Two pressure regimes, one verdict, how raters actually score the test.
Most duct leakage failures do not show up at rough-in. They show up at final, when drywall is up, when the HVAC startup is scheduled, and when the HERS Rater shows up with a Duct Blaster and a clipboard. By that point, sealing a boot or a chase costs roughly three to five times what it would have cost before the drywall closed.
The 2024 International Energy Conservation Code sets the binding ceiling at 4 CFM25 per 100 ft² of conditioned floor area for ducts outside the envelope, per R403.3.5 and R402.4.2. ENERGY STAR Multifamily New Construction is stricter at two stages: total leakage at rough-in must be ≤ 6 CFM25 per 100 ft², and post-construction leakage to outside must be ≤ 8 CFM25 per 100 ft². NGBS-2023 §704.1 references the same test method, ANSI/RESNET/ICC 850. None of these numbers are "target" values, they are pass/fail thresholds enforced by the rater, not negotiated with the HVAC contractor.
The builder error we see most often: assuming CFM50 readings can be directly compared to CFM25 limits. They cannot. CFM25 is roughly 1.5 to 2.0× higher than CFM50 on a typical residential system, because the lower test pressure opens fewer small leaks. A system that reads 4 CFM25 might read 2 to 2.5 CFM50, passing either way, but the conversions are not linear, and the test pressure itself must be specified in the rater's report.
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The CFM25 vs CFM50 distinction matters because the ENERGY STAR rough-in threshold is stated at CFM25, while many legacy contractor reports quote CFM50 because older Duct Blaster firmware defaulted to that pressure. A rater who sees a CFM50 number against a CFM25 limit has to reject the report or re-test.
| Test stage | Pressure | ENERGY STAR MFNC v1.3 Rev05 limit | 2024 IECC R403.3.5 limit |
|---|---|---|---|
| Rough-in, total leakage | 25 Pa | ≤ 6 CFM25 / 100 ft² CFA | n/a (code only addresses post-construction leakage to outside) |
| Post-construction, leakage to outside | 25 Pa | ≤ 8 CFM25 / 100 ft² CFA | ≤ 4 CFM25 / 100 ft² CFA |
| Legacy CFM50 reading | 50 Pa | Not accepted at face value, must be re-tested at 25 Pa | Not accepted at face value |
The 2024 IECC R402.4.1.2 blower door threshold (≤ 3.0 ACH50 in CZ 3-8 per Table R402.4.1.1) is a separate test from the duct test, failing one does not excuse failing the other.
HECS sequences duct leakage testing the way a senior tester would: rough-in first, then a targeted mastic-and-foil sealing pass on boots, plenums, and trunk seams, then a post-construction retest at the same 25 Pa reference pressure. Our duct leakage testing service uses a calibrated Duct Blaster with a manometer that reads at both 25 Pa and 50 Pa, so the report can be filed against either legacy or current limits without re-testing.
For projects targeting ENERGY STAR MFNC v1.3 Rev05, we recommend a rough-in target of 4 CFM25 per 100 ft² CFA, not the 6 CFM25 ceiling, because the post-construction threshold of 8 CFM25 leaves no headroom for the mastic joints that age and the boot connections that get disturbed during drywall hanging. A rough-in reading at 3 CFM25 typically posts at 6 to 7 CFM25 after drywall closes, based on HECS project data.
For Louisville-area projects, the climate zone is CZ 4A per the DOE Building America climate zone map, which means the binding whole-building blower door limit is 3.0 ACH50 under the 2024 IECC, a number that gets misquoted as 5.0 in roughly half of contractor submittals we review.
HECS handles duct leakage testing and verification for projects in Kentucky, Indiana, Ohio, Tennessee, Illinois, and Missouri. Our HVAC testing and balancing service includes the duct leakage test as a standalone deliverable or bundled with blower door and HVAC functional testing. For a scope call: (859) 983-7382 or hecsusa.com/contact/.
Every HECS duct leakage report includes the test pressure in pascals, the CFM25 reading (not CFM50 unless specifically requested), the conditioned floor area, the calculated CFM25 per 100 ft², and the pass/fail determination against the controlling standard. For ENERGY STAR projects, we also note the rough-in vs post-construction stage and the applicable MFNC v1.3 Rev05 limit.
The report identifies the sealing defects found: typically boot-to-drywall gaps, unsealed trunk seams, and the always-present plenum-to-air-handler interface. We do not perform the mastic work ourselves, HECS is a testing and balancing firm, not a mechanical contractor, but the report lists the locations so the installing HVAC contractor can seal and we can re-test within the same site visit.
For multi-family projects using sampling per ANSI/RESNET/ICC 380, HECS applies the sample size table by building height and unit count and reports which units were in the sample and which were sampled at reduced frequency.
Does ENERGY STAR MFNC v1.3 Rev05 require CFM25 or CFM50? ENERGY STAR MFNC v1.3 Rev05 requires the test at 25 Pa and reports in CFM25. A CFM50 reading from an older Duct Blaster default does not satisfy the program requirement without a documented conversion or a re-test. HECS reports always state the test pressure to remove ambiguity.
Can a rough-in reading of 5 CFM25 per 100 ft² pass if the limit is 8 CFM25 post-construction? It passes the threshold on paper, but it leaves no headroom for leakage added by drywall hanging, boot disturbance, and mastic aging. HECS project data shows rough-in readings of 3 to 4 CFM25 typically post at 6 to 7 CFM25 after construction, inside the limit. Anything rougher than 5 CFM25 at rough-in is a flag to seal before drywall closes.
Is the 2024 IECC R403.3.5 limit the same as the ENERGY STAR post-construction limit? No. The 2024 IECC R403.3.5 limit is 4 CFM25 per 100 ft² of conditioned floor area, stricter than the ENERGY STAR 8 CFM25 post-construction ceiling. On a project subject to both, the IECC controls and the 4 CFM25 figure is the binding number.
HECS provides duct leakage testing, blower door testing, and HVAC testing and balancing for residential and multifamily projects across Kentucky, Indiana, Ohio, Tennessee, Illinois, and Missouri. Our reports are formatted to satisfy 2024 IECC R403.3.5, ENERGY STAR MFNC v1.3 Rev05, and NGBS-2023 §704.1 on the same site visit. To schedule a rough-in or post-construction test, call [(859) 983-7382](tel
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