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Education of Architects in Solar Energy and Environment, section 2.2 page 1

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Section 2.2

Sunspaces and atria

2.2.1 IntroductionSunspaces and glazed atria are glazed spaces that are thermally separated from the buildings they

are connected to. These spaces are often partially heated, or not heated at all.

• A sunspace is defined as a relatively small glazed space attached to a dwelling.

• A glazed atrium is defined as a glazed space attached to a large building or placed betweentwo or more such buildings.

Historically, atria were uncovered courtyards providing a tempered climate and were valued asprotected, private outdoor spaces. The glazed atrium is a relatively recent evolution and we must ensure,by careful design, that the natural benefits of the traditional open space are not lost with its glazing.There is a strong belief that the incorporation of an atrium or sunspace in a building design leadsautomatically to reduced energy consumption. If properly designed as a passive solar feature, theatrium can save energy, but if the atrium is artifically lit or heated it may waste more energy than itsaves.Energy conservation is seldom the primary reason for incorporating atria in a building design, andamong the reasons for including a sunspace or an atrium in a building design are the following:to create a dramatic entry or central space, to facilitate circulation and provide more perimeter space,to increase amenities for the building users such as restaurants and recreation areas with interiorgardens. However, they can strongly influence the energy use characteristics of the buildings they areattached to and as part of a passive solar system, many environmental benefits can be achieved;including the heating, cooling and daylighting of the adjacent building.

2.2.1.1 Contents of the section

This section of the book provides information about the value of a sunspace or an atrium as part ofa passive solar system. Emphasis is placed on their potential for saving energy for the buildings theyare attached to.

Most of the description refers to atria, but, as sunspaces function much the same way, theconclusions drawn are generally valid also for these, smaller spaces. The following subsectionsdescribe what types of atria there are, what potential they have for saving energy, and what effect thatwill have on the cost and on the comfort in the building and in the atrium itself.

A few built examples are used to illustrate the issues. These are the ELA university building andthe Dragvold university building in Trondheim, Norway, a day care center in Alta, Norway, and theGateway II office building in the UK.

2.2.1.2 Types of atria


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Core atrium

This is the classic atrium type providing a glazed courtyard inthe center of the building surrounded by adjacent spaces on allsides. The external envelope of the atrium is limited to the areaof the roof glazing.

Integrated atrium

An integrated atrium is a glazed space that is positioned in thebuilding such that only one side faces the exterior. It may ormay not have a glazed roof.

Linear atrium

The linear atrium covers an open space between two parallelbuilding blocks ending with glazed gables on both sides.

Attached atrium

The attached atrium is a glazed space added to the external wallof the building envelope.

Envelope atrium

The envelope atrium is characterized by an entirely enclosedbuilding covered by glass representing a "house-in-house"concept. The large external envelope glazing may include onefacade of the building.

Atrium

Building


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2.2.2 StrategiesAtria can be used in strategies to heat, cool, and/or provide daylight to the building:

Heating

An atrium acts as a buffer reducing transmission losses fromthe adjacent spaces to the ambient and may also provide heat for theadjacent spaces. It is used to displace auxiliary heating by solar gaintransfer from atrium to the adjacent spaces. Thus, the predominantorientation of the atrium aperture should be south, and the glazingshould be vertical (to reduce overheating risks in summer). Collectedsolar radiation has to be stored in interior mass in buildingcomponents exposed directly to the winter sun. Nighttime heat losseshave to be reduced by using good thermal quality materials in theenvelope glazing and in the walls and windows separating the atriumfrom the rest of the building.

Cooling

An atrium can be designed to induce natural ventilation and toprevent undesirable solar gains. Natural ventilation can be facilitatedby a vertical stack effect and by proper placement of air inlets andoutlets. Inlets should be placed at the bottom of the atrium (and/orinduced cross circulation should be included), and sufficient exhaustair vents should be placed at the very top. Nighttime convective coolingof building mass structure can be achieved by cross ventilation, withair passing from the ambient through the adjacent spaces and out viathe atrium space.

Daylighting

An atrium can be used to provide additional light to theadjacent spaces. The key issues are daylight availability, distribution,and utilization. The glazing of an atrium reduces the amount ofavailable daylight inside, but as a consequence of the buffer effect ofthe glazing, the window area in the intermediate boundary can beincreased without penalties in the form of higher heating energyconsumption. Consequently, more daylight may be available in theadjacent spaces.

The amount available is determined by the overall design andby the properties of the walls and windows separating the atrium andthe adjacent spaces. Atrium dimensions (height, length, width),determine the potential daylight aperture, and the size and position ofwindows in the intermediate boundary, as well as the reflectivity of thewalls themselves, determine the amount of daylight penetrating into theadjacent spaces.


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2.2.3 Design considerations2.2.3.1 Heating strategies

The strategies for reducing heating energy consumption used by atrium buildings varyconsiderably. Atria act as buffers for their adjacent spaces, reducing their heat loss. The energy savingsin the adjacent spaces partially offset the atrium heating energy requirements. Some atria alsocontribute to the total building heating requirements. They act as buffers during the coldest parts of theheating season and contribute heat to adjacent spaces during the warmer parts, when more solar energyis available.

J F M A AJM J S O DN

0

+

- Atrium has potential to contribute useful heat toadjacent spaces

Bldg. heat lossNet atrium energy

(solar gain- heat loss)

Period in which atrium solar gains exceeds atrium heat loss.

Excess solar gainBufferContrib. Potential

BufferContrib. Potential

Fig.1. Energy gains and losses in an atrium.

Some of the key design factors that influence the ability of an atrium to function as a buffer or aheat source for the adjacent spaces are:

Atrium type

Most of the buildings that have core or linear atria have more spaces adjacent to their atria thanthe buildings with other types of atria. In these the potential for the atrium to act as a buffer issubstantial since it can affect a greater portion of the building. By contrast, the integrated atria mayperform well but do not substantially buffer the building as a whole, since they often are connected toonly a small portion of the building.


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Glazing type/insulation level

The atrium glazing properties significantly affect atrium energy consumption if the atrium isheated to a temperature near the comfort zone. For example, parametric studies of the atrium in theELA building, in Trondheim, with north and south facing glazing, show about a 50 % reduction ofatrium heating energy requirements when the U-value of the glazing is reduced from 2.1 to 1.0 and thesolar gains are kept constant. For the total building, this improvement in U-value results in about a 5 %drop in heating energy requirements.

A comparison of glazing options for the ELA atrium was made by the designers prior toconstruction. The results indicate that the double, low emissivity glazing produces about a 10 %improvement in building heating energy consumption compared to other glazing options, and about a20 % improvement compared to an open well (non-atrium) building when the atrium is to be heated to15oC.

A

B

C

D

E 100%

81%

91%

82%

92%Double glazing in roof and gable

walls, single in facades.

Double low-E glazing in roofandgable walls, single in facades.

Double low-E glazing in roofandgable walls, double in facades.

Double glazing in all

No glass roof, triple glazingin facades.

92 %

82 %

81 %

91 %

100%

Fig.2. Energy consumption for heating in the ELA building in Trondheimfor different glazing alternatives.

Parametric analyses were made of the atrium in the ELA building to study the effect of theinsulation level in the common wall on the atrium temperature. The results show that when theinsulation level of the common wall is increased, the resulting atrium temperature is lower in the winterand higher in the summer. In these simulations the atrium was not heated unless the temperature fellbelow 5oC.

Additional simulations were performed to examine the effect of changing the atrium width andatrium glazing U-value. Increasing the atrium width has the effect of increasing the area of the atriumexternal envelope, resulting in a higher heat loss and consequently in higher heating requirements. Inthese simulations the atrium was heated.


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420

440

460

480

500U=2.8

U=2.1

U=1.6

6m 10m 12m 14mWidth (m)

(MWh)

Fig.3. Calculated annual heating requirement for the ELA atrium of different widthsand for different U-values of the atrium glazing.

Glazing configuration

Most atria have glazing in almost the entire surface of the exterior envelope. Most of thisglazing is sloped to form a gable (saddle), shed, or mansard roof. These glazing configurations receivesolar energy from throughout the sky vault, providing light in the atrium even during periods of lowsolar availability. However, unlike vertical south facing glazing, sloped glazing receives more incidentsolar energy in summer months than in winter months. This contributes to overheating during hotportions of the year. In addition, measurements indicate that the sloped glazing loses more heat thanvertical glazing, partially due to nocturnal radiation. The magnitude of these losses is higher thansimple calculations indicate.

The effect of changing the amount and configuration of glazing in a typical atrium with slopedglazing in the external envelope was studied by computer simulations using a calibrated model of theELA atrium. The atrium base case (actual building design) has equal glazing areas facing north andsouth, and the glazing is double, with a low emissivity coating. By simulation, the north facing glasswas replaced with an opaque roof (U = 0.35W/m2 oC). This resulted in a decrease in annual atriumheating energy requirements of about 25 %, while there was no change in office heating energyrequirements.

To examine the effect of climate, the ELA building was moved (by computer simulation) toWashington, DC, a climate with about 2 400 heating degree days (base18oC) and about 900 coolingdegree days (base 24oC). This is similar to portions of southern Europe. In this location, withsubstantial cooling requirements, the north facing atrium glazing was left intact, and the south facingatrium glazing was replaced with an opaque roof. This resulted in a decrease in annual atrium heatingenergy requirements of about 27 %, and a decrease in cooling requirements of about 36 %.


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Atrium conditioning

There are different ways of conditioning the atrium space, from no heat input to almost fullcomfort conditioning. The approach chosen affects the times and purposes for which the atrium can beused, as well as the atrium heating energy requirements.

Most unconditioned atria are primarily used for circulation. Solar energy heats the atria but noattempt is made to maintain a specific temperature. When atrium temperatures are comfortable, theycan be used for exhibits, and other casual purposes. During the heating season, they act as buffers foradjacent conditioned spaces, but do not contribute heating energy, except by casual transmissionthrough the intermediate boundary.

Some atria, such as the day care center, in Alta, are designed to be used to preheat ventilation airduring a portion of the year. In this case, the incoming ventilation air passes first through a heatexchanger (extracting heat from the exhaust air), and then through the atrium before entering theadjacent spaces. During mid-winter, this results in the atrium being slightly heated by the ventilationair; in milder periods, solar energy gains in the atrium increase the temperature of the ventilation air.Based on building measurements, this process increases atrium temperature by 10-15oC above ambientconditions.

Fresh air

Exhaust air

Fig.4. Preheat of ventilation air.

On an annual basis, about one third of the atrium heating is obtained from the ventilation airpassing through the atrium; in mid-winter the percentage is higher. Simulations indicate that the energyuse would be less if the incoming air passed through the atrium first, but this would lower the atriumtemperature, reducing the time during which the atrium can be used as a play area for the children. Thevalue of semi-conditioned play space in this climate may be greater than that of the energy savings.


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Other atria are partially conditioned by ventilation exhaust air from the adjacent office spaces.In this way, no primary energy is used to condition the atrium, and it serves as a buffer for the fullyconditioned adjacent spaces. Others again have small, localized space heaters that, for instance, controlcold draughts along the glass. This has proved effective both in the Gateway II building and in theELA building.

Other atrium buildings use mechanical means to transfer heat from the atrium to the adjacentspaces or the reverse. This can be a refrigerant loop that recovers extra heat from the atrium duringspring and fall and provides heat to the adjacent spaces, or a heat pump that extracts heat from theatrium for water heating and ventilation air preheating purposes.

Thermal capacity

In free floating atria, a heavy construction will provide some thermal storage capacity, delayingthe passage of heat. This results in a more stable atrium temperature, but the stabilizing effect mayprevent the atrium temperatures from becoming high enough to provide useful heat to other parts ofthe building. In conditioned atria that maintain lower temperatures at night, the thermal mass lengthensthe time required to increase comfort conditions after temperature setback periods.

Computer simulations of the ELA building were used to assess the effect of increasing themass in the common wall in all climates. Replacing wood frame construction with concrete blockresulted in a very small reduction in atrium or adjacent space heating requirements in any climatestudied (typically less than 1 %).

Atrium temperature

The ELA building atrium is very well liked by students who use it for circulation and as a studyspace. The popularity of the space caused a need for the temperature to be increased from 15oC in thefirst year of use to 18oC in the second year. More students use the building than originally planned,because of an increase in admissions. Thus, the atrium provides much needed space for studying andsocial communication.

Measurements and simulations of ELA demonstrate that heating the atrium to 15oC results inan energy saving for the overall building, compared to a building with no atrium. This is due to thebuffering effect. In fact, there is little difference in total building energy consumption if the atrium isheated to 5oC or 15oC, while temperatures higher than this result in substantial increases in atriumheating energy requirements.

The effect of atrium temperature on building energy requirements was further studied insimulations for Trondheim and for Washington, DC. In both climates, the total building heatingrequirements increased dramatically when the atrium temperature was raised above 15oC. InTrondheim, the increase in temperature setpoint to 20oC caused the atrium heating energy use toincrease by about 60 %. However, the warmer atrium serves as a better buffer for the adjacent offices,reducing their energy requirements. For the overall building, raising the atrium temperature causedapproximately a 20 % increase in heating requirements, excluding ventilation heating. For Washington,the result is similar; the increase in temperature from 15oC to 18oC caused an increase in atriumpurchased heating of 100 % and an increase in total building purchased heating requirements ofapproximately 40 %.


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20

350

300

250

150

100

500

5 10 15

200

Atrium heating setpoint Temperature (o C)

He

atin

g C

ons

um

ptio

n (K

Wh)

University Building, Trondheim

350

300

250

150

100

500

5 10 15

200

Atrium heating setpoint Temperature (o C)

He

atin

g C

ons

um

ptio

n (K

Wh)

University Building, Washington DC

Total

Atrium

Office

Office

Total

Atrium

Fig.5. Heating requirement as a function of atrium temperature for the ELA buildingin Trondheim and in Washington DC.

It is worthwhile to note that in both locations an increase in atrium temperature from 5oC to15oC caused a relatively small increase in total building heating requirements. Within this range, theincrease in atrium temperature improves the buffering effect, decreasing the heating requirements ofadjacent offices. An additional benefit of the buffering effect is that the installed heating capacity of theadjacent spaces was reduced, saving on the construction cost. Increasing the atrium temperature above15oC provides little additional buffering benefit for the adjacent spaces, however.

The total building energy consumption for a conditioned atrium (heated to 18oC and cooled inclimates where cooling is required for comfort), a free floating atrium, and a building with no atriumwas also studied. These strategies were compared in the climates of Trondheim, Oslo, and Zurich(heating only); and Washington DC, Dallas, Texas, and Rome (heating and cooling). When the total


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energy density is compared, accounting for the difference in size between the building with the atriumand the one without an atrium, the most attractive strategy varies significantly with climate.

250

0

Trondheim Zürich Oslo

KW

h/m

2/y

r

Electricity

Cooling

Heating Hea

ted

atr

ium

Fre

e-flo

at

No

atriu

m

Hea

ted

atr

ium

Fre

e-flo

at

No

atriu

m

Hea

ted

atr

ium

Fre

e-flo

at

No

atriu

m

Fig.6a. Total energy consumption for a building with an atrium heated to 18 oC,a free floating atrium, and a building with no atrium; the building is only heated.

250

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Rome Washington Dallas

KW

h/m

2 /yr

Atr

ium

hea

t & c

ool

Fre

e-flo

at

No

atriu

m

Atr

ium

heat

& c

ool

Fre

e-flo

at

No

atr

ium

Atr

ium

hea

t & c

ool

Fre

e-flo

at

No

atriu

m

Fig.6b. Total energy consumption for a building with an atrium heated to 18 oC,a free floating atrium, and a building with no atrium; the building is both heated and cooled.

The effect of activity level and clothing on atrium comfort during the heating season wasexamined by simulating the ELA building with a free floating atrium on a cold winter day in each offour climates. Even on the coldest day, the atrium was comfortable to someone using it for circulationfrom one part of the building to another. In relatively mild climates, such as Rome or Washington, theatrium was comfortable to someone seated in it for part of the day, depending on the type of clothingbeing worn. In practice, comfort will be greater if the people are able to sit in a sunlit area. Thesestudies suggest that designers can save energy by partly conditioning portions of the atrium, such asusing radiant heat in seating areas, and allowing the other portions of the atrium to maintain a lowertemperature during the heating season.

2.2.3.2 Cooling strategies


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Cooling considerations are important in all locations. In cold climates, the cooling approachesused in atrium buildings are principally intended to avoid excessive overheating in summer months. Inwarmer climates, the atrium may be used to provide a cooling effect for the adjacent spaces.

Natural ventilation and shading unwanted solar gain are the most applicable methods of cooling atria inthe warmer climates.

An atrium should have adequate daylight, sunlight and view without overheating and discomfort.Excessive sunlight causes overheating and glare and controls are neccessary to limit the amount ofsolar radiation entering the building, which minimises the amount of cooling neccessary.

Solar ControlThere are three main methods of solar control:• internal blinds• external shading• solar control glazing

The most effective of these is external shading because it intercepts solar radiation before itenters the building. Horizontal shades are most effective on south-facing facades, vertical on east / westfacing facades. However, all permanent shading reduces daylight, so moveable shades are preferable,either manually or power assisted (refer section 2.1.1)

Internal blinds are most common, but they do allow almost half the solar radiation to enter thebuilding. Venetian blinds properly adjusted are most effective and specialised mirrored venetian blindscan now increase daylight at high level while lower slats are closed. Mid-pane venetian blinds allowabout one third solar radiation into the building.

The choice of glazing effects the daylight, solar gain and heat loss through an atrium. Solarcontrol glass, heat absorbing or heat reflective, reduces heat gain and loss, but can reduce light also.(refer section 3.1.5)

Stratification and ventilation

In cold climates with moderate summer temperatures, the most common cooling strategy is toventilate the atrium taking benefit from the stratification that occurs. Operable windows or hatches canalso be used. Outside air is usually admitted to the atrium at a low level and allowed to flow upward,exiting through smoke vents or other openings at the high levels of the atrium.

In the ELA building smoke vents are used for ventilation purposes. Measurements indicate thatthe atrium temperature can be maintained at about the outside air temperature if adequate ventilation isprovided using top and bottom openings. A temperature range of only 2 - 3oC was measured frombottom to top when the vents were open. However, when the vents were not open during the summer,the temperature of the top of the atrium rose to 10 - 15oC above the outside air temperature.Overheating of the offices in the adjacent space occur under these conditions, and the operablewindows of these offices are ineffective in obtaining ventilation. This shows that dependable operationof the ventilation openings is quite important if atrium comfort is to be maintained during summerconditions.

Simulations of the ELA building were carried out in order to examine the amount of ventilationrequired to maintain comfort conditions. Air change rates were varied from 1 - 25 per hour for bothpeak summer days and typical swing season days in different climates. The results show that it is notpossible to maintain atrium comfort during mid-day in Washington in the summer. A range of one toten ACH is sufficient to maintain comfort on a typical day in May. In Oslo, a ventilation rate rangingbetween one and ten ACH is sufficient to maintain comfort in the atrium even on a peak summer day,and during the milder months, a maximum of five ACH is sufficient.


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In most countries, there are fire code requirements for ventilation of the atrium space to controlsmoke accumulation. Although the specific regulations vary, a rate of 4-6 ACH is common. In someplaces, the regulations mandate a specific vent area, such as 10 % of the floor area. These requirementsare very close to the ventilation rates necessary to maintain atrium comfort without cooling. If theserequirements are met using either natural stack ventilation or fan induced exhaust, it may be possible touse the smoke venting system to maintain comfort during much of the cooling season in manyclimates.


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Using the atrium to cool the building

While the stratification, ventilation, and shading devices in most of the atrium buildings areused to control overheating in the atrium itself, they do not provide a cooling benefit to the adjacentspaces of the building. The Gateway II building, in the UK, was designed to eliminate the mechanicalcooling system by using the atrium to create a flow of air through the adjacent spaces and the atrium.This saved considerable construction and operating cost.

Fig.7. A section through the Gatway II building.

The offices are arranged around a core atrium so that there is only 14 meters from the exteriorwall of the building to the common wall with the atrium. Manually controlled operable windows in thebuilding exterior wall and louvered windows in the common wall permit the flow of outdoor airthrough the offices to the atrium. The atrium has large roof lights located near the top that allow theventilation air to escape. Stratification of the air in the atrium, aided by solar gains, enhances the airflow, as do small fans in some of the offices.

Sensitivity to climate

The atrium buildings described are located in climates where it is possible to avoid the use ofmechanical cooling in the atrium. To test the effect that warmer climates can have on atrium design, acalibrated computer model of the ELA building was used to examine building performance inWashington, DC, U.S. The base case assessment is for the building as designed, without mechanicalcooling in the atrium.The results show that the atrium comfort conditions are unacceptable during a typical summer day,based on the Fanger Thermal Sensation Index. This finding is consistent with other analyses, anddemonstrates why most atria in similar climates are mechanically cooled. However, the comfortconditions for the atrium in its actual location in Trondheim are quite acceptable. This is consistentwith measured data in the building.

A series of alternatives was examined to achieve tolerable atrium conditions with low coolingenergy use. First, the offices were cooled to 24oC, but the atrium was left uncooled. This did notimprove comfort conditions in the atrium, so other strategies that use mechanical cooling in the atriumwere tried.


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A mechanically cooled base case was established by simulating the cooling of both the officesand the atrium to 24oC. If the south facing portion of the roof glazing in the atrium is replaced withopaque roof, the cooling energy requirements drop significantly. Another way of achieving thiscooling benefit is to use a fixed or operable external shading device in order to reject the solar gainbefore it reaches the building. Shading devices can be designed to admit daylight without substantialsolar gain, which may be more architecturally desirable than an opaque roof.

2.2.3.3 Daylighting strategies

The glazing of an open well or courtyard reduces its potential as a daylighting strategy. Ageneral estimate is that the atrium external envelope reduces daylight factors by at least 20% andsometimes by as much as 50 % compared to an open well.

Thee amount of daylight entering the space depends on:• shape and height of the atrium• roof construction• transmission of the glazing• reflectance of the atrium walls.

The most important issues to consider in order to maximize the benefits of daylighting for theatrium itself and for the adjacent spaces are:

Atrium function

The light levels necessary to support the various activities planned for the atrium varysignificantly. In most atria the daylight levels are sufficient to serve most suitable functions throughoutthe majority of the year at most latitudes. In some cases, additional light may be required to maintainplant growth. (Daylighting mb, pg. 4, Fig. 4)

Adjacent space function

Differing functions require differing illuminance levels, (Daylighting mb, pg 5, table) and thatrequirement must be met in spaces adjacent to the atrium.

The ELA building has four floors of offices adjacent to the atrium. They require about 500 luxduring occupied hours. A combination of measured and simulated results shows a daylightingpotential for the top floor of the spaces adjacent to the atrium of about 50 - 55 %. The daylightingpotential for the second floor is estimated to be 30 - 40 %. These estimates do not consider thepossible influence of plants or ducts in the atrium, or contrast ratios.

When this building is moved by simulation to the Dallas climate, the daylighting potentialincreases to about 80 % for the top floor and 60 - 70 % for the second floor. To realize this level ofsaving in the electric lighting requires either high quality automatic dimmers, or highly motivated userswho manually control the lights to save energy. In practice, this does not occur in this building, andlittle or no daylighting energy savings occurs in the offices. However, light levels in the atrium arequite satisfactory.

Atrium proportions

There are two basic principles regarding the proportions of atria for maximising daylight inatria: a lower atrium will be lighter than a higher atrium of similar plan, and the smaller the perimeterwalls for similar roof area, the lighter it will be also. (image? BRE)


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Parametric studies using a physical model of the ELA atrium, as well as computer simulationswith the program Superlite, indicate that doubling the height to width ratio (that is, making the atriumtwice as high) results in a decrease in the daylight factor at the bottom of the atrium of between onethird and one half.

0 0.2 0.3 0.5 1.0 1.50.1 0.4 0.7 0.80.6 0.9 1.2 1.31.1 1.4

10

20

30

40

50

60

70

0

0.80.650.40.1

Street width : Building height ratio

Day

light

fact

or in

cen

tre

of s

tree

t

Fig.8. Calculated daylight factors at the center of the floor of a long glazed street.

For extreme ratios, i.e. for very tall and narrow atria, the windows at the upper levels should besmaller than those at lower levels, as they receive more direct light from the sky. While increasingdaylight to lower adjacent floors and minimising glare at upper adjacent floors, it also has theadvantage of reflecting more light downwards, as light coloured walls will reflect more light thanglazed surfaces. Splaying the walls of a high atrium can greatly increase the light which lower adjacentfloors receive and mirrors are ofter used to reflect light downwards also.


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0 0.2 0.3 0.5 1.0 1.50.1 0.4 0.7 0.80.6 0.9 1.2 1.31.1 1.4

0

2

4 0.90.40.1

0.9

0.4

0.1

0.90.40.1

Percent daylight factor in rooms facing glazed street

Street width : Building height ratio

75% window in building facade

50 % window in building facade



Fig.9. Measured daylight factors close to the windows on the first floor rooms adjacent to a four storyatrium, for different atrium proportions, window sizes, and facade reflectances (0.9, 0.4, 0.1).

Roof construction and external envelope glazing

The amount of daylight entering through the external envelope is influenced by the structuralelements of the roof and glazing and the optical properties of the glazing materials. (EIA, pg.147, Fig9.34)

Conventional single clear glass will transmit approximately 85% of the light that falls upon it.Double and triple glazing with low e coatings will reduce this to between 60% and 70%, however, itsincreased thermal qualities and solar gain reductions make it more suitable for large areas of glazing inatria. ( Green Design, pg. 51, fig. 7.1 chart )

Technical developments in recent years have now made it possible to specify the make-up of a glazingunit to meet the requirements of heat gain, heat conservation, light transmission and light direction, atdifferent latitudes and for different orientations. (see section 3.1.5. Daylighting and Solar ControlGlazing Systems)

Reflectance of atrium surfaces

In most cases, the colour of the atrium walls has some effect on the daylight factor on theatrium floor. A darker colour has a far more serious effect on a higher atrium than a low one, it mayreduce the daylight factor on the floor by up to half. A parametric study of the ELA atrium with 50 %glazing in the intermediate boundary was made for varying height to width ratios. The results showthat when the colour of the solid walls in the intermediate boundary is changed from black, to grey, towhite, the daylight factor at the center of the atrium floor improve by about 10 % for all ratios. Inpractice, most surface reflectance characteristics are similar to the grey curve; the black and whitecurves represent the extremes of performance.

Glazing in common wall


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For the most common atrium proportions, the amount of light available in the atrium itself isnot greatly influenced by the glazing in the common wall, although some interreflection does occur.However, the amount of light available in the adjacent spaces is directly affected by the amount (andtype) of glazing in the common wall. Since the daylight factor drops on the lower floors of an atrium, auseful design strategy is to increase the size of the windows in the common wall as the distance fromthe atrium external envelope increases.

For the Dragvoll building, physical model studies were conducted during the design phase todetermine the appropriate amount of glazing in the common wall on each of three levels in the atrium.In order to achieve balanced light in the offices adjacent to the atrium, the top level has 40 % glazing,the middle level 70 % glazing, and the bottom level 90 % glazing was applied. Measurements ofdaylight factors in the completed building confirms the validity of this strategy.


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2.2.4 Examples of sunspaces and atria2.2.4.1 Mid Europe and mediterranean climatesExample in Greece

« HELIOS 2 » - Passive Solar residence - Ekali, Athens

Architect : Alexandros N. Tombazisassisted by O.Diamandopoulou and C. Bitzaraki, architects

Design: 1979Construction : 1983 - 83

This single family house of 300 m2 is heated by passive solar energy. The house has twostories which wrap around a sunspace and courtyard on the side of the house, dug out of the terrainwhich slopes from south to north.

The first floor bedrooms are heated directly by direct gains and the warm air from the sunspacewheb doors are left opened. In summer, the sunspace is shaded by an interior awning and vented fromthe ceiling. Apart from being used as a pleasant living area the owner is using it for growing plants.

On average 59 % of energy demands have been calculared to be covered by solar contributions.

• The solar velum allow to reduce dazzling and direct irradiation and such to increase comfortconditions.• The height of the sunspace which imply air stratification is also a good design with upperventilation.• Vegetation participate in cooling the conservatory in summer and mid-seasons.

2.2.4.2. Case studies in the south of France

MAZOREL

REHABSOL is a project for the bioclimatic and socio-environnemental renovation of social housing inDrôme Department (France), in the Rhone Walley near Valence. The six buildings (on two sites -Mazorel, Crest and Val d’Or, Saint Rambert d’Albon) contain 88 apartments of varying sizes.

MAZOREL PROJECT :The designers, ARCHI.M.E.D.E.S., arranged new site planting to provide meeting and play spaces aswell as protection from sun and wind. External insulation with a rendered finish, double glazing andconservatories improve appearance and thermal performance. Trombe walls with transparent insulationact as passive solar collectors on South façades.

SUNSPACES :New conservatories have been added 15 m2 to living rooms, which were formerly only 18 m2 even inthe five-room dwellings. In winter they act as solar collectors, and in summer provide shading fromhigh-angle sun for the rooms behind. Inhabitants have somewhere to grow their plants kumquats,lemons and herbs.Solar panels on the roof terraces provide 60 % of hot water requirements. Mechanical ventilationincorporates a heat recovery system.The cost of this energy-saving retrofit is approximatively 13000 ECU per unit. Energy savings areexpected to be 70 % of previous consupmtion overall - 2.12 TOE / year per dwelling. Pollutionreduction should be 2.2. TOE / year per dwelling. The projected life of the retrofit components is 25 to30 years, and the calculated payback period 8 or 9 years.


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All interior work has been carried out and Trombe walls are being installed in March 1995.Monitoring is under way and will continue until March 1996.

2.2.4.3. Case studies in the south of France

CREST

The sunspaces (one per dwelling) are treated as buffer spaces, that means usable whenmeteorological conditions are favourable. It is a solar collector in winter, a living space in mid-seasonand in summer, when temperature and irradiation are low.

The thermal mass of the walls and floors allow to reduce temperature and to store energy.Energy saving for the sheme is supposed to be 20 % in comparison with standard housing and

13 % due to solar gains.

ALBON

Glass, concrete and wood are mixed to create a terrace architecture. Living room is largelyopened on the sunspace for both sun and landscape.

This design is typically the good solution to benefit of solar gains in winter and prevent overheating in Summer. This additional area can be used all the year long.Solar control, except for ventilation is made by the architecture itself (opaque roof and roof-overhangfto south orientation).

The design allow a 49 % of real data energy saving (the calculation gave a 15 % saving for theglobal performance).

2.2.5 ConclusionsMany atrium buildings save energy when compared to conventional buildings without atria. They

act as buffers for their adjacent spaces, reducing their heat loss. The energy saving in the adjacentspaces partially offset the atrium heating energy requirements. Some atria also reduce the total buildingheating requirements. They act as buffers during the coldest parts of the year and contribute heat to theadjacent spaces during the milder parts, when more solar is available.

Some of the most important conclusions that can be drawn regarding the atrium as part of a passivesolar heating strategy are:

- During midwinter an atrium primarily functions as a buffer. The buffer effect will naturally begreater the more surfaces the atrium and the adjacent spaces have in common.

- During the spring and fall the atrium also has the potential to contribute useful heat to the rest ofthe building. A prerequisite for this is that the atrium is not fully heated and that its temperature isallowed to fluctuate.

- In most climates, heating the atrium to a temperature in the range of 10-15oC results in little or noincrease in the overall building heating energy requirements. The heat put into the atrium partiallyoffsets the heating requirements in the rest of the building.

- Temperatures above 15oC result in substantial increases in atrium heating energy requirements.

- Tempering the atrium also reduces the size of the installed heating capacity in the adjacent spacesand reduces construction costs.


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- During the heating season, the U-value of the glazing is more important than the solartransmission for the energy consumption. Double low-emissivity glazing is particularly effective innorthern latitudes.

- Thermal mass in the atrium improves comfort, as it reduces temperature fluctuations, but it haslittle influence on the overall energy consumption.

- If the atrium is used for active functions such as circulation, occupants will be comfortable inanyof the climates considered without any heat being put into the atrium. If the atrium is used for moresedentary functions, such as seating, comfort conditions may not be acceptable and localized heatingwould have to be considered.

Some of the most important conclusions regarding the atrium as part of a passive solar coolingstrategy are:

- In most northern climates, no mechanical cooling system is required to maintain atrium comfort.Natural or fan assisted ventilation with ambient air is sufficient to maintain reasonable comfort.

- The smoke ventilation that is usually required for building reasons can be used to accomplish this,resulting in little effect on construction costs.

- Solar controls, such as awnings, shutters, fabric drapes, and movable blinds, can all be effectivelyused to control solar gain during the overheating season without hindering solar gains during thewinter.

- An atrium can act as a thermal chimney, drawing ventilation air through the adjacent spaces.Insome cases that can result in the elimination of a mechanical cooling system and thus in a building thatis less expensive to build than one with mechanical cooling, but with no atrium.

Some of the most important conclusions regarding the atrium as part of a daylighting strategy are:

- The frames and glazing of an atrium reduce the amount of available daylight inside. However, thewindow area in the common wall can be increased without penalties in the form of higher heatingenergy consumption because of the buffer effect of the glazing. Consequently, more daylight may beavailable in the adjacent spaces.

- Daylighting of the atrium results in lighting energy savings in many buildings, but few takeadvantage of the potential to save energy in the adjacent spaces, even if there is sufficient daylightavailable.

- The atrium's width to height ratio substantially influences the amount of daylight available in theadjacent spaces, while the color of the surfaces in the atrium have a somewhat smaller influence.

It is clear that the integration of an atrium in a building can result in energy savings when heating,cooling, and daylighting strategies are skillfully combined. In most cases, the inclusion of a welldesigned atrium also enhances the building's amenity value, and in some cases the use of an atriumlowers initial construction costs and operating costs.


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2.2.6 ReferencesØ.Aschehoug, Daylight in Glazed Spaces, Building Research and Information, Volume 20 Number 4,1992.

A.G.Hestnes et al, Atria, pages 329-429 in Passive Solar Commercial and Institutional Buildings: ASourcebook of Examples and Design Insights, John Wiley & Sons Ltd, UK 1993.

A.G.Hestnes, Four Norwegian Case Studies, pages 179-202 in Passive and Hybrid Solar CommercialBuildings. IEA Task XI - Basic Case Studies, ETSU, Oxfordshire, UK 1990.

European directory of sustainable and energy efficient building - 1995 - JAMES & JAMES

Architecture solaire en europe, Commission des Communautés Européennes - EDISUD

Règles professionnelles pour la conception des verandas, SNFA, 1989 - Documentation technique - 20P - Editeur : SNFA

Architecture, ambiances et énergie : prix 1989, Ministère de l'équipement - Direction de l'Architectureet de l'Urbanisme, 1989 - Ouvrage - 85 P - Editeur : Techniques et Architecture

Guide de l'éclairage naturel et de l'éclairage artificiel dans les établissements scolaires, P. CHAUVEL,CCTCT/ MINEDUC - Direction des Personnels d'Inspection et de Direction, 1989 - Ouvrage - 79 P -Editeur : Paris - CCTCT

Prise en compte du rayonnement solaire dans l'éclairage naturel des locaux : méthodes et perspectives,M. FONTOYMONT / ENSMP, 1987 - Thèse - Travaux universitaires Mémoire - 228 p - Editeur :Fontoymont

Concevoir et habiter : l'espace de la veranda, Plancon, 1987 - Ouvrage - 144 P - Editeur : Plancon

European solar passive handbook : basic principles and concepts for passive solar architecture(preliminary edition), P. ACHARD / R. GICQUEL, 1987 - Ouvrage - CCE

Passive solar energy dwellings, M. de Langen, Th. Reijenga, C. Boonstra, May 1989, final report,Goirle

Effets de serre : Conception et construction des serres bioclimatiques, I. HURPY / F. NICOLAS -Ouvrage - 206 P - Editeur : AIX EDISUD

Project monitor (CCE brochures, Cas study)

Energy in Architecture, The European Passive Solar Handbook, 1986, J. GOULDING / J.OWENLEWIS / THEO C. STEEMERS, Batsford

European Passive Solar Handbook, 1986, P. ACHARD / R. GICQUEL, CEC

Environmental Design Guide for naturally ventilated and daylit offices, 1998, D. RENNIE / F.PARAND, BRE

Working in the City, 1990, S. O’TOOLE / J.OWEN LEWIS, CEC