Blower Door ACH50 & Natural Infiltration Calculator
Convert blower door depressurization airflow (CFM50) to Air Changes per Hour (ACH50), calculate natural air leakage (ACHnat & CFMnat) via LBL N-factors, and verify IECC code compliance.
Blower Door Test & Building Geometry
CFM50
From Minneapolis Blower Door manometer.
cu ft
e.g., 2,250 sq ft home × 8 ft ceilings.
Building Tightness & Code Compliance
FAILS 2021 IECC (MAX 3.0 ACH50)
Air Changes per Hour @ 50 Pa
4.17 ACH50
Depressurized test standard
Natural Infiltration (ACHnat)
0.34 ACHnat
100.8 CFM natural air exchange
Effective Leakage Area (ELA)
125.0 sq in
Approx. open hole size (ASTM E779)
LBL / Sherman-Grimsrud N-Factor
12.4
Climate, height & wind divider
Mechanical Ventilation Requirement
ASHRAE 62.2 Ventilation Mandate:
Mechanical Ventilation Required (< 5.0 ACH50)
2021/2024 IECC Air Sealing Limit:
≤ 3.0 ACH50 (Climate Zone 5–8)
Required CFM50 Reduction to Comply:
350 CFM50 reduction needed (28%)
ASTM E779 & RESNET/ANSI 380 Standard
Building Science
ACH50 = (CFM50 × 60) ÷ Conditioned Volume. The Lawrence Berkeley National Laboratory (LBL) N-factor converts artificial 50 Pascal fan depressurization into year-round natural air infiltration (ACHnat = ACH50 ÷ N), accounting for stack effect and local wind exposure.