Advanced Naturally Ventilated Architecture

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    Architectural design of an advanced naturallyventilated building form

    Kevin J. Lomas *

    Institute of Energy and Sustainable Development, De Montfort University, Leicester LE1 9BH, UK

    Received 3 April 2006; received in revised form 4 May 2006; accepted 24 May 2006


    Advanced stack-ventilated buildings have the potential to consume much less energy for space conditioning than typical mechanically

    ventilated or air-conditioned buildings. This paper describes how environmental design considerations in general, and ventilation considerations inparticular, shape the architecture of advanced naturally ventilated (ANV) buildings. The attributes of simple and advanced naturally ventilated

    buildings are described and a taxonomy of ANV buildings presented. Simple equations for use at the preliminary design stage are presented. These

    produce target structural cross section areas for the key components of ANV systems. The equations have been developed through practice-based

    research to design three large educational buildings: the Frederick Lanchester Library, Coventry, UK; the School of Slavonic and East European

    Studies, London, UK; the Harm A. Weber Library, Elgin, near Chicago, USA. These buildings are briefly described and the sizes of the as-built

    ANV features compared with the target values for use in preliminary design. The three buildings represent successive evolutionary stages: from

    advanced natural ventilation, to ANV with passive downdraught cooling, and finally ANV with HVAC support. Hopefully the guidance, simple

    calculation tools and case study examples will give architects and environmental design consultants confidence to embark on the design of ANV


    # 2006 Elsevier B.V. All rights reserved.

    Keywords: Low energy buildings; Advanced natural ventilation; Ventilation areas; Case studies; Downdraught cooling

    1. Background

    The imperative of reducing the emission of greenhouse gases,

    and in particular CO2, caused by the burning of fossil fuels has

    stimulated interest in thedesign of low energybuildings.In the20

    buildings monitored by Bordass et al., in the well known UK

    PROBE Studies [1] there was a factor of 6 difference in the CO2emissions produced for space conditioning and lighting a given

    floor area (Fig. 1). Nine of the 10 highest CO2 emitters were air-

    conditioned (AC) or mixed mode (MM) (these used chilled

    beams, with displacement ventilation, etc. rather than full AC),and 9 of the 10 lowest emitters were naturally ventilated (NV) or

    advanced naturally ventilated (ANV). The term advanced

    natural ventilation was coined to encompass buildings which

    utilised the stack effect to drive an air flow and so has been

    adopted for the buildings which are the subject of this paper. In

    the ACand mechanicallyventilated buildings, the CO2 emissions

    resulting from the fans and pumps required to move air (and

    water and refrigerant) accounted for up to 50% of the emissions

    associatedwith space heating andcooling.Because ACbuildings

    tend to be deep-plan, the CO2 emissions for artificial lights were

    also substantial. Buildings which are particularly densely

    occupied, with long periods of usage and with high internal

    heat gains (e.g. from computers and other equipment) might

    justify the use of AC, but as the PROBE results show, some

    relatively lightly used buildings nevertheless had AC.

    NV and ANV buildings utilise naturally occurring wind

    pressures, and/or the buoyancy force generated by internal heat

    sources, to drive an air flow, thereby avoiding the use of fans.Admitting cool night air into a building, to purge daytime heat

    accumulated in exposed thermal mass, can avoid the need for

    mechanical cooling entirely or, in warmer locations, reduce

    cooling loads, energy use and associated CO2 emissions.

    Shallow-plans, which typify simple NV buildings, or the use of

    atria and lightwells in deeper-plan buildings, can improve the

    use of natural light reducing the CO2 emission associated with

    artificial lighting.

    Whilst global warming is seen as a treat to NV and ANV

    buildings, the overheating risk can be overstated. Current and Buildings 39 (2007) 166181

    * Tel.: +44 116 257 7961; fax: +44 116 257 7977.

    E-mail address: [email protected]

    0378-7788/$ see front matter # 2006 Elsevier B.V. All rights reserved.


    mailto:[email protected]://[email protected]
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    evidence for the UK, although rather weak, suggests that ANV

    can keep buildings comfortable though the next century in allbut the hottest (London) region [2,3].

    Conventionally conceived NV buildings are shallow plan

    with an extended perimeter, and facade openings which provide

    the fresh air inlet and exhaust air outlet (Table 1). These

    features can be incompatible with the planning constraints

    imposed by tight urban sights and the noise and pollution in city

    centres. The use of manually operated windows can

    compromise security, increasing concerns about theft by

    building occupants (a particularly important consideration

    for library buildings of the type described in this paper).

    Mechanically controlled perimeter windows enable night

    ventilation but the building may then be vulnerable to break-

    in or other malicious acts.At the design stage an ability to reliably predict the likely

    internal conditions in a building, for example by using dynamic

    thermal models and computational fluid dynamics programs,

    can be reassuring and it is important to have a clear idea of how

    the internal conditions in the finished building will be

    controlled. Relying, as they do, on variable and ill defined

    pressure differences set up across the building by the wind, the

    likely performance of simple NV buildings is hard to predict

    and control.

    ANV buildings that utilise the stack effect, in which air

    warmed by internal sources of heat drives the air flow, do not

    necessarily rely on wind pressures. If properly designed and

    controlled, an air flow can be assured at all times when there is

    an internal source of warmth, including at night. In fact, in anunconstrained displacement flow regimen, where heat sources

    generate isolated plumes of warm air, the flow rate is directly

    proportional to the strength of the source, and the interface

    between the cooler air the warm air above remains fixed [4].

    With heat sources distributed over a surface, the air flow is also

    dependent on the source strength in steady state conditions [5].

    This happy coincidence, between heat input and air flow rate,

    enables rather simple but robust control of air flow and makes

    prediction of performance at the design stage comparatively

    reliable. Further, the interface between the cool and warmer air

    can be designed to lie above head height.

    The benefits of control-ability and predictability, which stack

    driven natural displacement ventilation offers, can be lost if windpressures begin to dominate the flow. An inability to harness

    these pressures is not a disadvantage; after all it is during still

    warm summer conditions when it is most difficult to keep ANV

    buildings thermally comfortable. Therefore, designing the

    buildings to be wind neutral is a useful guiding principle.

    In a recent paper [2] a taxonomy was proposed, in which

    stack ventilated buildings were divided into four main types

    (Fig. 2). The edge-in, centre-out approach (E-C) is exemplified

    by the Queens Building at De Montfort University, Leicester,

    UK[69] and the edge-in, edge-out strategy (E-E) by the UK

    Building Research Establishments (BRE) Energy Efficient

    Office of the Future [10].

    Fig. 1. The CO2 emissions from 20 buildings and ECON19 [32] benchmarks.

    K.J. Lomas/ Energy and Buildings 39 (2007) 166181 167

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    In both building types, cold winter air can be drawn in over

    perimeter heating elements to pre-warm it and in summer

    operable windows can be used to enhance airflows, and create air

    movement, without disrupting the basic airflow strategy.

    Mechanically operated air inlets permit night ventilation (and

    in the Queens Buildings lecture theatres, also daytime

    ventilation). With centrally located stacks (E-C), deep-plan

    buildings are possible, as in the Queens Building. Whilst

    centrally located atria can, in principle, assist buoyancy driven

    flow, stacks require less space, have more reliable ventilation

    performance, can have terminations which are less susceptible

    to wind effects and can, if necessary, incorporate low-powered

    axial fans to encourage airflow under particularly adverse

    conditions (as in the BRE office). The disadvantage of the

    edge-in strategy is that the perimeter inlets are susceptible to the

    noise, pollution and security concerns associated with design on

    urban sites (Table 1).The three case study buildings described in this paper, for

    which the author provided strategic design advice and

    performance evaluations, on behalf of the client and the

    architect, Short and Associates, all utilise a centre-in ANV

    strategy: the centre-in, edge-out (C-E) strategy is exemplified

    by the School of Slavonic and East European Studies building

    (SSEES) building, London, UK [2,1113]; the larger, very

    deep-plan, Frederick Lanchester Library (FLL), in Coventry,

    UK employs both the C-E and C-C strategy [2,3,11,1319];

    whilst the Harm A Weber Library (HAWL), in Elgin, near

    Chicago, Illinois, USA [20,21] uses the C-E approach with

    localised E-E ventilation of perimeter offices.

    Table 1

    Characteristics of different simple and advanced natural ventilation strategies (after [2])

    Simple natural ventilation Advanced natural ventilation (ANV)






    edge-out (E-E)


    centre-out (E-C)


    edge-out (C-E)


    centre-out (C-C)

    Architectural implications

    Air inlet object


    No No No No Yes YesExhaust stacksb No No Yes Yes Yes Yes

    Plan depthc 2.5 (5) 5 10d 10d 5Deep plane No No No Yes Yes Yes

    Indoor air quality provided

    Occupant inlet control Yes Yes Yes Yes No No

    Close control No No Possf Possf Yes Yes

    Displacement vent possible No No Yes Yes Yes Yes

    Draught control Poor Poor Poor Poor Good Good

    Performance predictability Poor Poor Good Good Very good Very good

    Protection from local environment

    Urban noise attenuation Poor Poor Poor Poor Good Good

    Perimeter security Poor Poor Poor Poor Good Good

    Robustness to climate change

    Night vent cooling Yesf Yesf Yesf Yesf Yes YesPossible mech vent assistg No No Yes Yes Yes Yes

    Comfort cool Difficult Difficult Difficult Difficult Easy Easy

    Heat recovery No No No No Noh Noh

    a Such as plenum and lightwell.b Might utilise other feature, such as an exhaust air lightwell.c Rules of thumb (e.g. CIBSE, 2001)based on multiples of the floor-to-ceiling height. For single sided vent this is the room depth, but for cross-flow vent it is the

    floor plate width perimeter-to-perimeter.d With a row of centrally located stacks, exhausting both sides of the building (E-C), or a central air inlet shaft supplying both sides of the building (C-E), the

    perimeter-to-perimeter depth may be 10.e Exceeding, about, 20 m perimeter-to-perimeter.f If mechanically controlled perimeter air inlets are used.

    g E.g. fan in a stack or fan pressurised supply.h However, since the air is exhausted through discrete vertical stacks, heat recovery is possible when a mixed modevariant of the building is operated in mechanical

    mode (e.g. HAWL).

    Fig. 2. Schematic diagrams of the different forms of stack ventilation.

    K.J. Lomas/ Energy and Buildings 39 (2007) 166181168

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    The centre-in strategy has a number of strategic design

    advantages: it enables the external facade to be sealed, which

    overcomes security, noise and pollution concerns; the air supply

    route can become a lightwell if necessary, thereby introducing

    daylight into a deep-plan; the air inlet can be protected from

    wind effects, giving even greater confidence about the likely

    airflows (than in buildings with perimeter openings); and the

    fresh air can be pre-heated. Further, by locating the exhaust

    stacks at the perimeter of the building, as in the three case study

    buildings, the basic floor plate is left clear permitting more

    flexible internal space planning.

    Most interestingly, the supply air can be comfort cooled or

    fully conditioned enabling the same basic, but versatile, plan

    form to incorporate either an ANV or a mixed-mode (MM)

    approach to ventilation without the overhead of having two

    different air distribution systems (one for mechanical mode and

    the other for natural mode), Table 1. This offers the prospect of

    introducing measures to combat future climate change and for

    applying the basic design strategy to a range of climate types

    these advantages are illustrated by the case study buildings.

    2. Common features of the three case study buildings

    There are numerous geometrical differences between the

    case study buildings, as dictated by the client, site, budget,

    summer cooling strategy, etc., however, design considerations

    imposed by the natural ventilation mode of operation have a

    major impact on the overall built form and so there are strong

    generic similarities between them: it is these on which this

    paper concentrates. The geometry of the three buildings and the

    intended ventilation strategies, are illustrated in Figs. 38 and

    their key features, and dimensions, particularly those related tothe ventilation strategy, are tabulated in Appendix A.

    All three case study buildings were for educational

    establishments with clients who would own and operate the

    buildings and so were concerned about whole of life operating

    costs and particularly energy consumption. The buildings

    contain cellular offices for staff and teaching spaces and

    extensive areas for library books, which, for security reasons,

    and because of the noisy sites, required the building facade to be


    The climate to which the UK buildings were exposed is, of

    course, much less severe than that in the Chicago region (see

    Appendix A1). For example the UK sites have around 230

    cooling degree days (CDD) to base 15.5 8C, compared to 766for Chicago; the mean daily maximum temperature (MDMa) in

    the warmest month (July) is around 20 8C at the UK sites and

    28.7 8C in Chicago; and there were under 3% of working hours

    when the ambient temperature exceeded 25 8C (WH25) at the

    UK sites and over 15% in Chicago. Comparing the two UK

    climates it is evident that London, even without considering the

    urban heat island influence, is warmer than Manchester (CDD

    229 cf. 77; MDMa 22.4 8C cf. 19.4 8C; and WH25 2.9% cf.

    0.6%). Interestingly, the Chicago climate has a greater mean

    diurnal swing in both spring and autumn than the UK climates

    (over 9.5 K cf. under 8 K), which suggests that night ventilation

    cooling could be a useful energy saving resource. The diurnal

    swing for London is, in fact, likely to be less than the climate

    file indicates due to the urban heat island effect [13,22].

    To contend with these climatic differences, the three case

    study buildings illustrate a progressively more complex

    environmental control strategy: from pure ANV for the FLL

    (which is located in the UK Midlands); through ANV withcomfort cooling using passive downdraught cooling (PDC) in

    the SSEES building (because of the reduced summer night

    cooling potential caused by the London urban heat island and

    because the UK design guidelines relevant at the time [23]

    require the use of a near-extreme weather year for the design of

    naturally ventilated buildings2); to ANV with full HVAC

    support in the HAWL (because of the severe Chicago climate).

    As noted above, the summer-time mechanical cooling

    Fig. 3. Floor plan of the Frederick Lanchester Library (after [13]).

    1 AppendixA presents Manchester data for the FLL as this is the nearest TRY

    site to Coventry.

    2 The third hottest year recorded in London (Heathrow) between 1976 and

    1995: the London Design Summer Year is 1989.

    K.J. Lomas/ Energy and Buildings 39 (2007) 166181 169

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    equipment could be introduced into the SSEES and HAWL

    without compromising the basic centre-in ANV strategy.

    The buildings have a square (or in the case of the SSEES,approximately square) footprint, which yields a low surface

    area to volume ratio. This, together with the high insulation

    standards used in the roofs and walls (Appendix A), produces

    low specific fabric heat gains and losses. The windows are of

    clear low-emissivity double glazing to admit natural light to

    perimeter offices and work spaces, and the SSEES and FLL

    have artificial lighting which responds to daylight levels. The

    windows are well shaded to reduce perimeter heat gains: either

    by deep window reveals (HAWL); by adjacent buildings

    (SSEES); or by the stacks, stair towers and metal shading fins

    (FLL). Concrete (SSEES) or steel (FLL and HAWL) columns

    and beams support the exposed flat concrete ceilings, which are

    essential for effective night ventilation cooling. Castellatedbeams (FLL) or open trusses (HAWL) enable stratified warm

    air to move across the ceiling soffit. The plan forms, insulation

    standards and window designs represent good, energy efficient

    practice, irrespective of how buildings are conditionedbut the

    deep-plans are unusual for NV buildings.

    The lightwells are, of course, a critical and distinctive feature

    of the three buildings. These supply fresh air to each above-

    ground floor via low level openings to encourage a displacement

    ventilation flow. Higher floor-to-ceiling dimensions are advanta-


    to occupied spaces is control by either dampers (FLL) or

    windows (SSEES, HAWL) set below desktop level. Secondary

    Fig. 4. Frederick Lanchester Library showing air supply strategy (left) and air exhaust strategy (right) (after [13]).

    Fig. 5. Floor plan of theSchool of Slavonic andEastEuropean Studies Building

    (after [13]).

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    heating is provided by column radiators (SSEES, HAWL) or

    trench heaters (FLL) at the point where the air leaves the

    lightwell and enters the occupied spaces. Perimeter heating is

    used in all buildings to offset any fabric heat loss.

    The lightwells in the SSEES and HAWL are centrally located

    to so that the flow path from the inlets to the perimeter air outlets

    is approximately the same in all directions. In the larger(50 m 50 m) FLL, four lightwells areused, onein the centre ofeach quadrant, and a central lightwell acts like a large, glazed air

    exhaust shaft. The use of a triangular lightwell in the SSEES

    building was primarily an architectural choice, precipitated by

    the shape of the building and constructional considerations,

    rather than ventilation or environmental control considerations.

    The lightwells have clear glazing in the walls and at the top

    to admit natural light to the centre of the buildings and to

    provide visual connectivity between the interior and the

    outside. It is critical that the lightwell tops are sealed shut in

    winter to prevent warmed buoyant air within them leaking

    away. They must also incorporate summertime solar radiation

    Fig. 6. School of Slavonic and East European studies building showing the

    natural ventilation cooling strategy (after [13]).

    Fig. 7. Floor plan of the Harm A Webber Library (after [20]).

    Fig. 8. Harm A Webber Library showing the natural ventilation strategy (after


    K.J. Lomas/ Energy and Buildings 39 (2007) 166181 171

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    control to stop the ambient air, and in the case of the SSEES and

    HAWL buildings the cooled air, being heated. The FLL and the

    HAWL have an enclosed greenhouse formed by a horizontal

    glazed screen just above the air inlet to the top-most floor. The

    greenhouses have moving blinds and can be copiously

    ventilated to remove solar-originated heat in summer. The

    SSEES building uses the same principle, but the greenhouse

    takes the form of an upper and lower ETFE cushion. There is no

    blind system, but the space between the cushions can be

    ventilated and the lightwell top is tilted towards the north. The

    lower ETFE layer has dampers around its perimeter with

    cooling batteries below. In spring and summer the dampers

    admit ambient air for ventilation cooling and if necessary the air

    can be chilledpassive downdraught cooling.

    In all three buildings air is supplied to the lightwell(s) via a

    horizontal plenum located between the ground floor and the

    basement. The plenum feeds all sides of the lightwell, in the

    SSEES and HAWL, but just two sides of each lightwell in the

    FLL. Vertical drops from the plenum supply fresh air to

    basement areas in the SSEES and HAWL, which are exhaustedby stacks. Because the basements are partially earth-bound and

    are poorly day lit, they tended to house unique (support) spaces

    (e.g. book archives and computer rooms), some of which may

    require air conditioning. (For example, the whole of the FLL

    basement is a 24 h access computing suite.)

    The plena are supplied with ambient air via dampered slots

    at the buildings perimeter (FLL and HAWL) or by air supply

    corridors and discreet inlets (SSEES). Each plenum has inlets

    located on more than one side of the building so that the

    dampers at one or more inlets can be closed in adverse wind

    conditions whilst retaining an open air inlet elsewhere. The

    inlets are heavily louvered and incorporate either bird androdent mesh (SSEES, HAWL) or insect mesh (HAWL). Heater

    batteries pre-heat the air and are located either across the base

    of the lightwell (FLL) or behind the air inlets (SSEES, HAWL).

    The latter strategy avoids insulating the plenum and enables the

    bottom of the lightwell to be clear glazing, thereby providing

    natural light to the basement.

    The perimeter stacks are an architecturally striking feature of

    these ANV buildings and are crucial the ventilation strategy. In

    all three buildings they are reasonably uniformly distributed

    around the perimeter, which: assists with aesthetics; enables the

    stacks to contribute to solar shading of windows; creates thermal

    buffers between the inside and outside; helps to ensure zones of

    warm stale air do develop in the building; and offers planningflexibility by enabling perimeter cellular spaces to be easily

    locked into an exhaust stack. This latter benefit is fully

    exploited in theSSEES building,which hasmanyoffices:the rear

    stacks, of triangular shape, cover the entire back wall and a

    double facade runs right across the front face.3 There are draught

    lobbies to entrance doors which prevent the stacks drawing in

    ambient air, which is particularly important in winter when the

    stack forces are greatest and the draught risk higher.

    Exhaust air enters the stacks through dampered openings set

    below the ceiling soffit. The stacks are vertical and well

    insulated to keep the air in them warm and buoyant. They

    discharge above roof level to provide the necessary stack height

    and to position the terminations (which are to be neutral to wind

    effects) out of the turbulent airflow zone at roof level. The

    terminations are louvered to prevent the ingress of precipitation

    and they contain bird or insect mesh. Above the roofline the

    stacks have a rectangular cross section, which simplifies the

    design of the dampers which seal each stack at roof level.

    In the HAWL the stacks discharge into a sloped roof plenum

    which exhausts via five ridge-mounted terminations: a position

    which the client preferred to the more dramatic perimeter

    location for stacks.

    Experience from CFD analyses has indicated that, unless

    perimeter stacks extend above the level of the top floor inlet a

    long way, which can be costly and impractical, stale air from

    lower floors can re-enter the top most floor [15]. Thus dedicated

    top floor exhaust paths have become a feature of these ANV

    buildings: separate short stacks in the HAWL; short stacks pluspartitioned perimeter stacks in the FFL; and partitioned stacks

    at the rear, but dedicated partitioned chimneys at the front, in

    the SSEES.

    3. Preliminary sizing of advanced natural ventilation

    system components

    3.1. Preliminary design

    In the preliminary design phase the geometry of a building

    can be extremely fluid as the architect grapples with a multitude

    of design constraints and design driversstructure, spaceplanning, fire, safety, cost, etc. This might include considering

    the number of storeys, the disposition of open plan and cellular

    spaces and the external visual appearancein particular the

    position, size and number of stacks and the style of the roof top

    exhaust air terminations.

    Under such circumstances it is helpful if the design team can

    work with simple equations and guidelines. In the context of

    centre-in ANV buildings, these need to assist with sizing and

    locating the main components: the plena; the lightwell(s); the

    stacks; and the air inlets and outlets to and from these. The

    critical measure is the free area available for air flow and so the

    equations that follow generate target structural areas for

    preliminary design, i.e. the size of the opening to be created bythe architect (and into which any air flow control, heating and

    other equipment will be inserted). The target areas are

    expressed as a percentage of the floor area to be ventilated,

    which enables them to be used with buildings of arbitrary size

    and shape.

    The air flow rates required to maintain thermal comfort

    during warm, still summer conditions invariably dictate the

    maximum free area of opening required (some spaces have

    higher ventilation needs, see e.g. [24], but in these specialist

    buildings, natural ventilation is probably inappropriate).

    Initially, the free areas are calculated on the basis of the

    overall expected internal heat gain in the particular building

    3 The outer front facade was designed to harmonise with the Georgian

    architecture of the surrounding area.

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    type, tabulated values for which can be found in design guides

    (e.g. [24]).

    Assumptions about the likely internal temperature differ-

    ential and air speeds have to be made and, for preliminarydesign, values are chosen such that the calculated ventilation

    opening areas are conservative, i.e. over, rather than under,

    sized. Experience indicates that areas set aside for ventilation

    (stacks, lightwells, etc.) at preliminary design, can be readily

    surrendered for other purposes as the design evolves, but trying

    to reclaim space, to compensate for under sizing early in the

    design process, can be very difficult.

    The target areas calculated are, of course, merely the starting

    point. As the design evolves, the free areas can be refined, by re-

    using the equations but with the improved knowledge about the

    use which will be made of spaces and thus the likely heat gains.

    Detailed design can involve more accurate manual calcula-tions, such as the application of stack effect equations (see, for

    example [24]), and, later in the design process, the use of

    sophisticated computer-based methods such as thermal

    simulation and computational fluid dynamics analysis (see

    Appendix A for the analyses used in designing the case study

    buildings). Indeed, it is experience gained through the use of

    these methods that has informed the development of the simple

    equations and guidelines presented here.

    3.2. Displacement and stack ventilation

    The sizing equations are based on considerations of a simple

    stack-driven displacement ventilation regimen; that is, a lowlevel inlet supplying the space to be cooled and a high level

    outlet into a stack. The volume flow of air, m (m3/s), required to

    provide ventilation cooling for different internal heat gains is

    given by:

    m QA

    Cc DTm3=s (1)

    where Q is the heat gain (W/m2), A (m2) the floor area, DT(K)

    the allowable temperature rise, and Cc is the volumetric heat

    capacity of air (1200 J/m3 K).

    Typically, the supply air temperature would be 23 K below

    the target temperature for the occupied zone. The temperature

    close to the ceiling could be about 3 K, or in the case of higher

    ceilings, 4 K above the mid-height temperature [24,25]. Thus

    an assumption that the overall temperature difference, DT, is

    7 K is reasonable. Given this, the volume flows necessary fordifferent heat gains can be found (Table 2, cols 2 and 3).

    The allowable temperatures rise, DT, could be varied from

    the value used here, for example 5 K might be more appropriate

    in buildings with a lower ceiling height (and vice-versa). The

    effect would be to proportionally increase (or decrease) the

    target volume flows of air, and hence the target opening areas,


    The mass flow rate can be converted into a free area of

    ventilation opening, A, via:

    A m

    vm2 (2)

    where v (m/s) is the assumed air speed.

    The achievable air speed will decrease as the stack height

    decreases, all other factors being equal. Assuming that

    dedicated ventilation provision is made for the top floor of

    the building (as in the case studies) then the top-but-one floor

    will have the shortest stack height, say 6 mthe height of the

    floor above plus the height from the roof level to the stack

    termination. Using this value, and a DTof 7 K, it can be shown

    (e.g. equations in [24], pp. 411) that a value for v of 0.5 m/s is

    reasonable. Experience from CFD analysis corroborates this

    rough assumption (e.g. [21]).

    3.3. Location and size of lightwell

    It is generally most appropriate to position the lightwell in

    the middle of the floor plates which are to be ventilated:

    although circumstances can arise which dictate otherwise, for

    example when a building abuts its neighbours so that exhaust

    stacks cannot be located on all sides.

    The volume flow of air required up the lightwell, ml, can be

    calculated on the basis of total area of the building to be

    ventilated from the lightwell, Ab, and the expected daily

    average heat gain density in these areas, Qb. A building-average

    heat gain is appropriate for lightwell sizing even if peak gains in

    individual spaces are known, because, whilst some zones might

    Table 2

    Estimated structural inlet and outlet areas for use at the preliminary design stage of advanced natural ventilation systems

    Total heat gain


    Airflow rates Target structural areas as percentage of area of floors served (%)

    (ls1)a (ach1)b Lightwell

    outletscLightwell, plenum

    outlet and stacksdPlenum


    20 2.4 2.5 1.6 0.5 1.0

    30 3.6 3.7 2.4 0.7 1.440 4.8 4.8 3.2 1.0 2.0

    50 6.0 6.0 4.0 1.2 2.4

    60 7.1 7.4 4.8 1.4 2.8

    Values in bold are target area for preliminary design purposes.a Air flow rate required for ventilation cooling per m2 of floor area.b Assumes 3.5 m floor to ceiling height.c Eq. (8) assumes gross structural area is twice the free area and the air speed is limited to 0.3 m/s.d Eqs. (3), (4), (9) and (10) presume no obstruction by grills, dampers, meshes, louvers, etc. and an air speed of 0.5 m/s.e Eq. (7) assumes gross structural area is twice the free area and the air speed is limited to 0.5 m/s.

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    be at full occupancy, it is unlikely that all spaces will be so

    simultaneouslypeople move around redistributing the heat

    sources (and the stack system will automatically draw more

    air to the more densely occupied, and thus warmer, zones).

    Periods of particularly dense occupancy also tend to be short

    lived (especially at the whole building level) and a thermally

    massive building, in which occupants can radiate heat to a

    night-cooled ceiling slab, can simply ride out periods of dense

    occupation (the time history of thermally massive buildings can

    be in the order of several days).

    The lightwell cross sectional area, Al, can be expressed as a

    percentage of the whole building floor area by combining

    Eqs. (1) and (2):




    vCc DT 100 % (3)

    Using the values of 0.5 m/s, 1200 J/m3 K, and 7 K for v, CcandDT, respectively, yields the ratio of the lightwell area to that

    of the total floor area ventilated (Table 2), e.g. 0.7% and 1.2%,for heat densities of 30 and 50 W/m2, respectively.

    For a lightwell fed by a plenum (as in the FLL and HAWL),

    the cross sectional area calculated is, in fact, for the bottom of

    the lightwell. From an air supply standpoint, a bottom-fed

    lightwell could taper because the air volume to be carried

    diminishes floor-by-floor. However, from an interior day-

    lighting standpoint, it is better to have a lightwell with a large

    aperture at the top (and a larger aperture is almost certainly

    needed if the lightwell is (also) to be fed by from above in

    ventilation cooling mode, e.g. the SSEES building). Also, as

    will be seen later, it tends to be the available area of lightwell

    perimeter, rather than the cross sectional area of the lightwell,

    which begins to dictate its size. Therefore, in practice, airsupply lightwells (even those fed with air from the bottom only)

    tend to have vertical sides: this can also be less costly than

    sloping sides.

    3.4. Sizing the air inlet plenum

    If the lightwell is fed only from the bottom, the plenum

    needs to be able to deliver all the air the building needs. Thus,




    vCc DTbpo 100 % (4)

    where bpo is the proportion of the plenum outlet that is blocked

    (0, fully blocked; 1, no blockage), Apo the free cross-sectional

    area of the plenum outlet into the lightwell and v and DTare

    again 0.5 m/s and 7 K, respectively. If there is no obstruction at

    the interface between the lightwell and plenum and, in fact, no

    airflow control device is needed at this point, the free cross-

    sectional area Apo is equal to the gross structural area, giving:

    Apo Al (5)

    Because the building perimeter, which is the location for the

    plenum inlet, is longer than the lightwell perimeter, it tends to

    be the outlet from the plenum into the lightwell which

    determines the plenum depth, Dp:

    Dp Apo

    Plm (6)

    where P1 (m) is the length of the lightwells perimeter. Because

    a shallow plenum is desirable, as this reduces the overall

    building height (and increases the head height in a basementbelow), it is advantageous to make maximum use of the

    perimeter available by connecting the plenum to all sides of

    the lightwell (as in the SSEES and the HAWL).

    The plenum can be supplied with ambient air either by a slot

    around the buildings perimeter (FLL, HAWL) or by air

    corridors (e.g. SSEES). The structural opening to these should

    be sufficiently large that the necessary obstructions, dampers,

    heater batteries and bird or insect meshes, do not inadvertently

    reduce the free area. Therefore, the structural area of inlet to the

    plenum is given by:




    vCc DTbpi 100 % (7)

    where bpi is the proportion of the plenum inlet that is


    For preliminary design purposes, it is reasonable to assume

    that bpi is 0.5. Methods of reducing blockage at the air inlet

    include: raking the heater batteries (SSEES and HAWL) or

    enlarging the mouth of the plenum (HAWL). Whilst the plenum

    can carry services these should not unduly restrict the free area

    or hinder maintenance.

    If the air entering the plenum serves only the lightwell (and

    not, for example, the basement below) then Qb and Ab are the

    same as in Eqs. (3) and (4). However, if some of the air entering

    the plenum is used to ventilate a basement (as in the HAWL andthe SSEES) or other areas (such as the perimeter offices, in the

    HAWL) then the value of Ab in Eq. (7) will be larger than the

    value used in Eqs. (3) and (4). To produce the target areas in

    Table 2 it has been assumed that all air entering the plenum

    supplies only the lightwell.

    3.5. Sizing air outlets from lightwell

    The gross structural area of the outlets from the lightwell to

    each floor, Alo, can be given by:


    Af Qf

    voCc DTblo 100 % (8)

    whereAfis the area of the floor to be ventilated; Qfthe heat load

    density on the floor (W/m2); blo the proportion of the inlet that

    is blocked by obstructions; and vo is the speed of the air leaving

    the outlets (m/s). At preliminary design stage the value of Qfmight simply be taken as Qb and refined as the occupancy for

    each floor becomes better defined.

    The air outlets from the lightwell are located adjacent to

    occupied areas. In fact, the available daylight and fresh air,

    together with a structure (the lightwell) on which to mount

    services, makes the lightwell perimeter an ideal place to locate

    work surfaces. Care must be taken, therefore, to avoid cold

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    draughts, especially in winter. Thus the air speeds must be

    limited (and, in winter, the air temperatures not too low) but a

    displacement flow regimen, supplying all the occupied floor

    area, must be achieved. In general, guides (e.g. [24]) suggest an

    upper value of air speed of about 0.15 m/s. However, in summer

    cooling mode, which is the design condition being considered

    here, the speed can be higher (because the supply air

    temperature will be elevated) therefore, for these sizing

    purposes a value for the air speed, vo, of 0.3 m/s has been

    assumed (Table 2).

    The structure associated with the lightwell, the reheating

    devices, and the dampers with their framing and louvers (or

    other flow control objects), will introduce blockage. This might

    mean that only 50% of the structural opening is actually free

    area so a reasonable value for blo at preliminary design stage is


    The calculated free area for outlets, e.g. 2.4% of floor

    area for an internal heat gain of 30 W/m2 (Table 2) does not

    seem large. However, the inlets serving an entire floor will

    cluster around the perimeter of the lightwells and the top ofthe inlet may need to be no more than about 0.7 m above the

    floorto ensure a displacement flow and to fit below work

    surfaces. Thus, the length of a lightwells perimeter may

    limit the free inlet area achievable, which can lead to the

    lightwells being enlarged to accommodate the outlet areas

    required.4 In very deep-plan buildings, the lightwell

    perimeter may be insufficient and so multiple smaller

    lightwells may be used, rather than a single large lightwell

    (e.g. the FLL).

    3.6. Sizing the stacks and air outlets

    The stacks themselves are likely to be free of blockage and,

    as noted above, it is reasonable to assume an air speed in them,

    during ventilation cooling operation, of 0.5 m/s. Therefore the

    total area of the stacks As exhausting a floor can be given by:




    vCc DT 100 % (9)

    As noted above Qf might simply be taken as Qb for

    preliminary design. The required area can be distributed

    around a number of equally sized stacks or in some other way

    (the FLL has stacks and a central lightwell, the HAWL stacks

    of differing cross-section and the SSEES stacks and a double


    As successive floors exhaust into each stack, the volume

    flows of air to be carried increases, thus the cross-sectional

    areas could increase up the building. The central lightwell of

    the FLL visibly illustrates this; it enlarges from 36 m2 on the

    ground floor to 82 m2 at the roof topa form which is

    consistent with daylighting considerations. At the level of the

    stack terminations, the total area of all the stack outlets from the

    whole building (Ast) will be given by




    vCc DT 100 % (10)

    values for different Qb are given in Table 2.

    Ideally, the stacks should be vertical and straight and

    terminate above the roof line; as noted above, the top floor may

    need to be ventilated separately and/or the stacks might be

    internally partitioned.

    The stack-top terminations can become rather large because

    they must provide the same free area as the stacks which they

    surmount but also: prevent rain penetration; and include insect

    or bird mesh, dampers and devices to overcome unwanted wind

    pressures. They must also have a suitable aesthetic appearance

    as they may be the most striking visual feature of stack-

    ventilated buildings.

    The inlets to each stack will be located at high level, just

    below the ceiling soffit in order to drain warm, stale stratified

    air, and they will contain dampers to control the airflow. Thus

    the structural area of the openings into each stack will exceedthe area of stacks calculated from Eq. (9) in proportion to the

    degree of blockage caused by the dampers, e.g. by 50%. At

    preliminary design stage it is unnecessary to size these

    openings; however, if the number of stacks provided on a given

    floor is small and if the stacks present a narrow face towards the

    space, it may be difficult to get the necessary outlet area into the

    stack. The stacks in the HAWL and especially the SSEES

    building overcome such difficulties by presenting a wide stack

    face towards the occupied spaces.

    4. Comparison of as-built and target opening areas

    It is instructive to compare the as-built areas of the openings

    in the three case-study buildings with the target areas suggested

    by the above sizing method for a number of reasons: to

    understand which of the target opening areas are, in practice,

    the most difficult to achieve; to illustrate the extent to which

    opening areas deviate from the target figures; to illustrate how

    design ingenuity can enable the desired free inlets to be

    achieved in difficult circumstances; and, finally, to act as a

    springboard for discussion of the finer points of these buildings


    From the as-built dimensions (as given in Appendix A) it is

    possible to obtain: the total floor area to be ventilated from the

    lightwell, Ab; the cross-sectional area of the lightwell, Al; thefeasible maximum area of the outlets from the lightwell, Alo; the

    area of the outlet from the plenum into the lightwell, Apo; the

    gross area available for ambient air to enter the plenum,Api; and,

    finally, the cross-sectional area of the stacks available to exhaust

    the air from the building, Ast (see Table 3). These as-built areas,

    expressed as a percentage of Ab, are compared with the target

    areas intended for preliminary architectural design (Table 2) in

    Table 4. The target areas are calculated using Qb values of 30 W/

    m2 for the FLL and SSEES and 45 W/m2 for the HAWL, which

    were the values adopted at the preliminary design stage.

    It is evident (Table 4) that, in all three buildings, the as-built

    cross-sectional area of the lightwells exceeds the target free

    4 The authors and architect have considered articulating lightwell perimeters

    to artificially increase their perimeter lengthbut this can be costly, construc-

    tionally difficult and in conflict with interior planning ideas.

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    area by a considerable amount. For example, in the HAWL, the

    target area for preliminary design is 1.1% of the floor area

    ventilated, whereas the actual cross-section of the lightwell is

    3.2% of the area ventilated. The target free areas presume that

    there is no obstruction to airflow, however in the FLL, the heater

    batteries across the base of the lightwell reduce the effective

    free area by about 50%, i.e. to 1.0% of the floor area ventilated,which is much closer to the target of 0.7%. Nevertheless, it is

    evident that the cross-sectional area of these lightwells should

    not, in practice, act as a constraint to the flow of ventilation air.

    The air supply lightwells in the FLL occupy about 6.8%, of

    the gross area of the ground to second floor, the SSEES

    lightwell occupies 12% of the (reduced area) ground floor and

    5.4% of floors one to four, and the HAWL lightwell 6.3% for all

    above ground floors. In addition to providing fresh air, the

    lightwells admit daylight which brings both functional and

    physiological benefits to otherwise deep plan spaces.The feasible maximum area of outlets from the lightwells

    are much closer to the target areas, with the HAWL actually

    having a maximum feasible opening area that is less than the

    Table 3

    As-built areas of main airflow apertures in the three case study buildings


    Building Gross floor area of building (m2) 8161 3380 3468

    Gross areas of floors 2228 m2 (G-2) 303 m2 (G) 665 m2 (14) 1165 m2 (G-2)

    Lightwells Total building floor area serveda (m2) 4 1858 2936 2283

    Cross sectional area (m


    ) 4 38 36 73Perimeter lengthb 24.4 m (G-2) 12.2 m (3) 24 m (G-4), 19 (5) 34 m (G-2)Total feasible maximum outlet areac (m2) 4 60 76 71

    Plenum Depth at outlet into lightwell (m) 1.5 0.80 0.93

    Gross area at outletd (m2) 4 18 19h 32Depth at inlet at building perimetere (m) 1.4 n/a 1.45

    Gross area at inletf (m2) 4 36 15 55

    Stacks Cross sectional areasg (m2) 160 33 47.4

    Values in bold, rounded to the nearest 1 m 2, are used to calculate as-built statistics for comparison with preliminary design target values in Table 4.a Excludes the lightwell itself and all areas not ventilated from the lightwell(s), e.g. stair wells, mechanically ventilated areas (e.g. WCs), the basements and, in the

    HAWL, perimeter offices directly ventilated from the facade.b Stated perimeter length includes curved corners of SSEES lightwell, length excluding corners is 18 m (G-4).c Presumes inlet heights are a maximum of 0.7 m (i.e. below desktop). Curved lightwell corners are not a feasible outlet location in the SSEES building. E.g. FLL

    0.7 (3 24.4 + 1 12.2) = 60 m2; 24.4 m perimeter on G, 1 and 2 and 12.2 m perimeter on floor 3.d E.g. FLL = 1.5 12.2 = 18.3 m2 as only two sides of each lightwell served from plenum. The perimeter, excluding the curved corners, is used for SSEES.e Depth of plenum slot at the perimeter of the building for FLL and HAWL, not applicable at SSEES which has discrete air corridors and apertures as inlets.f Structural area, i.e. excluding any obstruction from louvers, dampers, grills, mesh, etc.

    g Area of all stacks at level of terminations, except HAWL which is area at entry to roof plenum, FLL includes area of the central lightwell.h Additionally there is 26 m2 of inlet at the head of the lightwell.

    Table 4

    Comparison of as-built areas and target structural areas for use in preliminary design

    Gross areas as percentage of floor area served (%)a


    As-built b Targetc As-builtb Targetc As-built b Targetc

    Lightwell Gross cross sectional aread 2.0i 0.7 1.2 0.7 3.2 1.1

    Feasible maximum area of outletse 3.2 2.4 2.6 2.4 3.1 3.6

    Plenum Outlets into lightwellf 1.0 0.7 0.7j 0.7 1.4 1.1

    Inlet from ambientg 1.9 1.4 0.5j 1.4 2.4 2.2

    Stacks Cross sectional area at terminationsh 2.2 0.7 1.1 0.7 2.0 1.1

    a Areas rounded to nearest 0.1%.b Areas used to calculate as-built values taken from Table 3, methods of calculation are given below.c Target areas from Table 2 using whole building design heat loads of 30 W/m2FLL and SSEES and 45 W/m2 HAWL. These do not, necessarily, concur with the

    heat loads used later in the design process when occupancy had been more accurately defined.d As percentage of floor area served, e.g. FLL 38/1858 = 2.0%.e Excludes curved lightwell corners in the SSEES, e.g. FLL 60/1858 = 3.2%.f Outlet from plenum to lightwell as percentage of floor area served by lightwell(s), e.g. FLL 18/1858 = 1.0%.

    g E.g. FLL 36/1858 = 1.9%.h Includes central lightwell in FLL, e.g. FLL = 160/(4 1858) = 2.2%.i But the horizontal heater batteries reduce the free cross-sectional area by 50%, to 1% of the floor area served.j

    Additional 26 m2

    of inlet in the head of the lightwell for ventilation cooling giving an extra inlet area of 0.9% of the floor area served.

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    target value (i.e. 3.1% compared to the target of 3.6%). This

    arises because the single lightwell is supplying air to a large

    surrounding floor plate. (Note, that in the HAWL the maximum

    horizontal air inlet to outlet distance is 15.7 m, but in the other

    two buildings it is only 12 m, see Appendix A.) The HAWL also

    has a higher design internal heat gain,i.e. 45 W/m2 compared to

    30 W/m2 in the other two buildings. To overcome the limitation

    in available area, top-hung canopy windows operated by long

    push bars are used at the outlets from the lightwell.

    Additionally, the top-floor, a design studio, has operable

    clerestory windows, so that it can be ventilated with cool air

    directly from ambient (see Fig. 8).

    Overall, these results illustrate the more general point that it

    tends to be the length of the lightwell perimeter necessary to

    achieve a desired outlet area, rather than the cross-sectional

    area of the lightwell itself, that determines the overall lightwell

    dimensions. Put another way, at the preliminary design stage

    the size of the lightwell is likely to be determined by the area of

    outlet needed around the perimeter (Eq. (8)) rather than the free

    area required for the lightwell itself (Eq. (3)).In all three buildings, the area of the outlet from the plenum

    into the lightwell is close to, or in excess of, the target area. In

    the SSEES building, the as-built area only just meets the target

    value (0.7% of floor area), but this free area was difficult to

    attain because the level of the basement and ground floor were

    set by the site levels and, of course, head height had to be

    retained within the basement.

    The inlets to the plenum at the building perimeter were

    larger than the target value in the FLL and HAWL but much

    smaller in the SSEES building (0.5% as-built, compared to the

    target value of 1.4%); again, this was due to the difficult site

    configuration. The vehicle delivery route at ground level, whichloops round the back of the building (see Fig. 6), prevented the

    use of a slot-type plenum inlet (as used in the FLL and HAWL);

    so air corridors were used at the back of the building and four

    large apertures at the front. The restricted area of inlet is

    overcome when the building is in natural ventilation cooling

    mode through the provision of air inlets at the head of the

    lightwellthese inlets provide an additional 26 m2 of opening,

    equivalent to 0.9%, of the floor area ventilated by the lightwell.

    Thus the total inlet area in this mode of operation is 1.4%,

    which is comparable to the target area. The area of inlets to the

    SSEES plenum is, in fact, just sufficient to meet the fresh air

    needs of occupants and it is during this winter time mode of

    operation that the air needs pre-heating before delivery to thelightwell.5

    Although, in the HAWL, the as-built gross inlet area to the

    plenum is marginally larger than the target value, not all the air

    serves the lightwell; some ventilates the basement and some is

    fed up to perimeter offices (Fig. 8). However, when the building

    is in natural ventilation cooling mode, the clerestory windows

    to the top floor studio and the operable windows to perimeter

    offices enable these spaces to be ventilated directly from

    ambient, thereby overcoming any restriction imposed by the

    plenum inlet.

    Because the plenum inlet area was barely large enough in the

    SSEES and HAWL, particular care was taken to reduce the

    blockage caused by heater batteries, by positioning them at a

    raked angle (e.g. Fig. 8). In the HAWL, the potentially severe

    restriction of the insect mesh, rather than bird/rodent mesh, was

    limited by folding the mesh (effectively extending the plan

    length over which it was distributed).

    In all three buildings, the cross-sectional area of the stacks at

    the level of the terminations (and in the case of the FLL, the

    stacks plus the central lightwell) are in excess of the target

    areas; in the FLL by a factor of 3 (2.2% compared to a target

    value of 0.7%). These larger cross-sectional areas are the result,

    in part, of adjusting the areas in line with the stack ventilation

    calculationsby enlarging the outlets, relative to the inlets, the

    neutral pressure level can be encouraged to settle higher up the

    stacks reducing the likelihood of back flow into upper floors.

    This was a particular concern during the design of the FLL.Whilst the stacks in the HAWL seem amply sized, they also

    ventilate the basement and the perimeter offices, which are fed

    with fresh air directly from the plenum rather than from the


    Overall, it is evident that the as-built structural opening areas

    in the SSEES building and the HAWL are rather closely aligned

    to the target areas proposed for preliminary design purposes,

    whereas in the FLL, the opening areas are conservatively larger.

    This general difference results, in part, from the growth in

    confidence of the design team with successive buildings. The

    SSEES and HAWL also have supporting mechanical summer-

    time cooling systems which can be activated when the naturaldisplacement ventilation cooling fails to achieve the desired

    internal temperatures.

    5. Design development

    The forgoing sections have shown how to calculate the

    overall sizes of the components of the ANV systems at the

    preliminary design stage. However, as designs develop, other

    factors must also be considered, for example: the provision of

    air transfer ducts, or labyrinths, to enable air to flow into (at low

    level) or out of (at high level) cellular spaces, as in all three of

    the buildings discussed here (Figs. 4, 6 and 8); the introduction

    of acoustic absorbers, especially between spaces with differentnoise level expectations (e.g. in the stacks of the HAWL

    between the design studio (floor 2) and the library and office

    floors below); and, of course, the fine adjustment of the areas of

    inlets and outlets, to and from, individual spaces to reflect their

    individual design heat loads and the different stack heights.

    These area adjustments can be made by recalculating the target

    areas in Table 2 using the actual heat loads in the individual

    spaces (in conjunction with the floor area of the space). The

    outlet areas into the stacks can be further refined by, in effect,

    using more realistic air flow velocities for the stacksthese can

    be calculated using the well known stack ventilation equations

    (e.g. [24]).

    5 Assuming a ventilation requirement of 10 l/s per person, and that each

    person occupies 10 m2 of building floor area, the target gross area of inlet

    required is 0.4% of the floor area served; which is comparable to the area

    provided of 0.5% (Table 4).

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    Other building, climate and client-specific factors might also

    need to be considered, for example, in the HAWL: the desire for

    operable office windows (which led to offices having dedicated

    stack outlets and inlets directly from the plenum); and the need

    to integrate the HVAC system, and thus the provision of a return

    air path from the exhaust stacks to the plant room (which led to

    the use of the roof plenum).

    Once the design is rather well developed, the likely

    temperatures and airflows need to be determined using more

    sophisticated analyses methods. Typically, as with the three

    buildings described here, this will involve dynamic thermal

    simulation modelling, to understand the time-varying

    behaviour of the building, and CFD analyses to evaluate

    airflows and temperatures under chosen critical conditions

    (e.g. [15], FLL; [21], HAWL). Other computer models might

    be used to evaluate solar heat gains and daylight levels and

    physical models might be used for wind tunnel tests (e.g. of

    termination designs) or for water-bath modelling of internal

    airflows. Such analyses can reveal critical design flaws, for

    example in the SSEES building it was found that the largerstacks at the rear of the building could draw air into the top of

    the double facade and across the floor plates, thus turning the

    double facade into an inlet rather than an outletwhich

    would generate very cold draughts, especially in winter, when

    the condition was most likely to occur. In the final design a

    glazed screen separates the front of the building from the rear

    (see Fig. 5).

    The building geometry and opening areas, as described

    above, are determined by the volume flows of air necessary for

    effective natural ventilation cooling in warm summer condi-

    tions under normal occupancy. Under other circumstances the

    openings required for airflow can be much smaller: e.g. inwinter when pre-heated air to heat only the fresh air

    requirements is needed; at times when the internal heat loads

    are low, outside of occupied periods; to provide appropriate

    night-time ventilation; when the spread of fire and smoke must

    be controlled; and, in hybrid buildings, to control mechanically

    driven airflows. A building energy management (BMS) system

    is, of course, the most appropriate system for effecting such

    control. It would take inputs from air (and possibly thermal

    mass) temperature sensors, CO2 (or volatile organic compound

    (VOC)) sensors, smoke detectors, and, in some hybrid

    buildings, possibly humidity sensors, and send output signals

    to control the dampers, windows (and possible shading

    devices): some insight into the control strategy from theHAWL is given in [20]. The definition of a suitable BMS

    control strategy, its programming, and its subsequent refine-

    ment during the commissioning and early post occupancy

    period, is an area of ANV building design which would benefit

    from further research.

    Preliminary data from the FLL illustrates the good

    summertime cooling performance and low energy consump-

    tion that is possible [3]. It also confirms the need for more

    post-occupancy performance data collection and analysis.

    And this exemplifies a rather more general pointthat there is

    rather little post-occupancy performance data for ANV


    6. Conclusions

    The attributes of two different forms of simple natural

    ventilation and four generic building types for exploiting

    advanced natural ventilation (ANV) have been summarised,

    highlighting, for each one: the architectural implications; the

    indoor air quality provision; the degree of protection from the

    surrounding environment; and the likely tolerance to climate

    change. ANV buildings, with a central airsupply andperimeter

    exhaust stacks, seem to offer benefits in each of these four

    areas. Such centre-in, edge-out (C-E) buildings can, in

    principle, be designed so they are essentially wind neutral,

    that is, wind pressures will not hinder, or assist, theairflow; this

    gives added reliability to predictions of their likely, as-built,


    Three case-study buildings which use the C-E ventilation

    strategy are described: the Frederick Lanchester Library,

    Coventry, which uses ANV; the School of Slavonic and East

    European studies, which uses ANV with passive downdraught

    cooling to combat the warmer central London micro-climate;and the Harm AWebber library, being built near Chicago, USA,

    which integrates and HVAC system within the ANV concept.

    They each have a central air-supply lightwell, fed with fresh air

    by a low level plenum and exhaust stacks arranged around the

    building perimeter. The sizes and other characteristics of these

    components are tabulated, along with synoptic climate data for

    each site.

    Based on experience gained through the design of these

    buildings, simple equations, for use at the preliminary

    architectural design stage, to roughly size the lightwell,

    plenum and stacks are presented. The sizes are determined

    by the volume flows of air needed for summertime NVcooling. The target structural areas to be provided at the

    preliminary design stage, expressed as a percentage of the

    total building floor area to be ventilated from the lightwell(s)

    are presented.

    Finally, the as-build structural areas in the case study

    buildings are compared with the target values. These

    comparisons illustrate that it is relatively straightforward to

    design a central supply route (e.g. lightwell) of sufficient great

    cross-sectional area but that it can be difficult, particularly with

    deeper floor plans and densely occupied buildings, to achieve

    the target structural opening areas for air supply around the

    perimeter of such lightwells. On constrained sites it can also be

    difficult to achieve the target structural opening areas for theplenum inlets. With design ingenuity, however, such difficulties

    can be overcome and strategies for doing this in two of the case-

    study buildings are described.

    The equations and tabulated structural opening areas are

    only a rough target for use at the preliminary design stage: more

    sophisticated analyses should be undertaken as the design


    It is hoped that this paper will give architects and engineers

    the added confidence necessary to embark on the design of

    ANV buildings. Their low energy consumption, relative to

    typical air-conditioned buildings, is valuable in attempts to

    combat global warming.

    K.J. Lomas/ Energy and Buildings 39 (2007) 166181178

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    The architects of the buildings described were Short and

    Associates, with whom the author has had a long-standing and

    fruitful working relationship, and without whose assistance this

    paper would not have been possible. The environmental design

    analyses were led by Dr. Malcolm Cook of the Institute of

    Energy and Sustainable Development.

    Appendix A. Comparison of key features of the three advanced naturally ventilated buildings

    Building name Frederick Lanchester

    Library (FLL)

    School of Slavonic and East

    European Studies (SSEES)

    Harm A Webber

    Library (HAWL)

    Client and context

    Client Coventry University University College London Judson College

    Location Coventry, UK Bloomsbury, London, UK Elgin, Nr Chicago, Il, USA

    Site City center City centre Green campus

    ANV Type C-E, C-C C-E C-E, E-E

    Cooling method Natural Natural and PDC Natural and HVAC

    Number of levels Basement + Ground + 3 Basement + Ground + 5 Basement + Ground + 2

    Completion date September 2000 November 2005 Winter 2006

    Structure Steel frame Concrete Steel frame


    Roof 0.18 W/m2 0.20 W/m2 0.25 W/m2 K

    Wall 0.26 W/m2 0.30 W/m2 0.25 W/m2 K

    Window 2.00 W/m2 2.00 W/m2 2.60 W/m2 K

    Footprint 50 m 50 m 31.5 m 27 m 34 m 34 m

    Gross floor areas 8161 m2 (G + 1 + 2 + 3) 3380 m2 (G-5) 3468 m2 (G + 1 + 2)

    942 m2 (B) 695 m2 (B) 1192 m2 (B)

    Floor to ceiling height 3.9 m (G, 1, 2, 3) 3.2 m (G), 2.9 m (14), 2.6 to 4.9 m (5) 3.35 m (G, 1), 3.86.5 m (3)Window shading Perimeter stacks, metal fins Adjacent buildings External window reveals

    Approximate cost 20 m 10 m $13.5 m


    By designers [2,3,11,1315] [2,1113] [20,21]

    By others [1619]

    Air supply lightwells

    Type 4 no Lightwells Lightwell Lightwell

    Levels serveda G, 1, 2, 3 G, 15 G, 1, 2

    Shape Square Triangular Square

    Top Sealed, glazedb Operable/ETFE Sealed, glazedb

    Shading Moveable blind None Moveable blind

    Bottom Opaque-heater battery Clear single glazing Clear single glazing

    Sides Clear single glazing Clear single glazing Clear single glazing

    Air outlet type Dampers Bottom hung windows Top hung windowsSecondary heating Trench heaters Column radiators Linear finned emitters

    Cross-sectional area 4 no 38 m2 36 m2 73 m2

    Gross perimeter lengthc 4 [25 m (G, 1, 2), 12 m2 (3)]d 24 m (G, 1, 2, 3, 4), 19 m (5) 34 m2 (G, 1, 2)Floor area servede 1858 m2 per lightwell 2926 m2 2283 m2

    Gross area of air inletf 18.6 m2 per lightwell 19 m2 bottom and 26 m2 top 32 m2

    Maximum airflow distanceg 12 m 12 m 15.7 m

    Air inlet plena

    Air inlet type Perimeter slots Corridors (side) and four apertures (front) Two perimeter slots

    Inlet depthh 1.4 m n/ai 1.45 m

    Outlet depth 1.5 m 0.8 m 0.93 m

    Gross plenum inlet area 36 m2 per lightwell 8.7 m2 (corridors), 6 m2 (apertures) 55.2 m2

    Air preheating Horizontal heater coils Raked heater battery Raked heating battery

    K.J. Lomas/ Energy and Buildings 39 (2007) 166181 179

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    Air exhaust pathsj

    Lightwell Stacks Stacks Double


    Stacks and

    roof plenumRear stacks Front chimneys

    Shape Square Square Triangular Rectangular Rectangular slot Rectangular

    Cross-sectional areask 81 m2 20 no 3.28 m2

    (G, 1, 2)

    10 no 2.1 m2

    (15)l4 no 1.2 m2

    (3 and 4)m1 no 7.2 m2

    (G, 1, 2)

    10 no1.64 m2

    and 10 no1.05 m2

    (G, 1)

    4 no 3.28 m2 (3) 3 no 6.5 m2 (2)

    Total outlet areas 160 m2 33 m2 47.4 m2 n

    Minimum and maximum stack height 7 m (2) 4.5 m (3) 6 m (5) 3.9 m (3)

    15.5 m (G) 18.5 m (G) 23 m (1) 12.5 m (G)


    Latitude/longitude 52.378/1.338W 51.488/0.08 42.038/88.278WHDDp (10 8C) 765 656 1745

    HDD (15.5 8C) 2163 1896 1274

    CDDq (18.3 8C) 13 69 426

    CDD (15.58

    C) 77 229 776

    Working hours

    Over 25 8Cr 0.6% 2.9% 15.2%

    Over 28 8C 0.0% 0.6% 7.3%

    Mean diurnal swing

    Springs 7.2 K 7.8 K 9.6 K

    Autumn 5.6 K 6.4 K 10.1 K

    MDMat 19.4 8C 22.4 8C 28.7 8C

    MDMau 7.2 8C 7.3 8C 0.4 8C

    Thermal analysis

    Summer design targetv 5%/27 8C 5%/25 8C Comfort envelope

    Weather filew Kew 67 London DSY Chicago TRY

    Dynamic thermal Sim x ESP-r ESP-r ESP-r

    Ventilation analysis


    CFX CFX and Water-bath model CFXSolar gain/daylightingz None Radiance Radiance

    a Basements either independently mechanically ventilated (FLL) or ventilated from plenum (SSEES, HAWL).b Ventilated greenhouse arrangement the bottom of which is sealed.c Length around the lightwell, in SSEES curved corners are not useable for supplying air length of straight sides is 18 m (G-4).d Only two sides of lightwell adjoin floor 3.e Excludes the lightwell itself and areas not ventilated from ite.g. stair wells, mech vented areas (e.g. WCs), and directly ventilated perimeter offices (HAWL).f Area of inlet from plenum to lightwells, and for SSEES also inlet at top.

    g Ie maximum air flow distance from lightwell to a perimeter stack.h Ie free height between insulation layers, in HAWL occurs at restricting downstand at lightwell edge.i Inlet is air corridors and apertures, not a plenum slot.j Excludes basement exhaust stacks (SSEES, HAWL).k Plan areas at termination of stack/lightwell/double facade (SSEES, HAWL), at entry to roof plenum (HAWL).l Internally divided stacks: Floors 1 and 2 combine, floors 3 and 4 combine and floor 5 linked separately.

    m Floor 3 offices and floor 4 offices internally partitioned chimney

    n Excludes exhaust from perimeter offices fed from perimeter (E-E) rather than the lightwell.o Data for FLL is Manchester TRY; for SSEES, London TRY; and for HAWL, Chicago TRY.p E.g. heating degree days to base 10 8C.q E.g. cooling degree days to base 18.3 8C.r E.g. annual percentage of hours between 8:00 and 18:00 over 25 8C.s Average of daily temperature swing (maximum to minimum) in March/April and October/November.t Mean of daily maxima for month of July for all climates.

    u Occurs in February for London (SSEES) and Manchester (FLL) and in January for Chicago (HAWL).v E.g., no more than 5% of occupied hours over 27 8C, for ANSI/ASHRAE comfort envelope, see eg [26].

    w See reference: [27] for Kew 67; [23] for London DSY; and [28] for Chicago TRY.x For all buildings, combined thermal and airflow modelling was used, for ESP-r, see [29].y CFX is a CFD code, see [30] for the water bath modelling see e.g. [4,5].z See e.g. [31] for description of radiance.

    Appendix A (Continued)

    K.J. Lomas/ Energy and Buildings 39 (2007) 166181180

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