1.5 Ventilation and air quality

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We will now look at the roles ventilation plays in promoting comfort and maintaining a healthy indoor environment.

By the end of this lesson you will have learned…

  1. Why ventilate?
  2. What determines “good” indoor air quality (IAQ)?
  3. External air quality
  4. Where do pollutants come from?
  5. What level of indoor air humidity is desirable?
  6. How much ventilation do we need?

1. Why ventilate?

There are many answers to the question of “Why ventilate?”. Ventilation serves many purposes, amongst the most important are the following:

  • Provide fresh air
  • Minimise the accumulation of and removing odours and pollutants
  • Maintain healthy levels of relative humidity
  • Remove heat in summer

One additional aspect of ventilation, which encompasses all roles above, is that ventilation helps to control indoor air quality. Providing and maintaining air with good air quality not only creates a comfortable environment but also a healthy one.

Air with low levels of pollutants and an acceptable relative humidity (more on these shortly), will be healthier for both occupants and buildings. So how do we design ventilation to ensure a high level of indoor air quality and how is defined?

2. What determines “good” Indoor Air Quality (IAQ)?

A short answer to how IAQ is defined would be to refer one to the EN standards that are applicable to non-domestic buildings (EN 13779) and general buildings, including domestic (EN1521).

Both standards judge IAQ on the basis of CO2 and recommend acceptable indoor concentrations of CO2 – relative to the concentration of CO2 in the outside air. Atlhough carbon dioxide isn’t as serious an internal air pollution issue compared with many other pollutants, it is an important issue affecting levels of alertness in offices and classrooms and has been known to affect comfort levels since early part of the 19th century. So one way to assess if we have sufficient ventilation is to provide good level of IAQ would be to base rates on those that will provide acceptable levels of CO2 in the air.

Although levels of CO2 in indoor air have become one of the main determinants of IAQ, there is one other seemingly harmless “pollutant” that we should control: moisture. Excessively high levels of moisture indoors can lead to an increased aggravation of asthma and favourable conditions for the growth of dust mites and fungi / moulds. Given the risks posed by high levels of moisture, ventilating at a rate sufficient to control moisture seems a sensible idea.

However, for good air quality, ‘source control’ (i.e. reduction of contaminants) is just as important. Ventilation on its own cannot remove 100% of the contamination if it is constantly replenished – it is only diluting it. In other words, ventilation is necessary, but not sufficient. Ventilation on its own cannot ensure safe, dry surfaces: if surfaces are too cold, and/or moisture is continuously entering the building from outside, damp is likely to result.

If damp, mould and other sources of contamination are present, they will contaminate the indoor air to a greater or lesser extent. Contaminated air can develop in pockets (‘microclimates’) within the building, often unseen. To ensure good indoor air quality, the conditions in these areas have to be improved (more on this in Modules 4 & 5) or ventilation will simply be circulating the contamination.

Sometimes the conditions leading to the contamination cannot be completely eliminated, particularly in some of the smaller non-habitable spaces such as crawlspaces under the ground floor. (For example, it is impossible to dry up the groundwater!) In these cases, the contaminated microclimates must be separated from the living spaces. Good ventilation can then assist in keeping the building fabric sound and healthy. The range of issues to do with getting and keeping a sound building fabric are looked at in detail in Modules 4 and 5. Here, however, we focus on the air quality we are aiming to achieve, focusing on occupant-generated air pollution and the role of ventilation in dealing with it.

3. External Air Quality

Outdoor air is a source of many pollutants and historically, external air pollution has received lots of attention and is a topic that arguably more widely known than IAQ. Although it may seem like old news, air pollution is still a very important and topical issue.

Public Health England has shown that around 29,000 people a year in the UK die early deaths because of air pollution. In 2015, the UK government was taken to the Supreme Court as they had been in breach of the EU limit on emissions of harmful nitrogen oxide (NO) particulates for years. In the same year, Volkswagen admitted to falsifying emissions tests on their cars, which means that far more automobile pollution has been released than previously thought. A study released in February 2016 by researchers at King’s College London has also shown that the effects of air pollution may be longer lasting than previously thought:

Recommended reading

  1. http://www.theguardian.com/environment/2015/apr/29/supreme-court-orders-uk-to-draw-up-air-pollution-cleanup-plan
  2. http://www.theguardian.com/environment/2016/feb/09/air-pollution-raises-risk-of-death-for-decades-after-exposure

(Accessed 9 Feb 2016)

Clearly, retrofit designers cannot control the larger air pollution issue. But there are design choices that are important to review during the retrofit design process.

Above, we have highlighted the importance of creating and maintaining a healthy indoor environment.

However, it is also important to avoiding worsening the local external environment for others through excessive fuel use and/or use of particulate emitting fuels such as burning solid or biomass fuels particularly in urban areas.

Furthermore, where outdoor pollution levels are high it is worth considering the installation of a balanced ventilation system (e.g. MVHR) so that the incoming air can be filtered to remove at least some of the outdoor pollutants from the indoor air supply.

4. Where do pollutants come from?

Indoor pollutants are primarily caused by occupants but can also come in with the “fresh” air from outside, unfortunately. The table below shows a range of pollution sources.

Table showing sources and types of indoor air pollution
Table showing sources and types of indoor air pollution

source: 3. https://www.nhbcfoundation.org/wp-content/uploads/2016/05/NF18-Indoor-air-quality-in-highly-energy-efficient-homes.pdf (accessed on 07/07/23)

5. What level of indoor air humidity is desirable?

The optimum range for indoor relative humidity is 40 to 60% – as shown in the image below. Drier air can aggravate respiratory problems and can lead to discomfort. Higher humidity can encourage mould growth and dust mites and will lead to discomfort at higher temperatures (‘muggy’ conditions restrict our ability to perspire and cool down). Furniture and flooring may also be damaged by high or low humidity.

Graph showing optimal humidity levels
Graph showing optimal humidity levels

In terms of thermal comfort, lower humidity makes it easier to cool down by sweating as evaporation will be more effective. The converse is also true – high humidity makes it more difficult to cool down by sweating as humid air cannot evaporate and hold as much moisture as drier air. This is one reason why hot, humid days typically feel more uncomfortable than hot, dry ones.

Further reading:

4. Dampness in Buildings and Health – Nordic Interdisciplinary Review of the Scientific Evidence on Associations between Exposure to ‘‘Dampness’’ in Buildings and Health Effects (NORDDAMP):

5. WHO Guidelines for Indoor Air Quality: Dampness and Mould http://www.aecb.net/knowledgebase/guidelines-indoor-air-quality-dampness-mould/

A much more detailed examination of the conditions in which moulds and bugs might flourish, and how retrofit can identify and minimise these, is found in Module 5.

6. How much ventilation do we need?

In order to answer this question effectively, we need to consider what type of building requires ventilation. Ventilation system design is based primarily on levels of occupancy and also influenced by the activity of those occupants. Different types of activity will require higher levels of ventilation. Think of how the ventilation requirements differ between a home and a gym, for example. The level of activity and associated production of heat and moisture will be much higher at a gym, so consequently the ventilation rates will be significantly higher to compensate.

Also important is the need to provide a minimum level of background ventilation to maintain air quality, deal with materials off-gassing and dissipate the baseline level of pollutants that are produced. There may also be issues in retrofit relating to the potential for gas leaks, radon and other soil gases that require additional consideration.

In looking at some typical recommended occupancy-related ventilation rates in the chart below, it is clear that by ventilating to control moisture accumulation, we provide more than enough ventilation to deal with the other categories of pollutants.

So one general answer to how much ventilation is required would be: “enough to control moisture build-up”.

Bar chart showing ventilation required to remove a range of pollutants
Bar chart showing ventilation required to remove a range of pollutants

Clearly there is also responsibility on the designer’s part to remove and minimise any substantial sources of moisture as part of the retrofit. So although one can see that between 7-8 l/s of ventilation would seem to provide “enough” ventilation, it is worth remembering that the figures below are averages. (The level of pollutants and the need for more or less ventilation always changes). We will take this discussion further in Section 7 – Building Services, where we will explore precisely how these ventilation rates per person translate into the design of ventilation systems.


This lesson has covered:

The reasons why ventilation and air quality are important

Sources of indoor pollutants

The average ventilation rates generally needed to control different pollutants


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