Residential Duct Leakage CFM25 (IECC R403.3.5)
Whether a residential duct system passes the energy-code tightness test: the measured total leakage at 25 Pa (from a duct blaster) normalized to the conditioned floor area, CFM25 per 100 ft^2 = leakage / area x 100, against the IECC limit (4 total / post-construction, 3 for a rough-in without the air handler). 80 CFM25 on 2,000 ft^2 gives exactly 4.0 -- a pass at the limit; a tighter 60 CFM25 gives 3.0, a leakier 100 gives 5.0 (fail). Distinct from the SMACNA leakage-class test for commercial duct. A field aid; the adopted energy code, the required test type, and the rater govern.
Formula and source
normalized = leakage_cfm25 / cfa_ft2 x 100; passes = normalized <= limit (default 4 CFM25 per 100 ft^2).
IECC (International Energy Conservation Code) §R403.3.5 (duct testing) and RESNET / ANSI 380 duct-leakage test methods, by name.
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