Architectural design of an advanced naturally ventilated ... design of an advanced naturally...

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Energy and Buildings 39 (2007) 166–181

Architectural design of an advanced naturally

ventilated 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

Abstract

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 in

particular, 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

buildings.

# 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 the design of low energy buildings. In the 20

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

PROBE Studies [1] there was a factor of 6 difference in the CO2

emissions 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 AC and mechanically ventilated buildings, the CO2 emissions

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

* 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.

doi:10.1016/j.enbuild.2006.05.004

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

associated with space heating and cooling. Because AC buildings

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

mailto:[email protected]

http://dx.doi.org/10.1016/j.enbuild.2006.05.004

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

K.J. Lomas / Energy and Buildings 39 (2007) 166–181 167

evidence for the UK, although rather weak, suggests that ANV

can keep buildings comfortable though the next century in all

but 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 an

unconstrained 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 wind

pressures 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 [6–9] and the edge-in, edge-out strategy (E-E) by the UK

Building Research Establishment’s (BRE) Energy Efficient

Office of the Future [10].

Table 1

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

Simple natural ventilation Advanced natural ventilation (ANV)

Single

sided

Cross

flow

Edge-in

edge-out (E-E)

Edge-in

centre-out (E-C)

Centre-in

edge-out (C-E)

Centre-in

centre-out (C-C)

Architectural implications

Air inlet objecta No No No No Yes Yes

Exhaust stacksb No No Yes Yes Yes Yes

Plan depthc �2.5 (�5) �5 �10d �10d �5

Deep 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 Yes

Possible 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 mode variant of the building is operated in mechanical

mode (e.g. HAWL).

K.J. Lomas / Energy and Buildings 39 (2007) 166–181168

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 Building’s lecture theatres, also daytime

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

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 [2,11–13]; the larger, very

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

UK employs both the C-E and C-C strategy [2,3,11,13–19];

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.

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


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. 3–8 and

their key features, and dimensions, particularly those related to

the 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

sealed.

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 766

for 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.

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

site to Coventry.

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 with

comfort 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

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

1995: the London Design Summer Year is 1989.

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

Fig. 5. Floor plan of the School of Slavonic and East European Studies Building

(after [13]).


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. Castellated

beams (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 conditioned—but 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-

geous with such a flow regimen. The flow of air from the lightwell

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

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

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

[20]).


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 are used, one in the centre of

each 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


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 chilled—passive 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 exhausted

by 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 and

rodent 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 planning

flexibility by enabling perimeter cellular spaces to be easily

‘locked into’ an exhaust stack. This latter benefit is fully

exploited in the SSEES building, which has many offices: 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.

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

architecture of the surrounding area.

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 plus

partitioned 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 drivers—structure, space

planning, fire, safety, cost, etc. This might include considering

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

spaces and the external visual appearance—in 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 by

the 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

Table 2

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

Total heat gain

(W/m2)

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

(ls�1)a (ach�1)b Lightwell

outletsc

Lightwell, plenum

outlet and stacksd

Plenum

inlete

20 2.4 2.5 1.6 0.5 1.030 3.6 3.7 2.4 0.7 1.440 4.8 4.8 3.2 1.0 2.050 6.0 6.0 4.0 1.2 2.460 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.


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 preliminary

design, 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 low

level 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 ¼ QA

Cc DTðm3=sÞ (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 2–3 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 for

different 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,

required.

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

ventilation opening, A, via:

A ¼ m

vðm2Þ (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 m—the height of the

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

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

(e.g. equations in [24], pp. 4–11) 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


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

simultaneously—people 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):

Al

Ab

¼ Qb

vCc DT� 100 ð%Þ (3)

Using the values of 0.5 m/s, 1200 J/m3 K, and 7 K for v, Cc

and DT, 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, air

supply 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,

Apo

Ab

¼ Qb

vCc DTbpo

� 100 ð%Þ (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 DT are

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 ¼ 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 ¼Apo

Pl

ðmÞ (6)

where P1 (m) is the length of the lightwell’s perimeter. Because

a shallow plenum is desirable, as this reduces the overall

building height (and increases the head height in a basement

below), 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 building’s 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:

Api

Ab

¼ Qb

vCc DTbpi

� 100 ð%Þ (7)

where bpi is the proportion of the plenum inlet that is

obstructed.

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 and

the 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:

Alo

Af

¼ Qf

voCc DTblo

� 100 ð%Þ (8)

where Af is the area of the floor to be ventilated; Qf the 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 Qf

might 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


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

0.5.

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 of

the inlet may need to be no more than about 0.7 m above the

floor—to ensure a displacement flow and to fit below work

surfaces. Thus, the length of a lightwell’s 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:

As

Af

¼ Qf

vCc DT� 100 ð%Þ (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

facade).

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 top—a form which is

consistent with daylighting considerations. At the level of the

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

4 The authors and architect have considered articulating lightwell perimeters

to artificially increase their perimeter length—but this can be costly, construc-

tionally difficult and in conflict with interior planning ideas.

whole building (Ast) will be given by

Ast

Ab

¼ Qb

vCc DT� 100 ð%Þ (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 exceed

the 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’

designs.

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; the

feasible 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

Table 3

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

FLL SSEES HAWL

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

Gross areas of floors 2228 m2 (G-2) 303 m2 (G) 665 m2 (1–4) 1165 m2 (G-2)

Lightwells Total building floor area serveda (m2) 4 � 1858 2936 2283Cross sectional area (m2) 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 m2, 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.


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.

Table 4

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

Gross areas as pe

FLL

As-builtb

Lightwell Gross cross sectional aread 2.0i

Feasible maximum area of outletse 3.2

Plenum Outlets into lightwellf 1.0

Inlet from ambientg 1.9

Stacks Cross sectional area at terminationsh 2.2

a Areas rounded to nearest 0.1%.b Areas used to calculate as-built values taken from Table 3, methods of calculac Target areas from Table 2 using whole building design heat loads of 30 W/m2—F

heat loads used later in the design process when occupancy had been more accurad 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 lightweg 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%j Additional 26 m2 of inlet in the head of the lightwell for ventilation cooling g

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

ign

rcentage of floor area served (%)a

SSEES HAWL

Targetc As-builtb Targetc As-builtb Targetc

0.7 1.2 0.7 3.2 1.1

2.4 2.6 2.4 3.1 3.6

0.7 0.7j 0.7 1.4 1.1

1.4 0.5j 1.4 2.4 2.2

0.7 1.1 0.7 2.0 1.1

tion are given below.

LL and SSEES and 45 W/m2 HAWL. These do not, necessarily, concur with the

tely defined.

ll(s), e.g. FLL 18/1858 = 1.0%.

, to 1% of the floor area served.

iving an extra inlet area of 0.9% of the floor area served.


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, which

loops 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

lightwell—these 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 the

lightwell.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

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).

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

calculations—by 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

lightwell.

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 natural

displacement 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 different

noise 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 stacks—these can

be calculated using the well known stack ventilation equations

(e.g. [24]).


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 larger

stacks 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 outlet—which

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. in

winter 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 the

HAWL 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 point—that there is

rather little post-occupancy performance data for ANV

buildings.

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 air supply and perimeter

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, the airflow; this

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

performance.

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 NV

cooling. 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 the

plenum 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

develops.

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.


Acknowledgements

The architects of the buildings described were Short and

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

Appendix A. Comparison of key features of the three advanc

Building name Frederick Lanchester

Library (FLL)

Sc

Eu

Client and context

Client Coventry University U

Location Coventry, UK B

Site City center C

ANV Type C-E, C-C C

Cooling method Natural N

Number of levels Basement + Ground + 3 B

Completion date September 2000 N

Structure Steel frame C

U-values

Roof 0.18 W/m2 0.

Wall 0.26 W/m2 0.

Window 2.00 W/m2 2.

Footprint 50 m � 50 m 31

Gross floor areas 8161 m2 (G + 1 + 2 + 3) 33

942 m2 (B) 69

Floor to ceiling height 3.9 m (G, 1, 2, 3) 3.

Window shading Perimeter stacks, metal fins A

Approximate cost £20 m £1

Publications

By designers [2,3,11,13–15] [2

By others [16–19]

Air supply lightwells

Type 4 no Lightwells Li

Levels serveda G, 1, 2, 3 G

Shape Square Tr

Top Sealed, glazedb O

Shading Moveable blind N

Bottom Opaque-heater battery C

Sides Clear single glazing C

Air outlet type Dampers B

Secondary heating Trench heaters C

Cross-sectional area 4 no 38 m2 36

Gross perimeter lengthc 4 � [25 m (G, 1, 2), 12 m2 (3)]d 24

Floor area servede 1858 m2 per lightwell 29

Gross area of air inletf 18.6 m2 per lightwell 19

Maximum airflow distanceg 12 m 12

Air inlet plena

Air inlet type Perimeter slots C

Inlet depthh 1.4 m n/

Outlet depth 1.5 m 0.

Gross plenum inlet area 36 m2 per lightwell 8.

Air preheating Horizontal heater coils R

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.

ed naturally ventilated buildings

hool of Slavonic and East

ropean Studies (SSEES)

Harm A Webber

Library (HAWL)

niversity College London Judson College

loomsbury, London, UK Elgin, Nr Chicago, Il, USA

ity centre Green campus

-E C-E, E-E

atural and PDC Natural and HVAC

asement + Ground + 5 Basement + Ground + 2

ovember 2005 Winter 2006

oncrete Steel frame

20 W/m2 0.25 W/m2 K

30 W/m2 0.25 W/m2 K

00 W/m2 2.60 W/m2 K

.5 m � 27 m 34 m � 34 m

80 m2 (G-5) 3468 m2 (G + 1 + 2)

5 m2 (B) 1192 m2 (B)

2 m (G), 2.9 m (1–4), 2.6 to 4.9 m (5) 3.35 m (G, 1), 3.8–6.5 m (3)

djacent buildings External window reveals

0 m $13.5 m

,11–13] [20,21]

–

ghtwell Lightwell

, 1–5 G, 1, 2

iangular Square

perable/ETFE Sealed, glazedb

one Moveable blind

lear single glazing Clear single glazing

lear single glazing Clear single glazing

ottom hung windows Top hung windows

olumn radiators Linear finned emitters

m2 73 m2

m (G, 1, 2, 3, 4), 19 m (5) 34 m2 (G, 1, 2)

26 m2 2283 m2

m2 bottom and 26 m2 top 32 m2

m 15.7 m

orridors (side) and four apertures (front) Two perimeter slots

ai 1.45 m

8 m 0.93 m

7 m2 (corridors), 6 m2 (apertures) 55.2 m2

aked heater battery Raked heating battery

Air exhaust pathsj

Lightwell Stacks Stacks Double

Facade

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

(1–5)l

4 no 1.2 m2

(3 and 4)m

1 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)

Climateo

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 (15.5 8C) 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 8CMDMau 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 analysisy CFX CFX and Water-bath model CFX

Solar 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 it—e.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 chimneyn 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 )



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