allowing buildings to breathe

Invited paper, Renewable Energy 2004, Sovereign Publications, p85-95 (2004) ISBN 19 03605 458

ALLOWING BUILDINGS TO BREATHE

Mohammed S. Imbabi1 and Andrew Peacock2

1School of Engineering & Physical Sciences, King's College, Aberdeen AB24 3UE 2School of Engineering and Physics, Heriot Watt University, Edinburgh EH14 4AS

ABSTRACT Breathing walls provide high quality indoor environments in buildings that use less energy and combats urban air pollution. They provide a simple, elegant and sustainable solution to the increasing and serious problem of self-inflicted environmental damage that many cities now face. To the casual observer, a breathing wall cladding panel will be indistinguishable in appearance from conventional building cladding. It ‘typically’ comprises ventilated outdoor external rainscreen and indoor wearing surface encapsulating air permeable dynamic insulation media, buffered and supported internally by special air distribution screens. Ventilation air flows indoors through a breathing wall when a pressure difference is applied, picking up conduction heat loss as it flows through the wall, with filtration of air borne pollutants taking place simultaneously. The present paper provides a scientific overview of how breathing walls work, the materials used and under development, and the latest research into life-long particulate filtration.

INTRODUCTION It is predicted that 70% of EU primary energy will be imported by 2028, as opposed to 40% currently [1]. The principal importers of primary energy to the EU will be the Middle East, West Africa and the former states of the Soviet Union. The implication for energy security of the current energy policy of EU member states is stark. In addition to increased energy insecurity, there is a need to decouple GDP growth from energy use if Kyoto commitments are to be met. This is especially prescient in terms of technology transfer to the developing world. The global warming gas emissions associated with a world population of 12 billion, with increasing levels of affluence, will be extreme by 2004 standards.

The response to these issues is encapsulated in the policy of sustainable development. At the heart of this policy is effort to minimise the burning of fossil fuel to meet energy requirements. In delivering this policy, the construction industry has a key role, since 50% of primary energy used in the developing world is associated with buildings, either through embodied or operational energy. This has led in the UK and elsewhere to alterations in the

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building regulations that reduce air leakage in the building envelope while increasing the level of insulation. The result has been a dichotomy in energy use, where delivery of ventilation air to the building is often typically achieved through high energy-use HVAC systems. This is why recent developments have seen a return to building design which re-visits passive strategies for delivering ventilation air [2]. While these are potentially successful, they are subject to problems relating to the quality of air delivered to the building, especially in polluted urban environments.

If one is to consider air quality in relative terms, the background air quality in the UK over the last 20 years has seen a remarkable improvement if cleanliness is determined by the amount of particulate matter found in the air below 10 microns (PM10) [3]. However, if one were to consider local effects rather than background measurements, the air quality is actually worsening in certain areas [4]. PM10s are produced by a variety of dispersed, even remote sources, which is why front-of-pipe solutions, such as local traffic calming, are unlikely to be effective. There is therefore a need for ventilation air delivered to the indoor environment to be filtered. This presents a practically insurmountable problem for purely passive ventilation systems that do not generate the airflows required for ventilation air to flow through traditional HEPA filter arrangements.

One method of supplying fresh filtered ventilation air to indoor spaces, that brings us close to the natural ventilation ideal, is through the use of dynamic insulation. Instead of bringing fresh air into the building at roof level for ducted distribution as in conventional HVAC, air is drawn directly into the building through an air-permeable, dynamically insulated envelop. The use of a large wall area to provide ventilation means that the flow velocity can be reduced by a factor of 100 (or more) compared to that in a conventional HEPA filter. This method of ventilation enables conduction heat loss recovery for enhanced energy efficiency, cleans up the air by transforming relatively sparse fibrous insulation material into a highly effective filtration unit, and significantly reduces the size, complexity and cost of plant required. Successfully deployed, it paves the way for a new type of energy efficient 'breathing' building in polluted urban environments that also cleans-up the outdoor environment, thus reversing the age-old trend of buildings as net polluters of the environment.

BACKGROUND & OBJECTIVES Fundamental research on dynamic insulation was undertaken in an EPSRC-funded project at The University of Aberdeen on the use of diffusive and dynamic insulation for combined heat recovery and ventilation in buildings. Further research and field studies into breathing wall systems using dynamic insulation, including the particulate filtration performance and service life of fibre-based dynamic insulation, have been undertaken since then. The Environmental Building Partnership (EBP) Ltd, a newly formed spin-off company, has been created to promote and disseminate the results of research in partnership with developers, architects, engineers, manufacturers, contractors, research organisations and governments worldwide. EBP has been awarded SMART funding by the Scottish Executive for a 12-month proof of concept project, to develop a modular, multi-layer dynamically insulated cladding panel to reduce energy consumption and filter particulate and other forms of air pollution in buildings. The project will also evaluate the beneficial effects of such buildings on the outdoor environment, and examine the socio-economic factors likely to govern early adoption of the technology. This will be followed by Stage-2 of SMART in July 2005 where, over a 3 year period live demonstration projects will be undertaken to showcase this form of construction and its benefits in preparation for industry-wide adoption.

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Dynamic Insulation and the Environmental Building System Approximately 50% of primary energy use and 30-40% of CO2 emissions in the UK are associated with buildings. Improvements to the energy efficiency of buildings often concentrate on improving the air tightness of the structure and increasing the levels of insulation. This is encapsulated in the Building Regulations Approved Document Part L. Heating and ventilation requirements are conventionally left to expensive mechanical systems that can be responsible for up to 90% of operational energy costs. There is also growing evidence to suggest that this approach to construction is, in part, responsible for the prevalence of sick building syndrome.

Dynamic insulation describes a construction technique that can provide controlled ventilation requirements to a building while improving the energy efficiency. In dynamically insulated buildings the building fabric is used as a counter-flow heat exchanger through which the movement of air from outside to inside is controlled by careful balance of internal and external air pressures. This pressure difference is created by a negative pressure induced internally, either through natural (e.g. stack) or mechanical (e.g. fan-assisted) means. The whole wall area can then be used to provide ventilating air to the building - see Fig. 1.

Figure 1. Heat and air flows in (a) breathing building, and (b) breathing wall.

Use of the building envelope to source ventilation significantly reduces ducting and plant requirements, and results in extremely low airflows across the wall face. Some of the heat usually lost by conduction to the outside is reclaimed by ventilation air being drawn into the building through the insulation. Operational energy can be reduced by up to 25% compared with an equivalent conventional building [5,6]. In a typical school this system would result in a reduction in CO2 emissions of 20 tonnes per annum.

The above outlines one of the benefits of the Environmental Building System (EBS) under development. Another important benefit of low airflow is that the insulation layer can now act as an extremely efficient filter. The insulation phase, when designed correctly, can provide filtration efficiencies of above 99% for the lifetime of the building, further reducing capital and maintenance costs associated with conventional Heating, Ventilation & Air Conditioning (HVAC). With EBS ventilation air is delivered to the building ready-filtered. Air exhausted from the building is, as a result, cleaner leading to the possibility of the building being used as

(a) (b)

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a clean-up technology in polluted urban sites. The filtration aspect of dynamic insulation has been covered by the authors elsewhere [7,8], and will now be developed to enable accurate predictions of multi-layer filtration performance and useful life before clogging to be made.

Dynamic Insulation for Air Filtration In general, the range of applications across filtration spectra spans the ionic (atomic radius) to the macro-particle (fine sand) ranges. Common air filtration media include membranes, cellular-based (foam or sponge) materials, pulps, and fibres. The latter present the most attractive choice for use in dynamically insulated buildings due to their efficacy in the PM2.5 – PM10 range at low flow velocity, wide availability, utility, low cost, and prevalence as conventional building insulation materials. Our earlier investigations reveal that potentially suitable natural and man-made fibre-types and products already exist, important for ecologically sound, cost-effective manufacture of multi-layer breathing wall panels.

The behaviour of fibre-based air filters can be approximately described using single fibre theory, which calculates the flow fields around a fibre taking into account the effects of neighbouring fibres. For glass fibre with a typical diameter of 10 µm, at the airflow rates used in dynamic insulation, diffusional deposition becomes extremely efficient for sub-micron particles. Particle penetration estimated for 20 mm thick glass fibre insulation shows that it can effectively prevent the ingress of smaller than 0.5 µm and larger than 5 µm diameter air borne particles into a building. Its relatively poor performance for particles in the range 0.5 to 5 µm is cured if thickness is increased to 60 mm, at which point particle penetration would be effectively zero for all particle sizes [7].

An elegant and potentially efficient, compact, longer lasting alternative to increasing thickness, however, would be to add a layer(s) of other material with low particle penetration properties over this range. The challenge then becomes to identify the correct materials and how they can be combined to achieve any desired filtration and insulation performance, insulation layer thickness, and added functionality, such as gas adsorption. In its simplest form, a dynamically insulated multi-layer permeable building envelope panel to filter air requires the correct combination of filtration and pressure drop properties of the component layers, good thermal and sound insulation, and resistance to environmentally induced degradation. The present research explores how the filtration aspect of dynamic insulation might be exploited, in a practical way, for next-generation breathing building construction.

Underpinning the investigation, a 1-D, multi-layer particle filtration model was developed and briefly outlined in [8]. This model has been further developed to investigate the filtration performance of a conventional, fibre-based insulation material (Glasswool) in filtering PM10s. The present paper does not focus on the concept of filtration per se. Instead, it shows how the filtration model has been developed to address the following questions:

− What efficiency of filtration can be expected from a commercial insulation layer under conditions determined by dynamic insulation and conventional concepts of internal comfort?

− What is the lifetime of the insulation layer - i.e., when will it become clogged? The paper describes the techniques that have been developed to provide the answers to these questions, the results obtained, and what other questions need to be asked.

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DESCRIPTION OF THE PARTICULATE FILTRATION MODEL The aim of the research is to model the performance of a fibrous filter and the effect of deposited particles on efficiency of filtration over time. In this section the salient features of our multi-layer filtration model are described. In brief, a single-fibre model, based on the work conducted by Davies [9], was used to derive efficiency. This was coupled with an iterative representation of clogging in fibrous filters, developed following the method outlined by Thomas et al [10]. The model requires calibration in experimental tests and field trials, to account for 3D effects, etc., and these will be reported in due course. It will effectively contribute to the development of design guidelines and tools, to enable implementation of exciting new breathing building projects, systems and products by practitioners.

The Single-Fibre Model The single-fibre model estimates the clean filter removal efficiency E, prior to particle deposition, using an expression of the form presented in Eq.(1) below:

−−−=

πααη

fdZE

)1(4exp1 (1)

where η, the collection efficiency, is the sum of the collection efficiencies ascribed to three different collection mechanisms, namely Brownian motion or diffusion (ηd), inertial deposition (ηin) and impaction (ηim). For clean fibres, this parameter is obtained as:

η = ηd + ηin + ηim (2)

The collection efficiency by diffusion is obtained from:

323

1

d19.2

−

⋅

−⋅= e

u

PKαη (3)

( )2

ln43

4

2 ααα −−−=uK (4)

TKCddU

P fpe ⋅⋅

⋅⋅⋅⋅⋅= 06 µπ

(5)

The parameter C in Eq.(4) is known as the Cunningham correction factor, needed to account for aerodynamic slip at the particle surface, and is obtained from:

⋅⋅

−⋅⋅⋅

+⋅⋅

+=λ

λλ2

exp221 p

pp

dBdQ

dAC (6)

where K is the Boltzmann constant (1.3805 x 10-23 J.K-1), and T the Kelvin temperature.

The collection efficiencies by interception and impaction are respectively: 2

,1

⋅

−=

f

lp

uin d

dKαη (7)

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( ) ( ) 2112tk

uf

ptk

uim S

Kdd

SK

⋅⋅−

+

⋅⋅

⋅−⋅=

ααααη (8)

f

pptk d

dUCS

⋅⋅

⋅⋅⋅=

µρ

18

20 (9)

Input particle distribution

The PM10 particle distribution upstream of the insulation layer can be conveniently split into 10 fractions, each assumed to contain a discrete particle diameter dp. The mass distribution shown in Fig. 2, which is typical, was used to generate these fractions.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

1 2 3 4 5 6 7 8 9 10Particle Diameter (microns)

Frac

tion

Figure 2. PM10 particle distribution.

Time-step selection

Although continuous, the evolution of dendrites has to be modelled in discrete time-steps, or increments. For this to work, the time increment t needs to be set correctly. Let t0,J be the time required for air to flow through a layer J of insulation of thickness JZ . If U0 is the airflow velocity at the filter face then:

0,0 U

Zt JJ= (10)

Logic suggests that the value of t must exceed t0,J for all values of J if each particle fraction is to go through each layer of the filter for selective filtration across the PM10 spectrum.

Multi-layer filter media layout

Filter media in the model is represented as thin sequential layers of equal thickness Z, with the option to assign different material properties such as fibre diameter and packing density as needed. This approach was adopted so that useful insights and understanding of depth filtration could be developed, in turn leading to accurate modelling and design of multi-layer dynamic insulation media that resist premature clogging. The basic arrangement is illustrated in Fig. 3, which shows a 9-layer model of fibre-based filter media and its evolution over 3

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time increments. In an ideal world, particle capture would be evenly distributed over the entire volume of media at any point in time, as shown in the figure.

In simulation, each layer is exposed to all of the discrete particle fraction inputs at every time increment. As particles from a fraction penetrate a layer, those particles that are not deposited within the preceding layer(s) become the input into the next layer and so on. The objective is to grade filter media properties so that ventilation air drawn into the building, once all of the layers have been traversed, is relieved of its entire particle load as uniformly through depth as possible. The optimum grading will be site-specific, since the choice of media properties and geometry depends on input particle distribution. It is noteworthy that, as the insulation layers become loaded with particulate matter, their efficiency of collection actually improves. The trade-off against useful life clearly requires careful consideration.

Figure 3. Model to facilitate evolution of filtration performance over time [8].

Extension to Dendrite Formation and Clogging In a loaded fibre filter the internal structure changes over time, as branch-like dendrites form through the agglomeration of particles within the filter media. The process of dendrite formation is extremely complex and difficult to predict, but the averaged effects on filtration performance, analogous to increasing fibre diameter and packing density in the early stages, with cake formation and terminal clogging ultimately, are more accessible.

With respect to the effects of dendrites a number of assumptions have been made in the model. They are (a) the particle aerosol will homogeneously load the filter, (b) all collected particles form dendrites but not all dendrites will be involved in further collection, and (c) the ones involved in further collection will be determined empirically once the model has been developed. Given the transient, non-linear nature of the process being modelled, an iterative solution, where the parameters governing filtration performance are updated at the end of each time-step (or increment), has been developed. Iterative expression for particle mass loading, packing density, mass of particles captured by fibres and dendrites, the particle fraction, filtration efficiency, collection efficiency, and pressure drop across each layer have thus been developed and applied to determine the filtration performance and rate of clogging

APPLICATION OF THE MODEL The model described in the previous section provides a powerful tool to evaluate the filtration performance of breathing wall panels employing fibre-based media. It will enable variable

Insulation layers →

t1

t2

Z1 Z2 Z3 Z4 Z5 Z6 Z7 Z8 Z9

Time ↓

t0

Airflow

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material properties to be specified in the evaluation and optimisation of graded or layered media for depth filtration, to prevent premature clogging and extend service life. Though based on established theory, the model needs to be calibrated and validated against experimental data, and this process will be completed in the near future. In time, the model will lead to the development of a range of breathing wall filtration panels possessing the longevity and durability need for use in the buildings of the future.

In the meantime, it was felt that applying the uncalibrated model could provide useful preliminary answers to the crucial questions of filtration efficiency over time and lifetime before clogging. The results would show if the model behaves in a predictable fashion and, assuming the results are encouraging, how the model might be further refined. The results will also contribute to the validation process, and ultimately confirm if the breathing wall filtration concept is practicable. A simple yet realistic scenario, built around a small office suite in a polluted environment and its ventilation requirements, was developed and used to generate a set of results from the model. This scenario will now be briefly detailed before the results are presented and discussed.

Description of the Airflow Conditions The model is designed to simulate the performance of the insulation layer in filtering PM10 under dynamic insulation conditions. To generate the conditions required for ventilation air, typical office suite conditions provided a convenient template. CIBSE guideline [11] indicates that 16 litres of air has to be provided per person per second in a smoking (i.e., worst case) environment. The test conditions are summarised in Table 1.

Table 1. Office suite parameters for test scenario.

Input Value

Number of People in the Office 5

Volumetric Flow Rate 80 litres.s-1

Unglazed External Wall Area 10 m2

Air Velocity 0.008 m.s-1

In order that the requisite volume of ventilation air is delivered to the office space, the flow rate is very low. This is due to the fact that 10 m2 of available external wall area is used as the ventilation source.

Description of the Pollutant Conditions There is a legal requirement of councils to monitor and publish information regarding the incidence of PM10 in their areas. A location that has received significant scientific research in recent years is Marylebone Road in London (opposite Madame Tussauds). The average yearly PM10 level at this location is 48µgm-3 [4]. In urban canyons of this type, the source of 80% of the PM10 is from incomplete combustion attributed to the internal combustion engines. The density of the pollutant was assumed to be 1850 kg.m-3, at the top end of the pollutant spectrum and 50% higher than carbon black. Temperature was taken to be 291K.

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Description of the Insulation Phase The effects of varying fibre diameter and packing fraction through depth of the insulation / filter were investigated for a layer thickness of 100 mm, divided into 5 slices. Packing fraction of fibre-based media is defined as the ratio of the volume of the fibres to the volume of the insulation / filter. This change with depth was defined by having two variables to describe the packing fraction, corresponding to an initial value that indicated the packing fraction in the first slice, and a gradient that indicated the change in packing fraction in each subsequent slice. The third variable was the fibre diameter.

Four sets of values were investigated, as shown in Table 2:

Table 2. Insulation / Filter media values examined.

Level Fibre Diameter (µ) Initial Packing Fraction Increment

1 10 0.008 0.0035

2 25 0.009 0.003

3 40 0.010 0.0025

4 55 0.011 0.002

In practice, the optimum packing fraction of the insulation / filter layer would be obtained with regard to the concept of clogging. Traditional definitions of clogging revolve around cake formation. At cake formation, excessive pressure drops are recorded across the face of the cake. While these can be tolerated in traditional HVAC systems, excessive pressure drops in dynamic insulation would result in the building technology being compromised. At pressure drops in excess of 40 Pa, dynamic insulation would no longer be viable. Difficulty would be encountered in opening doors and windows inside the building. With dynamic insulation, therefore, the definition of clogging is that point where the pressure drop required to provide acceptable levels of ventilation air exceeds 40 Pa.

The trial that has been outlined herein is a 3-variable, 4-level full factorial design with no replication, resulting in a total of 64 sets of results. For each time increment, the efficiency of each slice at filtering each particle diameter of pollutant was calculated. That pollutant not collected in the first slice was transferred to the next slice, etc. In this way, the efficiency of the whole layer was calculated.

RESULTS AND DISCUSSION From a practical perspective, the results of interest are the maximum pressure drop across the insulation / filter media over time, and the minimum efficiency of particulate filtration, as determined during the first time increment for fresh media.

The pressure drop maximum after a 60 year period was thus calculated, corresponding to the minimum life of a building as set by Public Finance Initiative (PFI) proposals.

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Filtration performance over time As the fibre diameter reduces in size so the efficiency of collection and the pressure drop increase, as shown in Fig. 4.

92

94

96

98

100

0 10 20 30 40 50 60Fibre Diameter (microns)

Effic

ienc

y (%

)

0

10

20

30

40

Pres

sure

Dro

p (P

a)

Efficiency

Pressure Drop

Figure 4. Effect of fibre diameter on pressure drop and efficiency.

The initial particle filtration efficiency for 10 and 25-micron fibres was greater than 99.8%, with pressure drops of 25 and 19 Pa at 60 years respectively. The evolution of pressure drop with time during a 60-year period is shown in Fig. 5 for an insulation layer of 55-micron fibre diameter, initial packing fraction of 0.011 and incremental increase of 0.002.

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60

Time (years)

Pres

sure

Dro

p (P

a)

Figure 5. Evolution of pressure drop with time.

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As fibre diameter decreases the pressure drop increases, in the same way that reducing the packing fraction increases pressure drop, all other variables being the same. These trends are quantitatively shown in Fig. 6.

5

10

15

20

25

30

0.007 0.008 0.009 0.01 0.011 0.012Initial Packing Fraction

Pres

sure

Dro

p

10 microns25 microns40 microns55 microns

Figure 6. Effect of packing fraction on pressure drop. Fig. 7 shows the effect of fibre diameter on filtration efficiency as a function of initial packing fraction.

90

92

94

96

98

100

0.007 0.008 0.009 0.01 0.011 0.012Initial Packing Fraction

Effic

ienc

y (%

)

10 microns25 microns40 microns55 microns

Figure 7. Effect of packing fraction on collection efficiency.

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55 micron Fibre diameter, 0.005 initial packing fraction

75

80

85

90

95

100

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007Incremental increase in Packing Fraction

% E

ffici

ency

Figure 8. Effect of filtration by depth on efficiency.

Fig.8 shows the effect of altering the incremental change in packing fraction on the efficiency of collection. The incremental increase factor indicates that for each slice of the insulation the packing fraction increases by that increment. An incremental increase of 0.1 and an initial layer packing fraction of 0.005 would result in a second layer having packing fraction of 0.006 and so on. As the incremental change increases, so the packing fraction of the internal side of the filter/insulation increases. Depth filtration increases the efficiency of collection. If we consider the lifetime of the filter with respect to this phenomenon, the effect is to prolong the lifetime of the insulation. This is shown in the Figs. 9 and 10 below.

0

5

10

15

20

25

0 10 20 30 40 50 60Time (years)

Pres

sure

Dro

p (P

a)

max Pressure @ 0max Pressure @ 0.001max Pressure @ 0.0025

Figure 9.Effect of depth filtration on pressure drop.

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0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0 20 40 60 80 100Insulation Depth (mm)

Exte

rnal

Fac

e of

Insu

latio

n

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014Internal face of Insulation

Increment = 0Increment = 0.0025increment = 0.01

Figure 10. Change in packing fraction with depth.

Effect of Pressure Drop on the Weight of a Door The figure below indicates the effect of increasing the internal pressure drop on the apparent weight of a door. This will in determine the allowable pressure drop and therefore will determine the lifetime of the panel. At 40 Pa for instance the door now weighs 7.8 kg, which is a hefty weight!

Figure 11. Effect of pressure on apparent weight of a door.

0

2

4

6

8

10

0 5 10 15 20 25 30 35 40

Pressure (Pa)

Wei

ght (

kg)

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Efficiency of Collection in the First Time Increment In order for the insulation layer to be developed as an effective filtration solution, the efficiency of collection has to be at HEPA levels of collection during the first time increment. This equates to efficiencies of 99% or greater. The figure below indicates the experimental space investigated during this experiment.

Figure 12. Filtration efficiency as a function of fibre diameter and packing fraction.

CONCLUSIONS A multi-layer model has been developed which will predict the filtration behaviour of fibre-based media under dynamic insulation conditions. In the example given, an optimum insulation layer was developed which performed with 99.44% efficiency or greater for a 60 year period.

The interaction between thickness, ventilation layer and packing fraction has not been fully investigated in this report. The results to date, however, suggest that there will be some synergy between these factors. One could therefore surmise that there will be an optimum office shape, layout, occupancy rate and building construction which will enhance both the performance and the lifetime of a dynamically insulated building.

It would certainly seem likely that given correct design guidelines, dynamically insulated buildings answer the questions initially posed by the research. It is possible to design energy-efficient breathing buildings that are 'naturally' ventilated and which deliver clean air to the internal envelope.

55

40

25

10

0.008 0.009 0.010 0.011

>99% efficiency

>95% efficiency

>92% efficiency

>90% efficiency

>97% efficiency

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ACKNOWLEDGEMENTS The authors wish to acknowledge the support of EPSRC and the DTI for funding some of the work outlined.

REFERENCES 1. European Commission, Green Paper - Towards a European Strategy for the Security of

Energy Supply, Luxembourg, Office for the Publications of the EU, 2001. 2. M. Fordham, Natural Ventilation, Renewable Energy, 19, p17-37, 2000. 3. The Scottish Executive, Building Standards (Scotland) Regulations, Technical Standards,

HMSO, UK 4. The UK National Air Quality Information Archive at http://www.airquality.co.uk/. 5. B. J. Taylor, R. Webster and M. S. Imbabi, "The Building Envelope as an Air Filter",

Building & Environment, 34(3), p353-361, 1998. 6. B. J. Taylor and M. S. Imbabi, “Dynamic Insulation in Multi-Storey Buildings”, Proc.

CIBSE A, Building Services Engineering Research & Technology (BSERT), 20(4), p175-180, 2000.

7. B. J. Taylor, R. Webster and M. S. Imbabi, "The Building Envelope as an Air Filter", Building & Environment, 34(3), p353-361, 1998.

8. M. S. Imbabi, J. Campbell and S. Lafougere, “Multi-Layer Dynamic Insulation Panels for Natural Ventilation and Filtration of Urban Air Pollution”, World Renewable Energy Congress VII, Frankfurt – invited speaker, July 2002.

9. Davies C.N, Air Filtration, Academic Press, London, 1973. 10. D. Thomas, P. Penicot, P. Contal, D. Leclerc, J. Vendel, "Clogging of fibrous filters by

solid aerosol particles Experimental and modelling study", Chemical Engineering Science 56, p3549-356, 2001.

11. CIBSE Guide, Volume B, Ventilation and Air Conditioning Requirements, London, 1986.

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allowing buildings to breathe

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