All buildings require ventilation. This is necessary for:
It is important to realise that the level of airtightness achieved within a building will have an important influence on the overall ventilation rates that will be achieved (higher levels of airtightness, lower ventilation rates) and the type of ventilation strategy that should be adopted. Typical levels of airtightness suggested for various different ventilation strategies are shown below.
Ventilation strategy as a function of air leakage
Therefore, the aim of good ventilation design should always be to minimise the uncontrolled infiltration by making the building as airtight as possible, and then supply sufficient purpose-provided ventilation. In other words:
‘build tight, ventilate right’
It should be remembered that a dwelling cannot be too airtight, but it can be under ventilated.
Under average weather conditions, the air infiltration attributable to air leakage can be approximated using a simple ’rule of thumb’, which involves dividing the air leakage rate in ach @ 50Pa by 20. For instance, if the air leakage of a dwelling was 10 ach @ 50Pa, then the air infiltration due to air leakage would equal 10/20 = 0.5 ach. However, it has to be remembered that this is only a simple ‘rule of thumb’ and will not always be applicable to all types of buildings all of the time.