Air permeability is essentially a function of the pressure difference between the inside and outside of the building and the air flow rate through the fan(s), necessary to produce a pressure difference. This is averaged out over the envelope area. The result takes account of environmental conditions.
The final air permeability at 50 Pa is based on a logarithmic graph of pressure difference and flow rate, the graph should:
- Have at least 7 points (ideally 10 or more).
- At least one building pressure >50Pa and at least on <50Pa, No building pressures >100Pa.
- The lowest figure should be at least 10 Pa or 5 times the ‘static pressure’ (the pressure difference between inside and outside without the fans)
- The readings should be no more than 10 Pa apart.
- The correlation coefficient r2 >0.98
- The gradient of the graph (n) should be between 0.5 and 1.0.
These are aspects that the building control should check carefully if choosing to accept air permeability results from non-accredited testing bodies.