Construction tab in model data under Airtightness header

The infiltration rate (rate of entry of unintentional air from outside through cracks, holes and through the porosity of the fabric) is assumed to be constant throughout the simulation and can be set in one of 4 different units depending on the setting of the Infiltration units model option:

- ac/h - the default option where infiltration is defined in air changes per hour (ac/h), i.e. zone volume per hour under standard operating pressures.
- m3/h-m2 at 50 Pa infiltration is defined as flow rate in m3/h per unit exposed surface area, when the pressure difference between inside and outside is 50 Pa. Unit commonly used in the UK.
- m3/h-m2 at 4 Pa infiltration is defined as flow rate in m3/h per unit exposed surface area, when the pressure difference between inside and outside is 4 Pa. Units commonly used in France and Belgium.
- n50 - ac/h at 50 Pa - infiltration is defined in air changes per hour (ac/h), i.e. zone volume per hour, when the pressure difference between inside and outside is 50 Pa. Unit commonly used in Europe.

The air flow rate in m3/s at standard pressure is calculated from the ac/h data using:

m3/s = ac/h * ZoneVolume / 3600

where ZoneVolume is the total air volume of the space including the volume of floor and ceiling constructions and any raised floor and suspended ceiling voids (if selected in Separate constructions) and including half the volume of partition walls.

Note: this total zone volume value may be different from the value used for calculating air flow rates for natural ventilation and mechanical ventilation which have model options for excluding floor/ceiling constructions.

The ac/h at standard pressure is calculated from other units using formulae in EN12831 and the m3/s calculated using the above formula.