Air permeability is essentially a function of the pressure difference between the inside and outside of the building and air flow rate through the fan(s) to produce that pressure difference.

This is averaged out over the envelope area.

The result takes account of environmental conditions.

The final air permeability at 50Pa is based on a logarithmic graph of pressure difference and flow rate, the graph should:

• Have at least 5 points (ideally 10 or more)
• The highest figure should be at least 35Pa but preferably would be in excess of 50Pa and ideally higher than 60Pa
• The lowest figure should be at least 10Pa or 5 times the ‘static pressure’ (the pressure difference between inside and outside without the fans) – which in itself MUST be less than 5Pa to limit uncertainty
• The readings should be no more than 10Pa apart
• The correlation of the graph should be at least 98%
• The gradient of the graph (n) should be between 0.5 and 1.0

These are aspects that the BCB should be checking carefully if choosing to accept air permeability results from non-accredited testing bodies.

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