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Energy and the city: density, buildings and transport

Koen Steemers*

Department of Architecture, The Martin Centre for Architectural and Urban Studies,

University of Cambridge, 6 Chaucer Road, Cambridge CB2 2EB, UK

Abstract

Cities by definition are a focal point of energy consumption. Their forms have a significant bearing on the balance of building and transport

energy use, which are the two sectors that are directly affected by urban planning (the third being industrial). This paper establishes the relative

magnitudes of building energy use in comparison to transport, and points out the interrelationships between the two in the context of the cities

and of a temperate climate. The main part of the paper assesses the building energy trends and implications of urban form, with a particular

reference to the effect of varying density, and presents strategic findings. It calls for continued research and development, particularly in the

field of modelling the urban microclimate as a function of design, as well as comfort research with an emphasis on outdoor comfort. Urban

microclimate and comfort are the themes of this journal, and this paper aims to set the scene.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Urban planning; Urban form; Density; Microclimate; Transport

1. Urban density—transport versus building energy

The relative low cost of energy during the last century has

allowed for the increasing dispersal of activities and decreas-

ing densities of many of our cities. These lower densities

have resulted in greater physical separation and diffuse

dispersal of activities, making mass transport increasingly

difficult. In combination with increasing wealth, this has

resulted in increased private vehicular traffic (currently

accounting for about 70% of Europe’s passenger transport).

As the private vehicle typically consumes more than twice

the energy per passenger per kilometer than a train, and

almost four times that of a bus, the energy (and pollution)

implications of an urban layout that does not maximise

public transport are likely to be very significant [1]. This

implies that it is essential to plan cities for efficient transport

use. Although this is undoubtedly true, it is important to

consider building energy use, which in UK accounts for over

half of the total energy consumed (in the European Union

this figure is 41%, and in US 36%) and an equivalent

proportion of pollution generated. This compares with less

than a quarter each for transport and industry.

Although these figure are notional figures and not specific

to urban areas, the 2:1 ratio of building:transport energy use

(for UK) is likely to be a conservative estimate for cities.

This is partly because of the availability of urban public

transport systems, the shorter distances for walking, the

increased traffic congestion and limited parking, which

suggest that there is likely to be proportionally less transport

energy use. For example, in central London only 10% of

commuters arrive by car compared to a national average of

40%. On the other hand, there is a concentration of office,

retail and other commercial building types in cities, which

have a greater level of energy consumption per square meter

than housing. An air-conditioned office building to current

building regulations consumes approximately six times the

energy per square meter of a house. So, it would be fair to

say that the energy and environmental implications of

buildings are at least twice as significant as those for

transport. For example, for London the ratio of energy used

in buildings to transport is approximately 2.2:1 (including

goods, taxis and air transport!) (Fig. 1) [2].

Despite the fact that building energy use is much greater

than transport energy, there are a number of factors that have

given the transport debate more urgency.

(i) First, the very local pollution—both atmospheric and

noise—are more immediately perceptible than those

associated with buildings (power stations being gen-

erally located on the edge of or outside urban areas).

(ii) Second, the rate of replacement of old vehicles with

new ones (increasingly more efficient) is rapid

compared to buildings (typically buildings may have

a design life about 10 times longer than that of cars).

Energy and Buildings 35 (2003) 3–14

* Tel: þ44-1223-331712.

E-mail address: [email protected] (K. Steemers).

0378-7788/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved.

PII: S 0 3 7 8 - 7 7 8 8 ( 0 2 ) 0 0 0 7 5 - 0

This means that transport policies will have a much

greater chance of having a short-term benefit com-

pared to building-related proposals, as the replacement

rate of cars is well within the time scale of targets such

as those set by the Kyoto agreement.

(iii) Third and finally, cars are not only associated with

environmental issues, but also with accidents, fatalities

and social erosion [3], bringing transport still higher

up the political agenda.

It has been shown that cities of a high density, such as, for

example Hong Kong, have a far lower transport energy

demand per capita than low density cities such as Houston,

by a factor of 18. On average, when comparing 10 major

cities in the US with 12 European cities, European cities are

five times as dense but the US cities consume 3.6 times as

much transport energy per capita [4]. The conclusion often

drawn from such data is that dense cities are low energy

cities. However, it is not clear to what extent density is the

cause, and increased energy use is the effect. For instance, it

can be argued that where private car use is less—due to, for

example economic conditions—cities are denser. Interest-

ingly, historic European cities, such as Paris, lie at a national

‘optimum’—achieving moderate energy use for modest

densities—whilst sustaining a rich urban life (Fig. 2). It is

not evident that moving towards increasing urban density

will lead to reduced car traffic—in fact, in the short-term the

opposite is likely to be the case. In the absence of extra

capacity in the form of effective integrated public transport,

increasing the density will inevitably increase traffic and

Fig. 1. Energy use breakdown for London [9].

Fig. 2. Transport energy vs. urban density for 32 cities [11].

4 K. Steemers / Energy and Buildings 35 (2003) 3–14

pollution. The challenge is, thus, to anticipate and imple-

ment public transport systems before increasing densities to

ensure that more existing and future inhabitants use public

transport instead of private cars and, thus, to shift the balance

away from the cars.

However, the long-term perspective of global environ-

mental concerns (climate change, ozone depletion, acid rain,

reducing fossil fuels, etc.) clearly means that building

energy use needs to be addressed, particularly in relation

to urban areas. This is combined with increasing urbanisa-

tion resulting in a predominantly urban global population. It

is, thus, essential to minimise the deleterious global and

local effects of urbanisation to ensure that the life nurturing

qualities of the city can be restored and maintained. Building

energy use is a predominant factor in this context.

2. Density and building energy

The concentration of activities and people in cities is often

perceived to be the main source of environmental problems.

However, such concentration can have environmental advan-

tages achieved, for example through the sharing of resources.

Most obviously, more intense use of land and sharing of

infrastructure—energy and water supply, drainage, roads,

buildings and public transport—reduces the energy per capita

associated with its construction (and possibly maintenance)

and benefits from an economy of scale by comparison to a

more dispersed urban configuration.

An example of this is the use of combined heat and power

(CHP) and district heating (DH) energy provision. Micro-

CHP has the potential to deliver thermal (55%) and electrical

(30%) energy locally to a neighbourhood at a high effi-

ciency—typically 85% or more overall—and reduces the

transmission and distribution losses of more centralised

power stations. This compares with an efficiency of approxi-

mately 30% for an average UK power plant to deliver

electricity. However, for the balance of heat and power to

be used optimally the energy demand should not only be

localised but also mixed, combining housing with other

commercial activities. Mixed use, high density neighbour-

hoods have the further potential advantage of local employ-

ment, commerce, retail, leisure, etc. which in turn may

reduce the distances that people need to travel to access

such facilities. The notion of ‘‘decentralised concentration

and high density mixed land-use’’, thus, seems to be rein-

forced by energy considerations [5].

However, when one considers the implications of high

density on the demand side of building energy use and on

building integrated renewable energy production (such as

photo-voltaics), does the balance begins to tip in favour of

lower densities? This balance is likely to depend on building

type, and for the purposes of this paper, we will consider

domestic and office use as the predominant types, in the

context of UK.

2.1. Domestic buildings

Taking UK as an example to demonstrate the point, the

energy demand in housing is dominated by space heating,

which on average accounts for 60% of the total energy

(Fig. 3) [6]. It is the space heating that will be most affected

by design, the remaining consumption being largely deter-

mined by occupant needs and not strongly dependent on

climate. In dispersed developments with the possibility of

greater solar access, passive solar design will have greater

potential to reduce space heating demands and, thus, overall

energy use in housing. In order to explore the effect of

increasing density on energy, we need to consider what

parameters can be altered to change the density of housing.

There are a number of ways in which the urban form of a

city can change to increase density:

(i) by increasing building depth;

(ii) by increasing building height or reducing spacing (i.e.

changing height:width ratios between buildings);

(iii) by increasing ‘compactness’ (e.g. apartments instead of

detached housing, where the building depth and height

characteristics may theoretically remain unaffected).

The first—increasing building depth—is perhaps particu-

larly common in terraced housing in UK cities, where

extensions to the rear in the form of conservatories, living

rooms, kitchens and bathrooms are commonplace. As a

result, the building envelope:floor areas ratio is usually

(though not necessarily) increased, and the availability of

daylight and sunlight to the interior is reduced. The energy

implications of conventional extensions (as opposed to a

passive solar extension) are an increase in both the heating

Fig. 3. Energy use breakdown for UK housing [13].

K. Steemers / Energy and Buildings 35 (2003) 3–14 5

and lighting loads. Overall, these increases are very small—

in the order of 5–10% for a two storey addition to a three

storey terraced house.

Housing densities can also be increased, by increasing the

average building heights (or reducing the spacing between

them), which can be defined by the obstruction angles to the

facades. An obstruction of 308 to the south facade of a

passive solar house1 can increase heating energy by 22%

compared to an unobstructed facade (Fig. 4). However,

conventional, non-solar housing will also require between

6 and 15% more energy for heating compared to a passive

solar house [7]. In an urban context, the solar potential for

housing is reduced, largely due to the presence of obstruc-

tions but also because of planning constraints on orientation

(Fig. 5). For the average UK dwelling, the heating contribu-

tion of solar energy is small, 10–15%, compared to a passive

solar house—in the order of 40%—thus, the energy con-

sequences of obstructions for a non-solar urban dwelling is

small.

Thus, there is in the above example, about a one-third

increase in heating energy for an urban house as compared to

a green field passive solar house. However, it should be

noted that there will be a significantly wider range of energy

use in an urban environment as some dwellings will be more

obstructed and facing north whilst others may benefit from

unobstructed solar gains.

The average one-third increase compares to a heat saving

of 40% while comparing detached housing with apartments

(Fig. 6) [7]. Thus, the way to increase density and energy

efficiency simultaneously is to increase ‘compactness’ of the

urban fabric whilst maintaining a limited building depth (in

the order of 10–12 m), and where an appropriate solar

orientation is ensured to access light, sun and air.

The conclusion would seem to be that for energy use in

UK housing the arguments for and against densification are

finely balanced and will probably depend on the infrastruc-

ture issues mentioned earlier. However, as the average

obstruction angles increase above about 308, the balance

will begin to swing against densification. An average

obstruction angle of 308 equates to a theoretical plot ratio

of up to 2.5 (assuming the terrace or courtyard form with a

plan depth of 10 m). Such a figure results in densities in the

order of 200 dwellings per hectare (DPH) (assuming 125 m2

per dwelling)—well above the 50 and 75 DPH discussed by

the Urban Task Force [8]. This compares with the current

density of new housing in UK of an average of 25 DPH [9],

Ebenezer Howard’s 45 DPH for the Garden City [10], the

new PPG3 guidance of up to 50 DPH [9], and Kowloon’s

1000 DPH in Hong Kong [10] (Fig. 7). The conclusion is that

relatively high housing densities can be achieved before a

negative impact on the energy demand becomes significant.

2.2. Non-domestic buildings

The non-domestic sector is very diverse, so for the

purposes of this paper, the focus is on UK office build-

ings—the predominant non-domestic building type. Unlike

housing, space heating for offices in UK is not the pre-

dominant issue in terms of primary energy use and energy

cost. Typically, the artificial lighting demand, and in the case

of an air-conditioned office, the fan power and refrigeration

loads, are often equally significant (dependent on the type of

office building).

If one excludes office equipment, as a non-design related

energy demand, the energy breakdown for a typical air

conditioned office in UK is: 44% for air conditioning,

34% lighting and 22% heating (Fig. 8) [11]. It is, therefore,

clear that avoiding, or at least reducing air conditioning, is a

primary objective to reduce energy demand. The secondary

approach is to reduce reliance on artificial lighting by

increasing and exploiting daylight availability. Finally, redu-

cing heat losses is likely to be the least significant energy

efficiency strategy and, therefore, should not be used as a

determinant for building form, except where it compliments

the previous two aspects (i.e. excessively compact and deep

plan forms may reduce heat loss but will dramatically

increase both mechanical ventilation and artificial lighting).

The first two aspects—avoiding air conditioning and

increasing daylight availability—are broadly complimen-

tary in terms of building form implications. They both point

Fig. 4. Effect of obstruction on space heating for housing [14].

1 A ‘passive solar’ house here is defined as a house with 75% of its

glazing to the south and 25% to the north, compared to a ‘non-solar’ house

which here is taken to be a house with west and east facing facades each

with 50% of the glazing. In both the cases, the assumption is that the

houses are designed according to, in 1990, Building Regulations (see [7]

for further details).


towards shallow plan building forms in order to enable

natural ventilation and daylight penetration. Typical plan

depths for naturally ventilated and day-lit office buildings

are 12–15 m. The implication of limiting the plan depths of

buildings might suggest that densities would decrease for

low energy offices, compared to more conventional deep

plan buildings. However, this is not necessarily the case. The

avoidance of air conditioning may save up to 1 m depth of

services zone per floor (i.e. 25% of the building volume

assuming an initial 4 m floor-to-floor height) and a further 3–

5% of the volume for reduced plant size [12]. Therefore,

avoiding air conditioning can save a maximum of approxi-

mately 30% of the building volume. This may more than

compensate for the reduced plan depths resulting in, for

example, courtyard or finger-plan forms.

The benefits of avoiding air conditioning through adopt-

ing shallow plan depths are typically that naturally venti-

lated offices use <40% of the primary energy. This is an

enormous potential advantage. However, there are limita-

tions in adopting natural ventilation in the urban context.

Apart from internal gains levels (which are independent of

context), the most often quoted constraints in cities are noise

Fig. 5. Effect of orientation on space heating for housing [14].

Fig. 6. Relationship between building form and heat loss [14].


and air pollution. Both these factors frequently result in the

adoption of air conditioning. The main cause of noise and

pollution is traffic and, thus, as mentioned before, this issue

needs to be addressed in advance of expecting a significant

reduction in energy consumption for office buildings. In the

meantime, there are building design strategies that can be

implemented which limit the need for air conditioning. The

most obvious is that it may only be necessary to mechani-

cally ventilate the street facing zone (and/or to locate less

noise sensitive accommodation there), allowing the rest of

the building to be naturally ventilated from a more quiet and

cleaner courtyard, garden or atrium. Such a ‘mixed-mode’

strategy to the servicing of offices will go some way towards

limiting energy use whilst maintaining the potential for

future adaptation to full natural ventilation.

Having established that it is in principle possible to

maintain a given density of development and significantly

reduced energy use by limiting air conditioning, it would

seem appropriate to investigate the energy impact of increas-

ing the density on office buildings. The primary energy

consumption of an average naturally ventilated office build-

ing in UK is dominated by artificial lighting and heating

Fig. 7. Dwellings and people per hectare for housing [3,15,16].

Fig. 8. Average energy use breakdown for air-conditioned offices [17].


loads (Fig. 9). As the level of obstruction to buildings will

affect daylight and sunlight availability, both the lighting

and thermal loads will be affected by increasing urban

densities.

If again we consider two possible strategies for increasing

density as being increasing building depths and heights,

some initial analysis can be carried out.

By increasing the building depth of offices the availability

of natural ventilation and daylight reduces, resulting in an

anticipated increase in mechanical ventilation and artificial

lighting. However, heat losses are likely to diminish because

the surface:volume ratio reduces with increasing plan

depths.

Using the LT method [11] to determine the energy use

implications, it can be seen that for a low energy naturally

ventilated office (with mechanical fresh air supply for deep

plan areas) the energy use doubles when the plan depth is

doubled from 12 to 24 m. However, for an efficient air-

conditioned building the energy use increase is only 20%,

although the total energy is almost three times larger than the

naturally ventilated option at a depth of 12 m (Fig. 10).2

Thus, increasing density by increasing building depths

will inevitably result in increased energy use for office

buildings, although the relative increase is less for air-

conditioned buildings. Nevertheless, on average deep plan,

Fig. 9. Average energy use breakdown for naturally ventilated offices [17].

Fig. 10. Effect of increasing the building depth on energy use for a mixed mode office.

2 Assumptions regarding the office buildings modelled include low

energy, optimised control systems, a 50% facade glazing ratio, with a

lighting datum of 300 lx and 30 W/m2 of internal gains, located in southern

UK (see the LT method for more details [11]).


air-conditioned buildings still consume typically twice as

much energy as equivalent mixed-mode buildings (naturally

ventilated and mechanical fresh air supply), suggesting that

the avoidance of air conditioning by improving the urban

microclimate is a key factor.

The alternative strategy for increasing density is to

increase the average building height (or reduce the spacing,

which will have the same consequence for sunlight and

daylight availability). Using the same assumptions as in the

example given previous, it is possible to predict the energy

consequences of increasing obstructions. From LT energy

analysis, it can be seen that the total energy use would be

expected to increase by 23% for air conditioned and 45% for

non-air conditioned offices for an obstruction of 308 com-

pared to an unobstructed situation (Fig. 11).

3. City texture

Despite the clarity and insights gained from looking at

urban density in terms of simple parameters, such as obstruc-

tion angles or plan depth, it is important to move from the

theoretical description to study the formal complexity of a

real bit of city. The study of urban form has been an ongoing

pre-occupation at the Martin Centre for Architectural and

Urban Studies for over 30 years—starting with the seminal

work of Leslie Martin and Lionel March [13,14]. The

current research builds on this work and focuses on the

relationship between urban ‘texture’ and environmental

characteristics. It has already been pointed out that the

energy performance of buildings is linked to the quality

of the urban environment, and although not the specific

focus of this paper, research into the solar, wind, pollution

and acoustic effects of urban texture is going on. A number

of recent research projects have developed analytical tech-

niques, which categorise the physical urban geometry and

assess the consequence of this on environmental perfor-

mance [15–17].

One of the techniques developed by the Martin Centre’s

urban environmental research team assesses energy demand

in terms of urban form. This technique, referred to as ‘LT

urban’, combines the LT energy analysis tool mentioned

previously with computer-based image processing [18] to

extract data related to building form for large urban areas

(typically 400m� 400 m). To demonstrate the technique

and its application to establishing the relationship between

urban density and building energy use, a 400m� 400 m part

of London is used (Fig. 12). As we are primarily interested in

built form, numerous assumptions have to be made about the

detailed characteristics of individual buildings, such as

glazing ratio, U-values, systems, etc. These have been

standardised, based on a detailed survey of the area and

making informed estimates where necessary. The base-case

urban form is altered by adjusting the building heights so

as to produce a range of urban densities from half to double

the existing. The energy consequences of increasing the

average obstruction angles are significant—for example, 108increase in obstruction results in approximately 10%

increase in energy (Fig. 13). For a density range of plot

ratios from 1.25:1 to 5:1 the results show that doubling the

Fig. 11. Effect of obstruction angle on energy use for offices [17].


Fig.12.A

digital

elevationmodel

(DEM)isusedto

determineandmap

energyuse

fortheLondonarea

under

investigation(left,aerial

view

ofLondonsite;middle,DEM

ofthesite;right,energydem

and

(kW

h/{

m}2per

year)).


density typically increases energy consumption by in the

order of 25% for this whole section of the city (Fig. 14).

It is interesting to note that optimising the glazing ratios in

response to the level of obstruction reduces the effect, so that

doubling the density now results in a 21% increase in energy.

This highlights that other parameters, at the level of indi-

vidual buildings, will change the relationship of energy to

urban density, and that they should respond to the specific

urban context. Glazing ratios in particular—as they repre-

sent the main interaction between building and climate—

Fig. 13. Effect of obstruction angle on energy use for naturally ventilated offices on the London site.

Fig. 14. Effect of density on energy use for naturally ventilated offices on the London site.


affect the energy performance and can be ‘optimised’ to

minimise energy demand. This is demonstrated by deter-

mining the optimum glazing ratio for the London site, which

for all orientations show that glazing should reduce as you

go up a building’s facade (Fig. 15). Optimum glazing ratios

vary less in response to orientation compared to green field

sites, as in an urban context the obstructions reduce the

differences in terms of daylight and particularly sunlight

availability.

4. Conclusions

The order of magnitude of energy implications in relation

to urban density has been demonstrated for domestic and

office buildings in UK’s temperate climate. For dwellings,

the energy implications of compact densification are

balanced between the benefits from reduced heat losses

and the non-benefits of reduced solar and daylight avail-

ability. For office buildings, increasing urban density

increases energy use because of the reduced availability

of daylight in particular. However, this increase is signifi-

cantly less than the energy increase of changing from a

naturally ventilated office to an air-conditioned office. This

change is only possible to prevent if the urban environment

is less polluted and noisy. This is where the link between

transport and building energy becomes evident, as cars are

the major cause of urban pollution. Although overall urban

transport consumes less than half the energy of urban

buildings, the implications of reducing private car use in

favour of clean and efficient public transport can be sig-

nificant for building energy use. Thus, the move should

initially be towards improving the urban environment, so

that the energy benefits will outweigh the non-benefits.

Increasing the urban population is not likely in the first

instance to improve the urban environment, and the worst

will be the increase in noise and pollution in the short-term

before the necessary level of investment in transport infra-

structure is made. The result may thus be increased non-

domestic building energy use due to the adoption of air-

conditioning. Combined with climate change, the same

effect may also occur in the domestic sector.

It is clear that the political will is to a large extent and

needs to be in place to encourage sustainable development.

However, there is a need for an analytical approach to

provide information on which the decisions will be based.

The potential role for research is evident.

‘‘Towns and cities should be well designed, be more

compact and connected, support a range of diverse uses

within a sustainable environment which is well integrated

with public transport and adaptable to change’’ [8].

It is not difficult to agree with this statement and its vision.

What is important is to develop the necessary techniques to

inform the balance, sequence and implementation of deci-

sions to achieve the desired results.

This issue of Energy and Buildings begins to provide the

required knowledge, with a particular reference to urban

environmental and related comfort issues. It is by making

our environments more comfortable that we can increase the

use of outdoor space for energy efficient movement (i.e.

walking and cycling) and exploit these beneficial urban micro-

climates to improve the energy performance of buildings.

Acknowledgements

Koen Steemers is joint Director of the Martin Centre

where he heads the urban environmental research team—he

particularly wishes to thank Nick Baker, Sam Lawton, Carlo

Ratti and Dana Raydan for their support. Dr. Steemers is co-

ordinator of EU research projects such as ‘‘Assessing the

Potential for Renewable Energy in Cities (PRECis)’’ and

‘‘Project Towards Zero Emission (ZED) Urban Develop-

ment’’, as well as ‘‘Sustainable Building Form’’ funded by

the Tyndall Centre for Climate Change Research, on these

aspects this paper is based.

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Fig. 15. Average optimum glazing ratios at different floor heights for the

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