tem perate climate (zone A) have smaller seasona lfluctuations than those in the m ore extreme climates (zonesB, C and D). Thus, temperature fluctuations in atria areclimatic d ependen t [Fig. 3]. Atria in more extreme climatesrequire more sophisticated controls to minimisetemperature fluctuations.

The m ajor temperature variations are found t o o ccu r inthe different generic types of atrium. Whe n com pared withcentralised and linear atria (types A and F), atria with south-facing facades (types B a nd D) have relatively high summe rtemperatures due to excessive heat gain but relatively lowwinter temperatures due to substantial heat loss. Thistemperature cha nge is particularly marked in summer. Thisis most no ticeable in the northern climate (zone B) wherewinter ambient temperatures are very low and sunlighthours are limited [Fig. 4]. Hence, the implications of externalatrium glazing in atria are more m arked in the extreme

N AF/BF =1:1 AF/ BF= 1:3 AF/BF = 1:I DR = 3.2 PR = 4.5 PR = 5.6nnnR = 1 . 3 PR = 1.6 PR = 1 . 8

Climatic responsiveatrium design inEuropeDennis Ho

I I II IIPR = 1.2 PR = 9 1 PR = 3 9

PR = 1.1 PR = 2.1Atrium

1 Main building

A. Centralized

B. Semi-enclosed

C. Semi-enclosed

D. Attached

E. Attached

F. Linear

G. Linear

1. Plans of thestandard genericatrium types used inthe analyses.AF/BF = atrium floorarea to main

building floor arearatio. PR =protectivity ratio, i.e.ratio of the totalinternal atriumfacade area to the

total external atriumglazing area.2. Europe showingthe four climaticzones used in theanalyses.



66arq: Vol 1: spr ing 1 996environment

Climatic responsiveatr ium design inEuropeDennis Ho

Continental climateNorthern climate

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DECMonth

Zone ASa

Zone B Zone CClimatic zones Zone D


Zone A5b

Zone B Zone C Zone DClimatic zones

cl imates. The m ore exp osed th e atr ia, and the colder theclimates, the lower the atrium temperatures in winter.

In addition, the fall in atrium tem perature in winterincreases as the atrium floor to main floor ratio (AF/BF)increases, i.e. increasing the size of the atrium withoutincreasing the main floor plate, particularly in a northernclimate. This is because the total heat loss through theexternal atrium glazing is often more than heat gainthrough th e sam e area dur ing the heating season. Thisleads to a net increase in winter heating load when theglazing is not protected from excessive heat loss [Rgs. 5aan d 5b ]. This temperature cha nge is more not iceable inenclosed atr ia. Hence, a balance is of ten needed betweenthe op t imu m a mou nt of glazing for passive solar heatingpurposes and that required to l imit heat loss.

Further analyses indicate that by elongating anattached atr ium ( type D) to twic e i ts length, summ ertemperatures increase markedly while winter atriumtemperatures increase by only a negligible amount. Thewinter thermal benefits of elongated atria are thus offset byadditional cooling requirements. This evidence furthersugges ts that energy savings d o not necessar ily improvewith larger atria.

In an urban environment whe re there is often a tendency toincrease the height of the buildings, it is interesting todiscover that by doub ling the height of the atriumbuildings, the average winter effects in all the genericatrium types are negligible, while the su mme r a triumtemperatures drop depending on the climatic zones andthe generic atrium types [Fig. 6]. The greatest summ ertemperature drop occurs in the change of height in thecentralised atria (type A) in the southern climate (zone D).This is mainly due to the self-shading effect the tallerbuilding is now providing. Thus, compact and taller formscan b e advantageous in reducing the overall summ ertemperature rise in the atria, particularly in low latitudelocations. The temperature at the lower levels within theatria will also be much lower than that at the higher levelsthus increasing the p ressure difference, which in turnenhances the natural buo yancy effect for natural ventilationpurposes.

It is interesting to compare the above observationswith protectivity ratio (PR) of the different generic atriumtypes. P rotectMty ratio is defined as the ratio of the totalinternal atrium fac ade area to the total external atriumglazing area.5 The greater the extent of external glazing



67arq: Vol 1: spr ing 1996envi ronment

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3. Graph showingtemperaturevariations for semi-enclosed atria withAF/BF=1:3inthefour climatic zones.4. Graph showingtemperaturevariations for theatrium types withAF/BF=1:3inthenorthern climate.5a. Bar chartshowing the effectof variation in theatrium floor to mainbuilding floor ratio(AF/BF)ofcentralised atria onJan/Dectemperaturedifference in the fourclimatic zones.Sb. Bar chartshowing the effect

Linear atrium

of variation in theatrium floor to mainbuilding floor ratio(AF/BF) of attachedatria on Jan/Dectemperaturedifference in the fourclimatic zones.6. Graph showingtemperaturevariations for atria ofdifferent heights inthe centralised andattached atria in thesouthern climate.7. Axonometricviews of four typesof atria. Theprotectivity ratio(PR) equals the ratioof the total area ofinternal atriumfacades to the totalarea of externalatrium glazing.

and the shallower the atrium, the smaller the protectivityratio [Fig. 1, Fig. 7]. Hence, the area of the main bu ildingprotected f rom the external environment is less for thesame amount of glass (Baker, Hawkes, 1987). Thissupp orts the results whic h indicate that centralised atria(type A) with high protectivity ratio have relatively smallseasonal temperature swings when compared w i th typesB, C, D and E which have lower protectivity ratios.

More detailed analyses indicate that the seaso naltemperature swings of atria in a temperate climate (zone A)are relatively small comp ared w ith those in the other threeclimatic zones. The centralised and linear atria (types A, Fan d G) have the smallest seasonal tempe raturefluctuations and their overall temperature performa nceremains relatively close to the neutral temperatures,particularly in summer. On the whole, the centralised andlinear atria pro ve to be the most favourable in thetemperate climate in terms of both summer and wintertemperature swings.

In the northern climate (zone B), precautions a gainstexcessive heat losses inflicted by the atria sho uld betaken. Solar gain in winter cannot reduce the peak heatingload by a substantial amou nt since solar energy isavailable in appreciable amounts only in the autumn andspring. However, high latitude locations can bene fit fromsolar energy for a longer period than the southerly regionsbecause of the longer heating season. This, on the whole,make s atria viable as passive solar systems. The analysesindicate that spring and autumn atrium temperatures in thecentralised and linear a tria (types A, D and E) are lowcompared wi th those in a south-facing attached atrium(type D). However, the centralised and linear atria have theleast seasonal temp erature fluctuations while the south-facing attach ed atria (type D) have very high summertemperatures due to solar gain from low angle sun . Ittherefore se ems that enclosed a tria with steeply inclinedroof glazing to optimise solar gain are most favourable.

The co ntinental cl imate (zone C) is characterised bythe large diurnal temperature swings and relatively highradiation in summer and winter. The analyses indicate thatthe cen tralised atria, with their highest winter tem peratureand summ er temperatures that fol low the co mforttemperature most closely, are most appropriate. South-facing sem i-enclosed atria (type B) can also be viableopt ions as their winter temperatures are high. Sh ading is,however, important during the cool ing season. Sou th-facing attac hed atria (type D) have relatively low wintertemperatures and very high summer temperatures, makingthem relatively undesirable.

For the southern climate (zone D), the results indicatethat all of the atrium types have the potential of raising thewinter atrium temperatures relatively close to the comfor tlevel. The p roblem , however, lies in summer overheat ing.Even with the centralised atria, which have the smallesttemperature fluctuations, the atrium temperatures insummer can sti l l be very high due to the large amount ofsolar radiation on the roof planes. The roof should ,therefore, preferably be north facing. On the who le, theredoes not seem to be a real energy-saving benefit inadopt ing atr ium design in low latitude locations. Thebenefit of a winter thermal buffer has to be balanced outby the need for mechanical cooling in summer. A south-






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facing glazed roof would also require a substantial amountof shading in summer which in turn reduces the overalldaylight penetration. It is therefore recommended that atriain the sou thern cl imate shou ld be designed so that theycan be converted into opened spaces in summer by usingretractable roofs. In atria where retractable roofs are notappropr iate du e to external condi t ions such as noise andrain, summ er atr ium vent i lat ion should be minimised toreduce the ingress of hot external air. Heat gain during theday ca n be col lected using thermal storage and released atnight which in turn encourages an increase in thermalbuoyan cy for night- t ime vent i lat ion.OrientationBuilding orientation affects the indoor cl imate in two mainrespe cts. First, solar radiation from different orientationsthrough the facades has a substantial effect on the internalenvironment. S econ d, the relationship betwe en thedirection of the prevail ing winds and the orientation of thebuilding w il l influence na tural venti lation within the building(Olygag, 1963). If we assume that venti lation takes placeunder natural buoyancy, it becomes the first factor that theanalyses are most concerned wi th.

In term s o f energy efficiency, orientation for the atriadepends on the heat ing requirements and the occupancyper iods. For max imum heat gain during heat ing season,the external atrium glazing should preferably be south-easterly orientated if morning heating is required, as in anoffice space. A south-west orientation is preferred ifafternoon heating is required in a house where the mainoccu pan cy per iod is of ten f rom the af ternoon to the nextmorning. Atria facing west may receive excessive solar gainin the afternoon, particularly in summer when the buildingsare already he ated an d am bient temperatures are high.Atria with their facades facing north require higher energyloads due to lack of solar gain and should, therefore, beavoided.

In the southern climate where the sun angle is relativelyhigh, horizontal glazing wil l receive mo re solar radiation insummer than an inclined roof. Horizontal glazing in lowlatitudes should, therefore, either be well protected fromsolar gain or avoided. In extreme southern climate, the

roofs should face away from the sun but catch theprevailing winds for ventilation purposes.

For the centralised and linear atria (types A, F and G),the design of the roof (its thermal performa nce and angle ofslope) is relatively important particularly in the northernclimate as discussed before. Trie angle of pitch shouldrelate to the sun angles during spring and autumn as theseperiods can benefit from solar radiation for a longer periodthan the winter seaso n. In general, the slope of the roofshould be perpendicular to the angle of incident.

The analyses also indicate that linear atria with theirlong axis running north/south have relatively high atriumtemperatures when compared with those in which the axisruns east/wes t. This is because the s un pe netrates into theground level throughout the winter, although it is limited tothe couple of hours about noon due to self-shading by themain building. This type of atrium may be beneficial increating a psychological w arm th effect particularly in thewinter of a northern climate.

After the first stage analyses, it becom es clear that thecentralised and the l inear atria seem to be the mostappropriate in the four cl imatic zon es. This suppo rts thenotion that covered courtyard buildings and arcades aremost effective climate modifiers, variations in the shapeand proportion of an atrium wil l also influence the heattransfer between the atrium an d the main bu ilding.Glazing ratioMost glass has relatively little insulation value and offerslittle protection from radiation. In designing atrium facades,the energy balance between useful natural light, usefulsolar gains, heat losses and other factors, su ch asventilation, view out and privacy, must be co nsidered. Here,the analyses focus o n the g lazed areas in the internalatrium facades that affect heat and l ight transmissionbetween the main building and the atrium.

In general, by increasing th e am ount of glazing in theinternal atrium facades, the atrium temp eratures in w interincrease slightly. This is due to heat loss from the warmermain building to the c ooler atrium. Fu rthermore, theincrease in glazing ratio mean s that less opaqu e surfacearea is now available for storing solar gain. The reverse is




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true for summer when the atrium temperatures lowersubstantially d ue to heat transfer from the warme r atrium tothe coo ler main building. It, therefore, seems that thehigher the glazing ratio, the smaller the temperaturefluctuations in the atrium and the more acceptable is theatrium environment. The overall effect results in greatertemperature s wings in the main building further increasingits heating and co oling loads. In the southern climate, thereduction in glazing ratio results in very high summer atriumtemperatures [Rg. 8]. This has further implications forroofing over an e xisting co urtyard of a heavyweightbuilding.

It is generally acknowledged that some forms oftemperature fluctuation are beneficial for thermal co mfort(Givoni, 1976). However, great and rapid temperatureswings cause discomfort and are subsequently moredifficult to deal wi th. In cases wh ere strictly controlledtemperature swing s are required in the main building, theuse of a high glazing ratio would therefore seem t o be evenless beneficial. Furthermore, atria with temperature swings

8. Graph showingtemperaturevanations fordifferent glazingratios in sem i-enclosed atria withAF/BF=1:3inthesouthern climate.9. Bar chart showingthe effect on atriumtemperatures ofchanging g lazingratios from 20 % to80 % in semi-enclosed andattached atria in thefour climatic zones.10. Cross sectionthrough a centralatrium. The thermal

performance of theglazing should varyaccording to thesolar gain receivedby each bay.11. Cross sectionthrough three centralatria.11a Shading isprovided to theoutside of the atriumglazing.11b. Shading isprovided to the t opbut inside the atrium.11c. Shading isprovided to theindividual bayaccording to solar

within acceptable levels wil l be perceived more astempered external spaces, thus reducing th e de man d forstringent com fort criteria.

Whe n the glazing ratio is tested ag ainst th e differentgeneric types of atrium, the analyses indic ate that, in all theclimatic zones , variations in glazing ra tio have th e greatesteffects o n the more enclose d atria [Fig. 9]. The reason l iesin the protectivity ratio. A larger amou nt of glazing in theattached atr ia tends to 'even out ' the tem pera turefluctuations. Hence, the glazing ratio in the more enclosedatria has a more important role to play in controll ingtemperature swings in the main buildings. Paradoxically,enclosed atria provide greater self-shading and thedemand for a larger amount of glazing for daylightpurposes can be relatively high.

The effects of variations in glazing ra tio in the internalatr ium facades are found to be more mark ed in the moreextreme climates. For example, the reduction in summeratrium temperatures due to an increase in glazing ratio in asemi-enclosed atrium (type B) is small in a temperateclimate bu t relatively high in a continen tal cl im ate. Th isreduction in summer atrium temperature may, init ial ly, seemto b e beneficial in the continental cl ima te. Th e result,however, is again deceptive as most of the heat has nowgone into the main buildings result ing in higher coolingload. Glazing ratio shou ld, therefore, be l imited to redu cewinter heating and summer cooling loads in the mainbuilding, particularly in the southern and northern climates.

In the analyses, it is diff icult to dete rmin e a n op timu mglazing ratio for each climatic zone give n all the variousparameters and requirements such as l ighting, view outand privacy. There is an argument for maximum glazingratio for maximum natural l ight. This, howev er, see msfragile. The LT curves6 show that there is a threshold l imitbeyond which an increase in glazing ratio would only resultin a very small saving in artif icial l ighting bu t co oling loadincreases substantially result ing in a n ett increase in energyconsum pt ion. However, in modern com merc ial b ui ldingswhere there are often high internal gains and l ighting is themain cause of energy consumpt ion, i t may be preferable toincrease the amount of glazing for daylight purposes butusing high performance glazing to limit heat gain and loss.




70arq: Vol 1: spring 1996environment

Southern climateSjemi-iencl6sed iatriiin -NprtheiSemi

rn cgmatfeii-enclqsed atrit

SbutheiCentr;'TndimatJBitised atriym--i-i-yf 7I \

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14Zone B Zone C

Climatic zones

One interesting idea is to vary the amount of glazingaccording to the solar gains on the internal atriumfacades. For example, areas of the facades at high levelwould require a smaller amount of glazing due to theincreased amount of daylight and solar gain. The glazingratio can be increased towards areas at the corners andat low level where daylight penetration and solar gain areoften small due to self-shading from the main buildings[Fig. 10]. Thermal performance of the glazing can also bereduced accordingly.Shading requirementThe primary reason for solar shading is to provide thermalcomfort by reducing unnecessary solar gain. Designparameters include or ientat ion, sun path, sun angle,daylight transmission, venti lation, user control,maintenance and cost . Al l these parameters vary

according to the climatic characteristics and the functionsof the spaces (Goulding, Lewis, Steemers, 1992).

A common dilemma in shading of atria is whether toshade the atrium roof and external atrium glazing or theinternal atrium facades [Fig. 11]. The justification for u singeither of the shading options is unlikely to be based onsolar gain and daylight factors alone, the case may also bema de on architectural or aesthetic reasons. In general, themost effective way to reduce solar gain to the atrium is toprovide external shading to the atrium roof glazing andexternal atrium glazing. As a consequence, the overallday light levels in the atrium and the main building aresubstantially reduced. Hence, this strategy is mostapp ropria te in places where levels of solar radiation anddaylight luminance are very high, i.e. the sou thern climate.

In the southern c limate, shading should primarily b econce ntrated on the roof due to the high angle sun.Op aq ue shading devices or egg-crate type shading maybe used without reducing daylight transmissionsubstantially. Alternatively, north-facing sa w-too th roof orclerestory roof l ights can also be use d. The analysesindicate that an extensive amount of shading is required tomaintain the comfort temperatures in the south-facingexp ose d atr ia. The extra cost of such facades over therelatively small benefit of winter thermal buffering mayprove to be a determining factor.

In places where there are smaller risks of overheatingin summer and l ighting is the major cause of energyconsumption, for example in off ice buildings in the northernclimate, shading should be located outside the internalatrium facades only where overheating and glare due todirect sunlight may occur. Devices such as translucentretractable blinds posit ioned away from the occupants tored uce re-emission of heat and a llowing diffuse daylight toenter the main building, may be used. This has theaddition al benefits of increased adap tabil ity an d individualuser control. Performance glass with a high solar shadingcoefficient and daylight transmittance can then be used inth e atrium roof glazing and external atrium glazing tocompensate for the loss of solar protection in the atrium.

In general, the analyses confirm tha t sh adingrequirement increases towards the lower latitude. Shadingrequirem ents for centralised atria in the s outhern climateca n be as much as three t imes more than those in anorthern climate. Shading is also required for a muchlonge r period of t ime in the southern climate. For sem i-en clos ed atria (type B), the propo rtional increments ofsha ding requirements f rom winter to summe r on theexte rnal vertical atrium glazing increase towards the highlatitud e locations [Fig. 12]. This is because the ratio of solarradiation on the vertical south-facing facades to the totalsolar radiation is greater in high latitude locations than inlo w latitude locations. This suggests tha t the flexibil i ty ofthe shading devices should increase as the amount ofexte rnal atrium glazing increases, particularly in thenorthe rn cl imate.

Heavyweight versus lightweight enclosuresThermal mass determines the rate at which heat isab so rbe d o r released from a material. It can assist inreducing indoor temperature swings and slow down a



71arq: Vol 1: spr ing 19 96envi ronment

building's response to a mbient temp erature fluctuations,reducing daytime cooling and night-t ime heatingdepend ing on the climatic characteristics a nd the users.The choice of whether thermal mass or insulated panel isused often depends on whether collected solar heat is tobe used to condition the atrium (Watson, Labs, 1983).

In the northern and tem perate c limates, the use ofthermal mass is considered to be less appropriate thanwell insulated panels, particularly in centralised atria wheredirect solar gain to the internal facades is limited. First, thetime lag property of the thermal mass may result in a lowutilisation of the winter solar gain. Second, the lack of solarradiation in winter make s the use of heavyweightconstruction as a storage element unreliable. Third, thewarm ing up period is greater for heavyweight tha nlightweight con struction. There will be less 'exces s' heatfor the main building to use . Fourth, well insulated panelshave potentially higher radiant temperatures than masonry.Thermal mass does not influence spaces with continuousheating and occupancy periods. Thermal mass, however,can retain the heat but extend the warming up period inspaces with intermittent heating and occupancy periods.Hence, well insulated panels may be the most appropriate.

For the continental cl imate, thermal mass can be usedto reduce temp erature fluctuations an d to act as a solarcollector. This thermal mass should be distributed in thefloor and the south-facing internal facades of expos ed atriawhich receive direct solar gain, while other internal facadescan be of well insulated materials. In more enclosed andcompact atria, lower thermal mass combined with suitableinsulation properties will lead to a faster response whichcou ld be more useful on days with poor solar radiation. Inaddition, the use of an exp osed slab soffit in the mainbuilding may b e sufficient to reduce tem perature swingsand cooling loads.

In the southern climate, hea vyweight con struction ismost appropriate. Peak temperatures may be shifted fromthe early to the late afternoon after the oc cup ied pe riod.The atria will then be com e relatively co ol during th e dayand warm during the night. Summer night-t ime venti lationmay also be used to cool the thermal mass, leaving theatria relatively cool for the next morning. This consequentlyreduces the radiant temperature of the wa lls and a higheratrium temperature may, therefore, still be relativelycomfortable. In most cas es, night-t ime venti lation can beachieved using natural venti lation. In places wh ich cannotnormally be reached by simple venti lation openings, suchas the central zones, plenums or mechanically drivenventi lation can be use d to direct air f low into the spa ces[Fig. 13]. The plenum s can be in the form of f loor voidswhere the thermal mass of the exposed structural slabsabso rbs heat and cools the incoming air further.

On the w hole, the analyses indicate that well- insulatedinternal atrium facades increase the atrium temperatures insummer. In the northern climate, the winter atriumtemperatures lower, hence less heat is transm itted from themain building to the atrium. In the southern climate, theeffect is negligible. Hence there is little benefit in using well-insulated panels in atria in the southern climate.

It must be emphasised that thermal mass is a veryimportant element in any direct or passive solar collection

Atriu m to bu ild in g Re -circula tion Building to atrium

Winter Mid-season Summ erRe-circulation Building to atrium Independent

16

17

Northern climate

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Cont inenta l d ims12. Graph showingannual shadingfactor variations forcentralised andsemi-enclosed atrian the four climaticzones.13. Cross sectionthrough an enclosedatrium showing howa plenum can beused to providenatural ventilation tocentral areas in

Temperate climateo\

LIJIe Southern climateenclosed atria whichcannot be reachedby conventionaltechniques.14. Bar chartshowing annualheating and coolingloads for the differentventilation modes inthe four climaticzones.15 . Schematic crosssection showingventilation modes for

the northern,temperate andcontinental climates.16 . Schematic crosssection showingventilation modes forthe southern climate17. Schematic crosssection showingdiagrammaticregional models forthe four climaticzones.

system such as an atr ium. Al though the ATRIUM mode lcannot examine the effect of thermal mass on atriumtemperature in detail, it al lows a simple and comp arativeanalysis to be made. Other important factors to beconsidered when uti l ising thermal mass include the thermalcapacity of the thermal mass, the strategic posit ion andarea of the m ass, the am ount a nd direction of air f low fornight-t ime flushing, occupancy pattern and period.Ventilation couplingDifferent mo des of venti lation coupling betw een the atriumand the main building may be used to optimise the thermalbuffering of the atrium. To select an app ropriate ven ti lationmode for the different seasons, one needs to consider theclimate of the s ite, and the e nvironmental criteria of theatrium and the main building (Baker, 1988). It is alsoimportant that openings be adjustable to vary the air f low






patterns and ventilation rates according to the comfortcriteria.The energy analyses indicate that the most effectiveventilation strategies for atrium buildings in the northern,temperate and continental climates (zones A, B and C) aresimilar [Fig. 14]. The greatest energy savings on the annualheating load of the main buildings can be made by usingan atrium to main building ventilation strategy inwinterwhen w arm air in the atria is used to supp lement heating inthe main buildings [Fig. 15]. A re-circulation strategy canbe used in autumn and spring where air is re-circulatedfrom the atrium to the main building. The period wh en there-circulation strategy can be used increases towards thehigh latitudes as the heating season for thes e locations isgenerally longer. Cooling load can b e minimised by usingpartial re-circulation ventilation in summer when air from

the main building is extracted through the atrium andexhausted to the outside through high level openings in theatrium roof. When the atrium temperature is much higherthan the m ain building temperature, independent ventilationshould be used to avoid mixing the warm atrium air withthe coo ler main building air.In a southern climate, the energy analyses indicatethat the total annual heating and cooling loads for theatrium buildings are similar to buildings without atria. Themajor concern lies in the cooling of the atrium [Fig. 14].In summer w hen daytime temperature is often wellabove body core temperature, ventilation should beminimised to keep the hot air out of the atrium . This

certainly contradicts the increased am ount of ventilationrequired to lower the atrium and main buildingtemperatures. He nce, air conditioning w ill be requiredwhich in turn increases the energy consumption. In lessextreme conditions, evaporative cooling can be achievedby placing water features outside the building to pre-coolincoming air. In addition, wa ter features placed inside theatrium can reduce the atrium temperature and create apsychological cooling effect. Broadly speaking, thebuilding to atrium ventilation mode is the mostfavourable for m ost of the year w hile the atrium tobuilding ventilation mode may only be used for a muchshorter period of time. In summer, the atrium and the

main building should be ventilated independently toavoid overheating [Fig. 16]. This clearly indicates that theadvantages of atria as climate modifiers in the southernclimate are significantly smaller than in the other threeclimatic zones.Regional modelsAll the above analyses confirm that energy efficient atriumbuildings are climatic responsive. Design parameters suchas geometry and orientation of the atrium buildings,performance of the envelopes such as glazing ratio,shading strategy and thermal inertia, and ventilationstrategy should vary according to the climaticcharacteristics and functional requirements, which in turnwill influence the energy consumption of the building.Based on the analyses, general observations can bemade on selected design elements for the four climaticzones. It must be stressed that the results are based on alimited number of tests and each model must also takeinto account factors such as rural or urban location,functional and aesthetic requirements. The four regionalmodels are summarised in tables 1 to 5 [also see Fig. 17].ConclusionsThe development of atrium buildings has so far beendiversified and inspiring. A wide range of strategies isnow available for the designers. In order to maximise thethermal buffering characteristics of atria, the designersmust understand the local climatic conditions and howdifferent atrium types respond to the climate. This in turnmay generate true regional forms which bring the internalenvironment closer t o nature. The current trend in atriumdesign tends toward a coherent approach that unites thebuildings and the a tria into single environmental systems.The building forms and fabric have to work m uch closerthrough the use of modern materials and analyticaltechniques. The choice of building envelope andventilation strategies should therefore be carefully chosenaccording to the nature of the site and the environmentalcriteria. Only through this process of selection andadaptation can an energy efficient and climaticresponsive atrium design be generated.

Atrium Type1 Overheating in summer2 Excessive heat loss in winter3 Seasonal temperature f luctuations4 Daily temperature f luctuations5 Effect from external environment6 Requirem ent for artificial lighting inmain building7 Requirement for shading8 Total energy consu mption

(to achieve neutral temperature)9 Summer temperature high10 Winter temperature low11 Points12 Ranking

A

* . .131

B. . . .. .. . . .. . . .. . . . . . . . *354

D. . . .

. . . .485

F. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .303

G.. .. .. . *. .* *242

( Low, High)




Atrium Type1 Overheating in summer2 Excessive heat loss in winter3 Seasonal temperature fluctuations4 Daily temperature fluctuations5 Effect from external environment6 Requirement for artificial lighting in

main building7 Requirement for shading8 Total energy consum ption


A

*

.111

B. . . .. . . .. . . .. . . .. . . .* . . . .. . . .. . . .. . .394

D

. . . . .

. . . . .

505

F. . .. . .. . .. . .. . .* *. .. . .. . .. .273

G . . .. . .

.

. . . .232

( Low, . . . . . H igh )


Atrium Type1 Overheating in summ er2 Excessive heat loss in winter3 Seasonal temperature fluctuations4 Daily temperature fluctuations5 Effect from external environment6 Requirement for artificial lightingin main building7 Requirement for shading8 Total energy con sump tion

(to achieve neutral temperature)9 Summer temperature high10 Winter temperature low11 Points 12 36 48 28 2412 Ranking

( Low, High)

Table 4: Zone D. Southern climate. Ranking of the different generic atrium types ac cording to their atrium environments

Atrium Type1 Overheating in summe r2 Excessive heat loss in winter3 Seasonal temperatur e fluctuations4 Daily temperature fluctuations5 Effect from external environment6 Requirement for artificial lighting

in main building7 Requirement for shading8 Total energy cons ump tion


A. .. . . ..

*

. .. . . .181

B. . . .. .. . . .. . . .. . . .. . . .. . . . . . .. .354

D.

. . . . .

. . . . .

. . . . .

.425

F. . .. . .. . . . .. . .

. .* *

. . .

. . .263

G. . .. . .. .* *. . .. . ..

282

( Low, High)




Table 5: Characteristics of the different atrium design e lements in the four climatic zones


12345

6

78

g10

11

12

LocationLatitudeZoneClimateForm

Orientation

ProportionSeparating skins

External envelopeRoof

Shading devices

Natural light quality

K ew5128'Mid European coastalMild winter, mild summerElongatedcentralised linearSouth

Medium height forsolar penetrationMedium-weightinsulated panelsThermal massSouth facing sloped roof

Internal shadingon vert ical planesUse clear glassor translucent materials

S t o c k h o l m5921'North European coastalCold winter, longmild summerElongated centralisedor l inear attachedSouth avoid extensivevert ical glazing

Shallow atrium formore solar penetrationLight-weight to med ium-weight insulated panelsThermal massSouth facingsteeply sloped roofInternal shadingto main buildingUse clear glass

S t u t t g a r t4850'ContinentalCold winter,hot summerElongatedcentralised l inearSouth avoidvert ical glazing

Medium he ight forsolar penetrationHeavy-weight constructionor medium-weightinsulated panelsThermal massSouth facing sloped roof

External shadingon vert ical andhorizontal planesUse clear glassor translucent materials

Mi lan4526'Southernand MediterraneanMild winter,hot summerCompact centralisedlinear: long axiseast/westSouth avoid extensivehorizontal glazing.Minimum glazing oneast, west, north sidesTaller atriumshaded by buildingHeavy-weightconstruction minimumopeningsThermal massNo south glazing, usenorth-facing clerestory ormonitor roofsExternal shadingon vertical andhorizontal planesUse translucent andopaque materials

13 VentilationIndependent mode insummer, venti lat ionpre-heating mode inspring and autumn

Recirculation in winter,independent mode insummer

14 Heating

in winter, inde pendentmode in summer,recirculat ion mode inspring and autumn

Ventilation pre-heatingwinter, independentm o d e for other seasons

Ventilation pre-heating inwinter

Auxiliary and ventilatio n Auxiliary & ventilatio n Auxilia ry and ventilation Not necessary unless

Notes1. The LT Method is a manual energy design tool for the calculationof energy performance in non-domestic buildings. It was developedby Cambridge Architectural Research Ltd., Cambridge.2. The ATRIUM m odel was developed by the Martin Centre,University of Cambridge. It is not a dynamic model and it responds tovarious parameters w ith certain limitations. The parameters includethe simplified nature of the site, the rectilinear shape of the atrium andits relationship to the main building, the nature of the atrium andbuilding envelope, the nature of the separating walls, and variousmodes of ventilation exchange between the atrium and the mainbuilding. Furthermore, the model does not respond to the detailedgeometry, such as curve, of the building, orientations other tha nsouth and non-south, the use of thermal mass, detailed configurationof the roof and complicated external obstructions. It is intendedprimarily for unheated atria. The reader should be aware that theresults are based on monthly average figures and there is no verticaland horizontal atrium temperature distribution. Certain assumptionsare therefore made in order to simplify the calculations. Theimportance, however, lies in the principles involved and the relativeperformance of the alternatives rather than actual predictions.3. The assumed values have been chosen to correspond to thefollowing criteria : The building is medium size, non-domestic with average internalgains, occupied during normal working hours and weekdays only. The base case has a plan of 24 m x 24m square plot and of threestoreys high in a suburban site, free from external obstruction.

The building envelope has a high standard of insulation and arelatively high glazing ratio with the minimum external and internalshading except that provided by the building structure. The main building has a base temperature of 18.5C. The atrium has minimum internal obstruction. The roof of the atrium is assumed to be flat. Independent ventilation for the atrium and the building is assumed.4. For the purpose of the analyses, criteria based on the neutraltemperature of the atrium, the temperature at which m ost people feelcomfortab le u nde r sedentary conditions, are used. The reason forusing temperature criteria rather than energy criteria is becauseenergy is always consumed to maintain thermal comfort conditions inthe atrium . The aim is, therefore, to maintain the neutral temperatureswith minimum energy consumption in the atrium and the mainbuilding. The following equation from M. A. Humphreys is used inorder to calculate the required monthly neutral temperatures in thefour climatic zones (Humphreys, 1971).T neutral = 11.9 + 0.534 x T ambient (+ /- 2.5C )T ambient is the mean monthly outdoor temperature.The +/-2.5C indicates the temperature zone over which 8 0% ofpeople judge to be between 'comfortably cool' and 'comfortablywarm' .5. See Baker, Nic k (1988). 'The Atrium Environment1, BuildingTechnical File, n o .2 1 , April 1988.6. See note 1 above.


12/12


ReferencesBaker, Nick (1992). Design Parameters and Performance: A Technical

Design Guide for Low Energy No n-Domestic Buildings.Unpublished paper, BRECSU.Baker, Nick (1991). Th e LT Method, version 1.2, CEC, Directorate-

General XII for Science, Research and Development.Baker, Nick (1988). 'The Atrium Environment', Building Technical Hie,

no.21, The Martin Centre for Architectural and Urban Studies,University of Cambridge, April 1988.

Baker, N., and Hawkes, D. (1987) 'Glazed Courtyards : an Element ofthe Low Energy City' in Energy and U rban Built Form, Hawkes,D., and Owers, J. (eds.), Butterworths, London.

Givoni, B. (1976). Man, Climate and Architecture, 2nd edition, AppliedScience Publishers Ltd., London .

Goulding, J., Lewis, O.J., Steemers, T.C. (eds.)(1992). Energy inArchitecture, The European Passive Solar Handbook, CEC, B.T.Batsford Ltd., London.

Humphreys, M. A. (1971). Theoretical and Practical Aspects ofThermal Comfort, BRE current papers, 14/71, Garston, UK.

Olygag, Vincent (1963). Design With Climate, Princeton UniversityPress, Princeton.Watson, D.,and Labs, K. (1983). Climate Design, McGraw-Hill Book

Company, New York.Titled File for the diagrams.AcknowledgementsThis paper is based on research undertaken in 1992-93 for a Masterof Philosophy degree at the University of Cambridge. Financialsupport for this was given by the Architects Registration Council ofthe United Kingdom, the Eastern Region Energy Group an d theKettle's Yard Fund.BiographyDennis Ho is an architect working with Richard Rogers P artnership inLondon.


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