Blower door test and calculations of air exchange rates
To measure the leakage rate of a building a pressure difference of 50 pa is created between the inside and the outside of the enclosure system. The pressure difference causes air to flow through openings (cracks, gaps and bad sealed joints). Local leaks are determined with a smoke candle.
With pressure gages and flow meters at the blower setup the amount of air changes per hour can be determined using the following data and formulas.
- CFM50: The flow rate when a building is depressurized -50pa
- ACH50: Air changes per hour when a building is depressurized -50pa or the volume of air through the blower door during a one hour period divided, by the house volume
- CFMNatural: The estimated flow rate of a building in a natural state
- ACHNatural: Estimated air changes per hour of a building in a natural state during a one hour period, divided by the house volume
Conversion of CFM50 to ACH50
ACH50 = CFM50 x 60/heated house volume
Natural air infiltration
To estimate natural infiltration, factors from the LBL chart below should be included in the following calculations.
ACHnat = ACH50/N = [CFM50x(60)]/[voume x N]
LBL Correlation factor N for Zone 2
| #Stories | 1 | 1.5 | 2 | 3 |
| Well shielded | 22.2 | 20.0 | 17.8 | 15.5 |
| Normal | 18.5 | 16.6 | 14.8 | 13 |
| Exposed | 16.7 | 15 | 13.3 | 11.7 |