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The n50 is expressed in air changes per hour (ac/h). It is the relationship between the total volume of air in m³ and how often leaks in the building envelope allow the air to exchange at 50 Pascals air pressure difference. This reference is often used when installing a mechanical ventilation system.
Air permeability rate per hour at 50 Pascals pressure difference which is called the q50.
The q50 is the air permeability rate and is expressed as m³/hr/m². It is an expression of the amount of air leaked from the dwelling in relation to the total exposed surface area of the building (i.e. the ground floor, all exposed ceilings and all external walls) at 50 Pascals air pressure difference.
Critically it is the air permeability rate that is relevant for Irish Building Regulations.
The q50 test result is then divided by 20 and inputted into the DEAP calculation to improve the BER and prove compliance.
Best practise in domestic dwellings calls for an ac/h rate of:
- 3.0 ac/h – natural ventilation (i.e. with wall vents) – very easy to achieve
- 1.5 ac/h – with mechanical ventilation
- 0.6 ac/h – Passive House Standard