Conversion factors

Relationship between 4 Pa and 50 Pa results

Within the UK, both the Standard Assessment Procedure (SAP) and CIBSE TM23 define standard formulas to convert between a 4 Pa and 50 Pa air permeability result.

Converting air permeability at 4 Pa to 50 Pa

To convert from an air permeability at 4 Pa to the same measurement at 50 Pa, use the following formula:

$$AP_{50} = 5.2540 × AP_{4}{ }^{0.9241}$$

Converting air permeability at 50 Pa to 4 Pa

To convert from an air permeability at 50 Pa to the same measurement at 4 Pa, use the following formula:

$$AP_{4} = (AP_{50} ÷ 5.2540)^{(1 ÷ 0.9241)}$$

These conversion equations were established following the analysis of nearly 300k fan pressurisation tests carried out by the University of Nottingham.

An investigation into the efficacy of the pulse method of airtightness testing in new build and Passivhaus properties

In this paper, experimental investigations were performed involving the pulse method to assess its repeatability and accuracy. In addition, two empirical models were applied to the data set to explore the conversion of air permeabilities between high and low pressures.

Converting air change rate (ACH)

To convert an air change rate from 4 Pa to 50 Pa, and vice versa, it must first be converted to an air permeability to factor in the geometry of the dwelling using the building volume and envelope area.

$$AP = ACH × V_{Build} ÷ A_{Env}$$