Blower-Door Air-Tightness (ACH50, Natural Infiltration, Code Check)
Turns a blower-door reading into the number every weatherization job turns on.
Example
You enter
- Blower-door reading CFM50 (cfm) 960
- Conditioned volume (ft³) 12800
- LBL N-factor (ACH50 -> natural) 17
- Target ACH50 (IECC limit) 3
You get
- ACH50 4.50
- Natural air change 0.265 ACH
- Natural infiltration 56.5 cfm
Checked against published worked examples from: Fine Homebuilding / GreenBuildingAdvisor (Randy Williams); Building Performance Association (Randy Williams).
Details, formula, and sources
ScopeACH50 = CFM50 x 60 / conditioned volume, checked against the IECC 2024 R402.5.1.3 limit (4.0 ACH50 in climate zones 0-2, 3.0 in 3-5, 2.5 in 6-8; IECC 2021 allowed 5.0 and 3.0), plus the year-round natural infiltration (the LBL divide-by-N rule, in air changes and cfm) that the ASHRAE 62.2 ventilation sizing and the infiltration load both consume. ASTM E779 / E1827 are the test methods; the N-factor varies with climate, height, and shielding. A field normalization, not a rater's signed test report.
Formula or tableach50 = cfm50 x 60 / volume_ft3; verdict = ach50 <= target_ach50 (PASS) else FAIL; ach_nat = ach50 / n_factor; cfm_nat = ach_nat x volume_ft3 / 60.
Source and editionThe ACH50 normalization (CFM50 x 60 / conditioned volume) and the LBL natural-infiltration divide-by-N rule, by name; IECC 2024 R402.5.1.3 [R402.4.1.3 in IECC 2021] sets the prescriptive limit at 4.0 ACH50 in climate zones 0-2, 3.0 in zones 3-5 and 2.5 in zones 6-8 (2021: 5.0 in zones 0-2, 3.0 in 3-8), and the performance paths at 4.0 ACH50 or 0.22 cfm/ft^2 of envelope (2021 R402.4.1.2: 5.0 or 0.28); ASTM E779 / E1827 are the blower-door test methods.
Free accessThe air-leakage limit is in the published IECC 2024 R402.5.1.3 (R402.4.1.2-R402.4.1.3 in 2021); the normalization and infiltration relations are public building-science arithmetic. The blower-door test methods are ASTM E779 / E1827.
What governsEstimate. AHJ and licensed professional govern.
API field namescfm50, volume_ft3, n_factor, target_ach50, ach50, ach_nat, cfm_nat
- ACH50 ach50 = CFM50 x 60 / conditioned volume normalizes the gauge reading to air changes per hour at 50 Pafirst principles
- Code check verdict compares ACH50 to the editable target (IECC 2024 R402.5.1.3: 4.0 ACH50 in CZ 0-2, 3.0 in CZ 3-5, 2.5 in CZ 6-8; 2021 R402.4.1.3: 5.0 and 3.0)IECC 2024 R402.5.1.3
- Natural infiltration ach_nat = ach50 / N (LBL N-factor, default 17); cfm_nat = ach_nat x volume / 60 feeds the 62.2 ventilation and the infiltration loadLBL / ASHRAE Fundamentals