Natural Infiltration (ACH50 → ACHnat) Calculator

Inputs

View results
Air changes per hour at 50 Pa from the blower-door test report.
ACH50
Climate/height/shielding correction: SMALLER for tall, wind-exposed homes (more natural infiltration), larger for short sheltered ones. 20 is the classic default.
Conditioned air volume used in the blower-door calculation (floor area × average ceiling height).
ft³

Results

Default result
Edit inputs
Natural infiltration(ACHnat)
0.35ACH
Caution

Mid-band envelope (3-7 ACH50): typical existing housing; air sealing still pays.

Average annual air changes per hour under natural pressures.

Also computed

Natural airflow(Q_nat)70cfm

The average infiltration airflow a load calculation or ventilation balance uses.

Test airflow(CFM50)1,400cfm

Method notes 4 notes
  • ACH50/N is a screening conversion: the LBL-family N factor compresses climate, building height and wind shielding into one divisor (taller/exposed → smaller N → more natural infiltration). Hour-by-hour models replace it in detailed work.
  • Natural infiltration is an ANNUAL AVERAGE, not a design-day value — winter design infiltration runs higher; load calculations per Manual J handle that separately.
  • Common tightness anchors: ~3 ACH50 modern energy-code tier, 5-7 typical existing stock, 0.6 ACH50 the Passive House benchmark.
  • Below roughly 3-5 ACH50, code-level mechanical ventilation (62.2-style) is doing the fresh-air work — infiltration credit toward ventilation should be taken cautiously.

Powered by MachineCalcs ↗