Module 116: CIBSE guidance for delivering good IAQ in the built environment

This module gives an overview of how ventilation to improve indoor air quality (IAQ) is covered specifically in CIBSE guidance documents

When designing, installing and operating building environmental services, a principal requirement is to maintain indoor air quality (IAQ) that is healthy and provides an environment that is effective. Air quality is not just a challenge for the building services engineering practitioner; increasingly, it also excites interest among government, the general public and the media. Ventilation as a means of delivering appropriate IAQ pervades much of the CIBSE’s guidance documents; this CPD article will present an overview of how this is reflected in the various Guides, TMs, AMs, and Knowledge Series publications.

For the building occupant, there are many airborne gases and particulates that combine to create a perception of the internal environment, together with the more traditional measures of comfort, such as temperature, humidity, air movement, noise and light. The consequences of poor IAQ may not affect perceived comfort, but can still lead to loss of productivity, ill health, disease and – potentially – premature death (see the April 2011 CPD article for more details of environmental contaminants). The building systems designer and operator need to have a holistic understanding of how this cocktail of parameters can be controlled properly to deliver appropriate IAQ.

In its newly added chapter zero, CIBSE Guide A (2015) Environmental Design identifies that one of the main criteria in building environmental design is occupant health and wellbeing, and that this is explicitly linked to IAQ. Guide A also introduces the concept of ventilation effectiveness as a means of estimating the efficacy of a given rate of ventilation to dilute and remove pollutants for a particular air-distribution pattern. For example, this indicates that a (low-level supply, high-level extract) displacement system (in an appropriate application) is likely to be 50% more effective at ventilating the occupied space than a (high-level supply, high-level extract) mixing system.

A table links the basic ventilation rates (in L·s-1 per person) to the EN 13779 Ventilation for non-residential buildings categories of IDA 1 – high IAQ  – through to IDA 4 – low IAQ (these categorisations are often referred to in design specifications). Typical ventilation rates are also included for specific spaces in dwellings and offices – which generally coincide with those in the UK Building Regulations – together with an extensive list of references that define ventilation requirements for other spaces, such as call centres, cleanrooms and prison cells. The classic ‘pollution concentration equation’ is introduced in section 4.2.3, which can give useful, but simple, insight into the potential ventilation capacitance – or ‘reservoir effect’ – of a space as a means of controlling levels of transient pollutants; a worked example is included that considers occupant CO2 emissions and polluted outdoor air.

room concentration at time t,


qv – the ventilation rate (m3·s-1)

qe – pollutant emission rate (m3·s-1)

cO – concentration in outside air

V – room volume (m3)

A spreadsheet that applies this relationship is downloadable for free from the CIBSE Knowledge Portal, through the web page describing AM10 Natural Ventilation in Non- Domestic Buildings. This sheet can also be adapted to other pollutants, such as nitrogen dioxide (NO2 ), as illustrated in the panel.

Chapter 8 includes a commentary on IAQ and gives typically required fresh-air supply rates to maintain acceptable CO2 levels. Introductory material on indoor pollutants and their health (and sensory) effects are included, with links to the relevant occupational exposure limits. A notable table 8.3 indicates the influence of poor system practices, such as unclean ductwork, on occupant respiratory symptoms. This section considers the importance and impact of outdoor air and the typical relative pollution of the ‘fresh’ air and the internal space. Filtration is discussed as a means of moderating the impact of poor outdoor air quality. The chapter concludes with a brief discussion of the impact of IAQ on sickness and productivity.

CIBSE Guide B supplies the underpinning reference material when considering the systems needed to deliver a controlled environment. Mechanical ventilation systems are introduced as a means of improving IAQ by delivering ‘adequate clean supply air’, but it is cautioned that the systems can be detrimental to IAQ when not maintained appropriately, and that poor IAQ will impair occupant performance. Section B2.2.2 considers contaminant control, with some expanded consideration of indoor and outdoor pollutants. In particular, it clarifies that the ventilation rates that satisfy EN 13779 Ventilation for non-residential buildings relate to comfort air quality and do not necessarily reflect health-related requirements. Where natural or passive cooling is used, section B2.2.4 indicates that the ventilation rate is likely to surpass that needed for IAQ. It is noted that the natural ventilation provided by the frequently applied ‘openable area equivalent to 5% of floor area’ does not necessarily satisfy requirements for air quality needs, but reiterates that indoor air quality can be moderated using the reservoir effect provided by the volume of the space (as illustrated in the earlier example).

B2 section 2 discusses the different ventilation modes, including the impact on IAQ, and notes that ‘the maintenance of air quality may often require air-cleaning methods’. The section gives extensive comparisons of the pros and cons of natural, mechanical and mixed-mode ventilation, and, again, picks out displacement systems as potentially delivering improved air quality over those that rely on ‘mixed air’ dilution.

Section 2.3.3 introduces filtration, and offers some detailed information on contaminants and the opportunity to control them effectively in ventilation systems with appropriate filter types, noting that the available guidance on external air quality is subject to ‘periodic review’. Methods of delivering ventilation air are discussed, with a final section on maintenance that includes specific commentary on ‘air quality and health issues’.

The Air Conditioning and Refrigeration Guide B3 (2016) notes that occupants’ perception of the effectiveness of the system will normally be influenced by the air quality in the breathing zone, and that systems with higher airflow rates and little or no recirculated air are likely to promote good IAQ perceptions. Contrary to what may be expected, CIBSE Guide F (2012) Energy efficiency in Buildings suggests that the potential for improved air quality can be synonymous with low-energy buildings, and states explicitly that the object should be to deliver acceptable indoor air quality with the minimum use of energy. It continues with a methodical examination of ventilation modes to deliver appropriate conditions efficiently, with particular emphasis on controlling minimum fresh air to maintain air quality, including a concluding discussion on demand-controlled ventilation.

Spreadsheet examples

This example considers a large, well-constructed, city-centre, open-plan office – 12m x 30m x 5m high (that is, room volume 1,800m3) – with a ventilation rate that is normally based on the number of occupants (10L·s-1 per person). The AM10 spreadsheet has been used as a basis and adapted to provide a basic simulation of internal levels of NO2 drawn in through ventilation systems.

As an example, Figure 1 indicates the NO2 levels in the internal space, with a constant ventilation rate – resulting from nitrogen oxide (NOx) emissions from congested rush-hour traffic – for when there is a constant ventilation rate of 400L·s-1 (0.4 m3·s-1) throughout the working day (based an occupancy of 40 people), and almost no infiltration outside working hours.

Figure 2 shows the same scenario, but this time applying variable ventilation rates that increase to purge the space of NO2 at night, when outdoor NO2 levels are lower. The ventilation rate is also increased at other times when outdoor NO2 levels are low, and then reduced as outdoor NO2 levels increase. Without applying variable ventilation rates, this space would contravene the threshold hourly limit value of approximately 0.105ppmv, so it is likely that some NO2 filtration would also be needed.

Figure 1: An example of using working hours constant ventilation rate (orange) in a building with low air leakage, and the resulting internal NO2 levels (red) arising from traffic-related NOx emissions

Figure 2: An example of leveraging the ‘reservoir effect’ and using variable ventilation rates (orange) to reduce internal NO2 levels (red) arising from traffic-related NOx emissions

As discussed in CIBSE Guide H (2009) Controls, although the primary function of a building control system has traditionally been the control of temperature and humidity, the increased awareness of sick building syndrome and other building-related illnesses has emphasised the requirement to ensure good IAQ. Section 3 includes an overview of controlling ventilation for air quality, and includes some basic information on the application of sensors. (The market for – and the availability of – low-cost IAQ sensors has blossomed in the eight years since this guide was published.) Control strategies are included for dealing with poor outdoor air quality, with the warning that an indoor IAQ sensor that controls ventilation rate may react to any ingress of outdoor pollutants by increasing the ventilation rate, so making the situation worse. The link between elevated temperatures (resulting from climate change) to potentially increased outgassing of pollutants – for example, volatile organic compounds (VOCs) – from structure and furnishings affecting IAQ is identified in Guide L (2007) Sustainability (this guide is currently under revision).

Section 16.1 of CIBSE Guide M (2014) Maintenance engineering and management recommends that air quality indicators be routinely reviewed, to assess seasonal variations and to confirm the validity of any initial assessments. CIBSE AM10 (2005) Natural ventilation sets out by defining that natural ventilation systems are intended to provide sufficient outside air to maintain IAQ standards, and includes illustrations of the efficacy of different natural ventilation strategies on air quality (using CO2 as a proxy). The applications throughout this guide are led by IAQ considerations, noting the significance of the position of air inlets and highlighting that, as ventilation air passes through a space, it accumulates increasing levels of pollutants. It notes that the amount of ventilation to control winter IAQ may be a tenth of that needed to provide summer cooling – and during winter unoccupied periods, even less ventilation may be desirable. It warns that suitable, representative IAQ sensor locations may be more difficult to find in naturally ventilated buildings than in those with mechanical ventilation extract ducts. There is a useful example of applying ‘reservoir’ capacitance to maintain IAQ in section 4.5, and the spreadsheet as (adapted and) used in the example in this article is associated with AM10 to explore this further.

CIBSE AM11 (2105) Building Performance Modelling section 4.6.6 notes the challenge of modelling natural ventilation – and so IAQ – when the building users have control of ventilation openings. It discusses how this can compound the uncertainty in modelling performance. The chapter ‘Ventilation modelling’ gives an extensive account of the techniques, tools and applications that can provide that modelling, including simple tools and estimation techniques, analytical methods, zonal network methods and CFD. Although nearly 20 years old, CIBSE TM21 (1999) Minimising pollution at air intakes still maintains currency in its underlying physical descriptions and overview of the impacts of poor IAQ. Section 4 examines how to estimate the effect of external pollutants on IAQ, taking account of the room effects, including room air recirculation, the reservoir effect and ventilation effectiveness. CIBSE TM26 (2000) Hygienic maintenance of office ventilation ductwork highlights the difficulty in defining absolute standards for air quality in terms of the numbers of fungi or bacteria present in indoor air, and warns these must be treated with caution, as there was little data available at the time of publication on the health implications of exposure at these levels. Useful background information is provided, but specific data should be cross-checked with more recent publications.

CIBSE TM30 (2003) Improved life-cycle performance of mechanical ventilation systems notes that, as filters age, there is likely to be a reduction in air quality, as well as an increase in fan power required. Generic ventilation systems are compared in section 3.2.3, confirming earlier Guide advice that appropriate installations of displacement ventilation have superior ventilation effectiveness and so can improve the value of a scheme. CIBSE TM36 (2005) Climate change and the indoor environment predicts that, for spaces such as classrooms – that require high fresh-air ventilation rates to maintain good air quality – it will become increasingly difficult to provide comfort standards through use of passive systems alone.

Chapter 4 of CIBSE TM40 (2006) Health issues in building services delivers one of the most consolidated collections of CIBSE references for air quality related to ventilation. It considers a number of aspects related to the determination and control of air quality – including legal (as at 2006), physiological, environmental, anthropogenic impact, occupational and security – as well as offering practical calculations to determine the rate of fresh air. CIBSE TM53 (2013) Refurbishment of Non-domestic Buildings lists typical failures in existing buildings, including: limited cross-flow in naturally ventilated buildings because of enclosed spaces; changes to the building layout; and building services (specifically HVAC systems) no longer being fit for purpose – all of which result in poor air quality, which is cited as one of the largest potential sources of occupant dissatisfaction. It also indicates that night-purge ventilation can improve air quality for the following day, and can represent an important measure to strengthen the case for retaining a natural ventilation strategy for both naturally ventilated and mixed-mode buildings.

In CIBSE TM55 (2014) Design for Future Climate: Case Studies, the impact of changing climate on effectiveness –and thus IAQ – pervades all of the models that are discussed. CIBSE TM57 (2015) Integrated school design confirms that poor IAQ in learning spaces not only affects the health and comfort of pupils, but can also impair learning performance and increase absenteeism. It reports that older windows often have poor airtightness, so providing higher infiltration rates and, perversely, potentially improving IAQ – whereas modern buildings have much better airtightness, which can lead to a marked rise in CO2 levels. It suggests that ‘traffic light’ air quality indicators can be used to show when windows should be opened to improve IAQ.

The CIBSE Knowledge Series books provide summary information and aim to deliver this in a concise, accessible format. In this area, CIBSE KS17 Indoor Air Quality and Ventilation offers a great starting point to gain insight on the information incorporated in the CIBSE Guides, TMs and AMs. It augments this with a contemporary list of regulations and standards affecting IAQ, plus additional material on measuring ventilation and IAQ.

© Tim Dwyer, 2017.

All CIBSE publications are available through the CIBSE Knowledge Portal.