Ventilation Definitions Guide

Air permeability is a measure of how much air leaks out of your building. It is measured in cubic metres per hour per square metre (m3/hr.m-2). What this means is that it measures the volume of air that is leaking per square metre of your buildings outer walls & roof each hour. When designing a building, architects will often set a design air permeability value as a target.

Airtightness means the resistance to infiltration when ventilators are closed. Higher airtightness means lower infiltration.

Automatic control is where a ventilation device operates when some electronic device detects a stimulus. Stimuli could be air humidity, carbon dioxide levels or air pressure.

Closable or ventilation opening is any opening between the interior and exterior of a building that can be opened or closed.


Continuous operation is where a ventilation device runs all the time. This does not mean that the flow rate is constant, just that the device is switched on.

Equivalent area is a way of comparing different ventilator units. The performance of the ventilation unit is measured and a circular area that has the same performance statistics is calculated. This allows different ventilator units to be compared on a single metric; the equivalent area, rather than attempting to judge on the basis of a set of different specifications.

Extract Ventilation describes ventilation flow of any kind from the interior to the exterior of a building.

Free area is the size of the actual opening within the ventilator unit.

Infiltration is when air moves between the interior and exterior of a building through cracks and other unintentional openings. This air movement is caused by wind, difference in air pressure/the stack effect.

Intermittent Operation is where a ventilation device is not continuously active.

Passive stack ventilation (PSV) is where ducts are installed in the ceilings of rooms that connect to openings on the roof. This type of system requires no active ventilation and works by the stack effect and the pressure changes induced by wind. Wind is the movement of air from a high pressure to a low pressure area. As air is expelled from a building, it increases the pressure of the air around the opening. Wind ensures that the exterior air pressure remains balanced and that a pressure difference remains.

Purpose-provided ventilation is when ventilation devices within the building cause ventilation.

Purge ventilation is when air is quickly exchanged between the exterior and interior of a building. Opening a window is an example of a purge ventilation.

The stack effect is the difference in air pressure between the outside and the inside of a building. When air is warmer it expands as the air particles have more energy to move further away from each other. Expanded air is less dense as there are less air particles for any given volume. Less dense air has a lower pressure. Thus the warmer the air is, the lower the pressure. When the air temperature between the inside and outside of a building is different there will be a pressure difference, and air will flow from high to low pressure – this is the stack effect.

Ventilation is when air is supplied or removed from the interior of the building. Both purpose-provided and infiltration count as ventilation.

Whole building ventilation  is calculated by combining the total purge ventilation, extract ventilation and infiltration for the whole building.


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