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1Natural ventilation in non-domestic buildings11.1IntroductionGeneralThis publication is a major revision of the ApplicationsManual first published in 1997(1). At that time, there was asignificant expansion of interest in the application ofengineered natural ventilation to the design of nondomestic buildings. The original AM10 sought to capturethe state of knowledge as it existed in the mid-nineties andpresent it in a form suited to the needs of every member ofthe design team.Some ten years on from the time when the initial manualwas conceived, the state of knowledge has increased, andexperience in the design and operation of naturallyventilated buildings has grown. This revision of AM10 istherefore a timely opportunity to update and enhance theguidance offered to designers and users of naturallyventilated buildings.The first edition of AM10 devoted its first section tosetting natural ventilation into the context of the range ofavailable design solutions. This aspect is now dealt with inCIBSE Guide B2: Ventilation and air conditioning(2). TheGuide sets out the various approaches to ventilation andcooling of buildings, summarises the relative advantagesand disadvantages of those approaches and gives guidanceon the overall approach to design. This edition of AM10 isintended to complement Guide B2 by providing moredetailed information on how to implement a decision toadopt natural ventilation, either as the sole servicingstrategy for a building, or as an element in a mixed-modedesign(3).This edition of AM10 should also be considered alongsideother major sources of relevant guidance, and in particularthose in support of the requirements of the BuildingRegulations. For England and Wales, the key documentsare:—Approved Document F1: Means of ventilation(4)—Approved Document L2: Conservation of fuel andpower in buildings other than dwellings(5).At the time of writing (January 2005), both these parts ofthe Regulations are the subject of major review, and so theguidance in this document will need to be interpreted inthe light of the requirements prevailing at the time of use.CIBSE Guide A(6) complements the guidance in ApprovedDocument F, and provides much fundamental data onminimum ventilation rates and thermal comfort criteria.1.2Structure of this publicationFollowing this introduction, the manual is divided intothree main sections. These chapters progress from areview of the strategic issues to a detailed development ofdesign techniques. As such, the material becomes increasingly technical in scope. Consequently, non-technicalreaders will probably wish to concentrate on section 2,which deals with developing the design strategy. Section 3deals with a review of ventilation components and howthey should be integrated into an overall design philosophy. This section will be particularly relevant to allmembers of the design team, and elements of it will berelevant to the client and the facilities management team.Section 4 concentrates on design calculations, and isprimarily targeted at the building services engineer whohas responsibility for engineering the design. Briefoverviews of the chapters are provided in the followingsections so that readers can identify the material that willbe relevant to their own requirements.1.2.1Section 2: Developing the designstrategyThis section focuses on the strategic issues. It begins bysummarising what functions natural ventilation candeliver, and the key issues that need to be considered aspart of delivering a successful design. The section containsa detailed flow chart that can be used to assess the viabilityof natural ventilation.Natural ventilation systems are intended to providesufficient outside air to achieve appropriate standards ofair quality and to provide cooling when needed. Since thecooling capacity of natural ventilation is limited, a keydesign challenge is to limit heat gains through good solarcontrol and careful management of the internal gains. Thesection explains how naturally ventilated buildings do notaim to achieve constant environmental conditions, buttake advantage of dynamics to provide comfortable, controllable conditions for the occupants.The section continues by reviewing the different types ofventilation strategy. The most appropriate strategy isshown to depend on the type of space (i.e. open plan,cellular) and whether wind or buoyancy forces are likelyto predominate. The section aims to provide a conceptualunderstanding of how the various system concepts work,and how different design features can enhance theflexibility and robustness of the design.Because of the increase in summertime temperaturescaused by global warming, the achievement of goodthermal comfort with low energy consumption willbecome increasingly challenging for all summer coolingstrategies (both natural and mechanical). The effectiveapplication of natural ventilation will increasingly require

2Natural ventilation in non-domestic buildingscareful integration with other design measures (bothpassive and active), especially in the south-east ofEngland. Global warming does not mean that the importance of natural ventilation diminishes; it will still have avery important role to play as part of an integrated designapproach, as a key element in a mixed mode building, andas the lead strategy in the cooler parts of the UK. Inaddition, it might be the case that, as the climate warms,occupants will adapt themselves to that changing climate,and so the threshold at which people find conditions toowarm will also increase.examples, and guidance on where the relevant input datamight be found.1.2.22Developing the designstrategy2.1Satisfying design requirementsSection 3: Ventilation componentsand system integrationThis section is mainly about tactics. Having used section 2to develop the strategy, this section looks at the selectionand specification of the various types of ventilationcomponent (i.e. windows, ventilators and dampers) andhow they should be integrated into an overall system.As well as considering the technical issues of design andspecification, the section also discusses the important‘softer’ issues, such as the division of responsibilitybetween members of the design team and the componentsuppliers and system installers. This is particularlyimportant since many issues relating to the successfulimplementation of natural ventilation cross traditionalboundaries of design responsibility.Another key issue is the inter-relationship between thesystem and the occupants. A key aspect of natural ventilation is to empower the occupant to make suitableadjustments to window opening etc. to maintain personalcomfort without prejudicing the comfort of others. Thismeans that automatic control strategies need to becarefully integrated with user behaviour. Such issues aredeveloped in section 3.As an adjunct to this manual, a spreadsheet tool* has beenprepared that implements many of the design calculationsincluded in section 4. This is intended as an illustration ofhow the methods could be implemented. Users will needto confirm that the tool meets their own requirements,and adjust it as necessary to meet the particular circumstances of the design issue they are investigating.Natural ventilation is one of a number of strategies thatare available to the designer. CIBSE Guide B2(2) containsan overview of the various approaches and gives guidanceon their applicability to different situations.Natural ventilation systems need to be designed to achievetwo key aspects of environmental performance:—ventilation to maintain adequate levels of indoorair quality—in combination with other measures, ventilationcan reduce the tendency for buildings to overheat,particularly in summer.The natural ventilation strategy must also be integratedwith all other aspects of the building design. Key issuesfor consideration are:—A satisfactory acoustic environment: natural ventilation openings also provide a noise transmissionpath from outside to inside, and this may be adetermining factor in some building locations. Inaddition, naturally ventilated buildings ofteninclude large areas of exposed concrete in order toincrease the thermal capacity of the space. Suchlarge areas of hard surface will require carefulattention to achieve a satisfactory internal acousticenvironment.—Smoke control: since smoke can follow naturalventilation paths, the integration of the fire safetystrategy must be an important part of design fornatural ventilation.—Health and safety(7): many natural ventilationopenings will be at significant heights above floorlevel and so the proposed Work at HeightsRegulations(8) will be particularly relevant.Because of the important link between the design and theway the user operates the building, section 3 stresses thebenefits of post-completion fine tuning to ensure the fullpotential of the building is being realised to the benefit ofthe occupants.1.2.3Section 4: Design calculationsSection 4 is the most technical part of the manual. Itbegins by reviewing the calculations that will need to becarried out and reviews the type of calculation techniquesthat are available.The section suggests that for basic design purposes, a classof tools known as ‘explicit envelope flow models’ are themost appropriate. They allow basic dimensioning of thesystem components. It then explains how other, moresophisticated tools (such as implicit envelope flow models,combined thermal and ventilation models, computationalfluid dynamics and physical scale models) can be used tocheck the performance of the sized system under a varietyof operating modes.Because implicit envelope flow models are the most usefultool to the designer, this aspect is developed in depth,showing how the basic textbook equations can be manipulated to provide solutions to most design problems. Thesetechniques are then illustrated with a number of worked2.1.1VentilationThe principle role of ventilation is to provide an appropriate level of indoor air quality (IAQ) by removing anddiluting airborne contaminants. Guidance on achievingadequate levels of IAQ (to avoid mould growth and healthhazards) is given in Approved Document F(4). Higherrates of ventilation may be provided than proposed in the* The spreadsheet may be downloaded from the CIBSE website(www.cibse.org/venttool)

2Natural ventilation in non-domestic buildingscareful integration with other design measures (bothpassive and active), especially in the south-east ofEngland. Global warming does not mean that the importance of natural ventilation diminishes; it will still have avery important role to play as part of an integrated designapproach, as a key element in a mixed mode building, andas the lead strategy in the cooler parts of the UK. Inaddition, it might be the case that, as the climate warms,occupants will adapt themselves to that changing climate,and so the threshold at which people find conditions toowarm will also increase.examples, and guidance on where the relevant input datamight be found.1.2.22Developing the designstrategy2.1Satisfying design requirementsSection 3: Ventilation componentsand system integrationThis section is mainly about tactics. Having used section 2to develop the strategy, this section looks at the selectionand specification of the various types of ventilationcomponent (i.e. windows, ventilators and dampers) andhow they should be integrated into an overall system.As well as considering the technical issues of design andspecification, the section also discusses the important‘softer’ issues, such as the division of responsibilitybetween members of the design team and the componentsuppliers and system installers. This is particularlyimportant since many issues relating to the successfulimplementation of natural ventilation cross traditionalboundaries of design responsibility.Another key issue is the inter-relationship between thesystem and the occupants. A key aspect of natural ventilation is to empower the occupant to make suitableadjustments to window opening etc. to maintain personalcomfort without prejudicing the comfort of others. Thismeans that automatic control strategies need to becarefully integrated with user behaviour. Such issues aredeveloped in section 3.As an adjunct to this manual, a spreadsheet tool* has beenprepared that implements many of the design calculationsincluded in section 4. This is intended as an illustration ofhow the methods could be implemented. Users will needto confirm that the tool meets their own requirements,and adjust it as necessary to meet the particular circumstances of the design issue they are investigating.Natural ventilation is one of a number of strategies thatare available to the designer. CIBSE Guide B2(2) containsan overview of the various approaches and gives guidanceon their applicability to different situations.Natural ventilation systems need to be designed to achievetwo key aspects of environmental performance:—ventilation to maintain adequate levels of indoorair quality—in combination with other measures, ventilationcan reduce the tendency for buildings to overheat,particularly in summer.The natural ventilation strategy must also be integratedwith all other aspects of the building design. Key issuesfor consideration are:—A satisfactory acoustic environment: natural ventilation openings also provide a noise transmissionpath from outside to inside, and this may be adetermining factor in some building locations. Inaddition, naturally ventilated buildings ofteninclude large areas of exposed concrete in order toincrease the thermal capacity of the space. Suchlarge areas of hard surface will require carefulattention to achieve a satisfactory internal acousticenvironment.—Smoke control: since smoke can follow naturalventilation paths, the integration of the fire safetystrategy must be an important part of design fornatural ventilation.—Health and safety(7): many natural ventilationopenings will be at significant heights above floorlevel and so the proposed Work at HeightsRegulations(8) will be particularly relevant.Because of the important link between the design and theway the user operates the building, section 3 stresses thebenefits of post-completion fine tuning to ensure the fullpotential of the building is being realised to the benefit ofthe occupants.1.2.3Section 4: Design calculationsSection 4 is the most technical part of the manual. Itbegins by reviewing the calculations that will need to becarried out and reviews the type of calculation techniquesthat are available.The section suggests that for basic design purposes, a classof tools known as ‘explicit envelope flow models’ are themost appropriate. They allow basic dimensioning of thesystem components. It then explains how other, moresophisticated tools (such as implicit envelope flow models,combined thermal and ventilation models, computationalfluid dynamics and physical scale models) can be used tocheck the performance of the sized system under a varietyof operating modes.Because implicit envelope flow models are the most usefultool to the designer, this aspect is developed in depth,showing how the basic textbook equations can be manipulated to provide solutions to most design problems. Thesetechniques are then illustrated with a number of worked2.1.1VentilationThe principle role of ventilation is to provide an appropriate level of indoor air quality (IAQ) by removing anddiluting airborne contaminants. Guidance on achievingadequate levels of IAQ (to avoid mould growth and healthhazards) is given in Approved Document F(4). Higherrates of ventilation may be provided than proposed in the* The spreadsheet may be downloaded from the CIBSE website(www.cibse.org/venttool)

Developing the design strategy3Approved Document, and this may enhance the perception of freshness, but in most cases this will come at aprice because energy costs will increase correspondingly.In order to achieve adequate IAQ, Approved Document Fadopts a three-stage strategy as follows(a)Extract ventilation: to remove pollutants at source,with the extracted air being replaced with outsideair.(b)Whole-building ventilation (supply and extract): todisperse and dilute other pollutants.(c)Purge ventilation: to aid removal of high concentrations of pollutants released from occasionalactivities such us painting, or the accidentalrelease via spillages etc. Purge ventilation istypically an order of magnitude greater thanbackground ventilation. As well as helping toremove high levels of contaminants, purgeventilation can also help to remove excess heatfrom the space, thereby assisting thermal comfortin summer.The whole-building ventilation rate recommended byboth the 2005 edition of CIBSE Guide A(9) and the draftApproved Document F(10) is 10 litre·s–1 per person. This isbased on the correlation between ventilation rates andhealth. Since naturally ventilated buildings cannot1200CO2 concentration / ppmv1000InternalExternalAverage duringoccupancy800600400200004812Time / h162024(a) Constant mechanical ventilation rate1200provide a constant ventilation rate, it is necessary todemonstrate that an equivalent level of air quality hasbeen provided. This can be done by showing that the IAQachieved by the natural ventilation is equivalent to thatprovided using a constant ventilation rate of 10 litre·s–1per person during occupied hours. One way of doing thisis to use the CO2 level in the space as a proxy for generalIAQ levels. By calculation, the CO2 levels in the occupiedspace can be determined based on a constant ventilationrate of 10 litre·s–1 per person during occupied hours. Asimilar calculation can then be carried out using thevariable ventilation rate typical of a naturally ventilatedscheme. In both cases, the boundary conditions of externalCO2 concentration, occupancy levels etc. must be thesame. The naturally ventilated design would be acceptableif the average CO2 concentration during occupied hours isno greater than that achieved by the mechanicallyventilated design, and the maximum concentration in thenaturally ventilated scheme is never greater than anagreed maximum threshold figure. The IAQ tool in thespreadsheet (see section 1.2.3) illustrates how thesecalculations can be carried out. This is illustrated inFigure 2.1.Figure 2.1(a) shows the CO2 profile for a constant ventilation rate of 10 litre·s–1 per person (equivalent to 1.2 ACHin this example), coupled with a background infiltrationrate of 0.1 ACH. Figure 2.1(b) shows a naturally ventilatedscheme having three levels of ventilation: a night-timerate of 0.25 ACH, an initial daytime rate of 1.0 ACH and aboosted rate in the middle of the day of 1.5 ACH. Theaverage concentration of CO2 in the two cases is 986.2 and971.9 ppmv respectively although, as can be seen, thenatural ventilation peaks at just over 1100 ppmv, compared to the constant mechanical case of 1005 ppmv.In a similar way, if the volume of the space is sufficientlylarge, then the pollutants from the activities in the spacewill only degrade the IAQ in the occupied zone slowly,especially if a pure displacement type ventilation strategyis adopted, with the pollutants being concentrated in astratified layer above occupant level. As an illustration,consider ventilating a theatre, where there the designoccupancy is 1000 people. This occupancy will only lastfor the duration of the performance, but will build up tothat peak for the hour or two preceding ‘curtain-up’.Figure 2.2 shows the evolution of CO2 concentration inthe space when ventilating at a constant rate between17:00 to 22:00 equivalent to 8 litre·s–1 per person based on1200InternalExternalAverage ge duringoccupancy1000CO2 concentration / ppmvCO2 concentration / ppmv10008006004002004812Time / h1620(b) Varying natural ventilation rateFigure 2.1 Comparison of constant and variable ventilation rates onindoor air quality24004812Time / h1620Figure 2.2 Effect of volume and airtightness on indoor air quality24

4Natural ventilation in non-domestic buildingsthe design occupancy. This is 20% less than the wholebuilding ventilation rate recommended by ApprovedDocument F(4) but the average concentration in the spacepeaks just above 1000 ppmv at the end of the performance.If a displacement flow regime is in place, then the concentration in the occupied zone will be significantly lower.This relatively simple example illustrates the importanceof the dynamics of ventilation.2.1.1.1Control of ventilationIf natural ventilation is to be adopted, then the system hasto be able to provide controllable ventilation rates across awide range, from say 0.5 to 5 ACH or even more. Indeed, itshould be possible to shut down the ventilation rate tonear zero when the building is unoccupied, especially ifoccupancy is the principal source of pollutants. The widerange of flowrate that is required means that the differentmodes of ventilation (whole-building, purge etc.) arelikely to be provided via different devices such as trickleventilators, opening windows and/or purpose providedventilators. Such considerations will have considerableimplications for the façade design and the control strategy,requiring a high degree of design integration. This isconsidered in detail in section 3.As well as providing the required ventilation rates, theventilators should be designed so as to minimise discomfort from draughts, especially in winter. In office-typebuildings, this usually involves placing the inlets at highlevel, typically 1.7 m or more above floor level.2.1.22.1.2.1Compliance with Part L2 of the Building Regulations(5)requires that designers demonstrate that the building willnot overheat due to excessive solar gains. The aim of thisrequirement is to prevent the tendency to retrofit mechanical cooling.The compliance procedures for checking for solaroverheating are developing with subsequent editions ofApproved Document L2. In the 2002 edition, the compliance check was a rather coarse filter that checked theaverage solar gains over a design July day. This set a limitof 25 W·m–2 for the average solar load in a six metre deepperimeter zone, and assumed that internal gains were amodest 15 W·m–2 total, and that other mitigating factorssuch as effective thermal mass and night ventilation werepresent. The proposals for the 2005 edition, as publishedin the ODPM’s consultation paper(11) are that detailedcalculations will be required as part of the whole-buildingcalculation approach required by the European Directiveon the energy performance of buildings(12). This newapproach means that the benefits of thermal mass andnight ventilation can be properly credited.The forthcoming CIBSE TM37: Design for improved solarcontrol(13) will provide guidance on the solar controlperformance that will be needed to limit overheating to adefined number of hours as a function of the key designparameters. Solar control can be achieved throughmeasures such as:—Size and orientation of the glazed areas: this will beinfluenced by the general organisation of thebuilding on its site. Shading of the windows bysurrounding buildings, and through self-shadingfrom other parts of the same building can alsocontribute to reduced solar gains.—Tints, films and coatings in/on the glass: recentdevelopments in glass technology means thatspectral-selective coatings are able to reduce solargain without unduly reducing visible light transmittance.—Blinds: internal, mid-pane or external.—Overhangs, side fins and brise-soleil: the performanceof these forms of solar control are

ventilated buildings has grown. This revision of AM10 is therefore a timely opportunity to update and enhance the guidance offered to designers and users of naturally ventilated buildings. The first edition of AM10 devoted its first section to setting natural ventilation int