Blower-Door Air-Tightness (ACH50, Natural Infiltration, Code Check)
Turns a blower-door reading into the number every weatherization job turns on: 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.
Formula and source
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.
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