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)
Air flow per unit of floor area (l/s · m²)
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.