Indoor air quality is determined by the supply rate of different pollutants, their chemical interaction, the dilution of these substances through ventilation - and their resulting effects on IAQ (the human health or the perceived air quality). These effects are, to a very large extent, unclear or unknown and the IAQ is generally defined through specific air flow rates or through IAQ indicators
(see section 2.4.2)
. So most tools for the evaluation of IAQ are based on ventilation rates. These flow rates may be converted to IAQ indicators such as CO2 levels by combining an assumed supply rate of e.g. CO2 and the dilution due to the ventilation.
General calculation methods for natural ventilation are detailed, requiring information on the location and area of window openings, local wind speed, local wind direction, building geometry and interior building design. Surrounding buildings and the local landscape influence the local wind speed and direction. Based on this information, the pressure differences across each window opening are calculated, and the resulting airflow across each opening and through the entire building are determined. Simple cases of the general calculation methods are available for specific building design cases, such as window openings placed on only two opposite sides of a building.
The general methods are described in several guidelines (AIVC, 1996; SBi 202; CIBSE AM 10, 2005) and are also
implemented in a number of detailed building simulation tools, such as
IDA-ICE, VELUX Energy and Indoor
Climate Visualizer (EIC Visualizer),
TRNSYS, EnergyPlus, IES VE, etc.
The VELUX EIC Visualizer contains state-of-the-art methods for evaluation and illustration of ventilation flow through windows – and can be used
Traditionally, simplified methods for calculating natural ventilation have been very conservative, only taking
limited account of the full potential of natural ventilation. Typically, a worst-case approach is used that assumes only single-sided ventilation, which will lead to severe underestimation of the ventilation flow rate. However, the French institute CSTB has developed
a simplified calculation method for
evaluating natural ventilative cooling. The method will be implemented in the French Building Code for summer
comfort in 2014.
As illustrated in the table below, huge differences arise depending on which type of calculation method is applied.