Blower-Door Air-Tightness (ACH50, Natural Infiltration, Code Check)

ACH50 = CFM50 x 60 / conditioned volume, checked against the IECC R402.4.1.2 limit (<= 3 ACH50 in climate zones 3-8, <= 5 in zones 1-2), 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.

ach50 = 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.

The ACH50 normalization (CFM50 x 60 / conditioned volume) and the LBL natural-infiltration divide-by-N rule, by name; IECC R402.4.1.2 states the air-leakage limit in ACH50 (<= 3 in climate zones 3-8, <= 5 in zones 1-2); ASTM E779 / E1827 are the blower-door test methods.

The air-leakage limit is in the published IECC R402.4.1.2; the normalization and infiltration relations are public building-science arithmetic. The blower-door test methods are ASTM E779 / E1827.

Estimate. AHJ and licensed professional govern.

Field names used by the API: cfm50, 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 R402.4.1.2: <= 3 ACH50 in CZ 3-8, <= 5 in CZ 1-2)IECC R402.4.1.2
• 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