In most countries, the building codes express requirements to IAQ by a minimum outdoor air flow through a building. These requirements can be expressed in different ways, e.g. as:
Air change per hour (ACH)
Air flow per unit of floor area (l/s · m²)
Air flow per person (l/s · pers)
In general, the regulatory requirements for ventilation flow rates in residential buildings are based on a humidity balance of the building: All the humidity generated inside the building must be extracted by ventilation to prevent rot or mould damage to the construction and to avoid the hazard to human health that follows such mould formation. In many countries, building code requirements for ventilation of residential buildings are around 0.5 ACH.
Fixed airflow rates are difficult to guarantee with natural ventilation, due to the dependence on the outdoor climate. To provide a simple way of integrating natural ventilation into buildings, the requirement for a specific air change rate in Denmark have been translated into specific opening areas to the outdoor, depending on room usage and size. The openings in living rooms, bedrooms etc. supply fresh air to the building, whereas natural extract air ducts lead the “used air” to the outside from wet-rooms, such as toilets, bathrooms and kitchens.
IAQ indicators are another way of using (automated) natural ventilation to obtain a certain level of IAQ. Unfortunately, almost all building regulations for residential buildings are based on fixed minimum air flow rates, allowing only IAQ indicators to be used for increased ventilation flow rate.
National standards and guidelines describe various criteria and levels of IAQ and different calculation methods for natural ventilation.
Criteria for the indoor environment, for design and for energy performance assessment of buildings are described in EN 15251. The standard describes criteria for thermal environment, indoor air quality, lighting and acoustics.
Several ways of describing criteria for the IAQ through airflow rates or IAQ
indicator levels are included, depending on the desired use and comfort level. The criteria are defined by classes I to IV, where class I is very high performing and generally for people with special needs. Class IV is low performing and should only be accepted for a limited period of time.
There are several main philosophies for determining the right air flow rates:
Health-based, where the criteria are based on the impact of ventilation on human health
Perceived Air Quality-based for un-adapted persons (persons entering an occupied room from outside)
Perceived Air Quality-based for adapted persons (persons staying in the same room for a longer period of time)
Experience-based requirements that are believed to provide an acceptable humidity level under typical use of the house
A marked difference in required airflow rates is observed between adapted persons and unadapted persons – and between the health-based approach and the Perceived Air Quality for unadapted persons. Using criteria for perceived air quality for unadapted persons will lead to the highest ventilation flow rate –
often twice as much or more as for adapted persons. EN 15251 includes
examples of airflow rates for adapted and unadapted persons.
For residential buildings, it is reasonable to assume that people are adapted to the indoor air and that air flow rates can be determined accordingly – or according to the health based approach. For office buildings, restaurants, shops etc. where people frequently enter and leave the rooms, it is reasonable to
assume that people are unadapted.
The criteria for IAQ in EN 15251 are mainly based on studies of perceived air quality in offices where criteria for the IAQ in residential buildings are less studied. So, in most cases, the regulatory requirements for ventilation flow rates in residential buildings are based on a humidity balance of the building.
The health effects from ventilation have been studied only to a limited extent, but there is some indication that a minimum total air flow rate of 4 l/s pers should always be available for health reasons. This value is included as the minimum recommended ventilation flow rate in EN 15251.