Air-sealing the duct system is one test, two pressure regimes, and a code citation that builders routinely misread.
Duct leakage is reported two ways and the distinction matters. CFM25 measures total leakage at a 25 Pascal pressure difference across the duct system — the figure captured with a duct pressurization test at rough-in, before drywall closes the chassis. CFM50 captures leakage at 50 Pascals and is the pressure regime tied to building envelope blower-door testing per 2024 IECC R402.4.1.2. Mixing the two is the single most common error HECS finds on Kentucky and Indiana verification reports.
Builders routinely submit rough-in CFM50 numbers against rough-in CFM25 limits, or post-construction CFM25 numbers against envelope ACH50 thresholds. The pressure regimes are not interchangeable. Based on HECS project data, a duct system that measures 4.0 CFM25 per 100 ft² typically reads around 8-10 CFM50 per 100 ft² on the same envelope — roughly a 2:1 ratio on leaky systems and closer to 2.5:1 on tight ones. A pass at one pressure does not imply a pass at the other.
The code language is specific: the 2024 IECC ties duct tightness to a 25 Pa test (2024 IECC R402.4.2 and R403.3.5). The blower-door envelope test is a separate procedure with its own ACH50 limit. Conflating them voids the rating.
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Call (859) 983-7382 Get a QuoteThree programs drive most duct leakage verifications in the HECS service area: 2024 IECC, ENERGY STAR Multifamily New Construction, and NGBS 2023.
2024 IECC R403.3.5 caps verified duct leakage to outside at 4 CFM25 per 100 ft² of conditioned floor area when ducts run through unconditioned space. The test method references ASTM E1554 or ANSI/RESNET/ICC 380. For Kentucky and Indiana — climate zones 4A and 5A per the DOE Building America climate zone map — the envelope blower door is bound at 3.0 ACH50 per Table R402.4.1.1. Both tests are independent and both must pass.
ENERGY STAR MFNC v1.3 (Rev05) sets a two-stage threshold: total duct leakage ≤ 6 CFM25 per 100 ft² at rough-in, and ≤ 8 CFM25 post-construction. ENERGY STAR uses CFM25 only — never CFM50 — because the program aligns with RESNET sampling rather than envelope blower-door regimes. RESNET's ANSI/RESNET/ICC 301-2022 governs how multi-family samples are drawn.
NGBS-2023 §704.1 requires ducts to be sealed and tested per ANSI/RESNET/ICC 850 or equivalent, with third-party verification per §702.1.1. Tier upgrades (Silver, Gold, Emerald) tighten the duct leakage target rather than the test method. NGBS does not publish a single numeric threshold in §704.1 — HECS verifies against the project-specified tier on the verifier agreement.
| Program | Test Pressure | Threshold | Test Method |
|---|---|---|---|
| 2024 IECC R403.3.5 | 25 Pa | ≤ 4 CFM25 / 100 ft² CFA | ASTM E1554 / RESNET 380 |
| ENERGY STAR MFNC v1.3 Rev05 | 25 Pa | ≤ 6 rough-in / ≤ 8 post | RESNET sampling |
| NGBS-2023 §704.1 | 25 Pa | Tier-dependent | RESNET 850 |
HECS performs duct pressurization with a calibrated NCI Residential Air Balancer protocol. At rough-in, we mask all supply and return registers with taped film, pressurize the duct system with a duct blaster to 25 Pa, and read total leakage from the fan ring. The reading is normalized to CFM25 per 100 ft² of conditioned floor area — that is the number that goes on the ENERGY STAR checklist and the NGBS verifier log.
For post-construction tests on conditioned-space ducts, the same procedure applies but the threshold tightens to 4 CFM25 / 100 ft² under 2024 IECC R403.3.5 if any portion of the duct runs through unconditioned space — attic, crawlspace, garage, or exterior wall cavity. HECS flags the routing on every report so the code official can match the test scope to the duct routing shown on the as-built.
When builders ask for CFM50 readings, we explain that CFM50 is the envelope blower-door regime and is not the right metric for ENERGY STAR or NGBS duct compliance. CFM50 is reported separately on the blower-door test, normalized to ACH50 for envelope compliance.
HECS handles duct leakage testing for projects across Kentucky, Indiana, Ohio, Tennessee, Illinois, and Missouri. For a scope call on a rough-in or post-construction test: (859) 983-7382 or hecsusa.com/contact/.
Every HECS duct leakage deliverable includes the test pressure (25 Pa), the measured total CFM25, the conditioned floor area, the normalized CFM25 per 100 ft², the pass/fail call against the governing program threshold, and a routing sketch showing which duct sections were inside versus outside the thermal envelope. Photographs of register masking and the manometer reading are appended for the rater file.
For ENERGY STAR MFNC projects, the report cross-references the HVAC Functional Testing Checklist sections that require a passing duct leakage entry. For NGBS, the report maps to the §702.1.1 verification log and notes the verifier credential on file. HECS does not certify units — that is the provider's role — but the test data HECS supplies is what the provider registers in the HERS registry.
A common pitfall HECS catches: mastic-only sealing at the trunk-to-boot joints. On Lexington and Louisville custom homes, we routinely see boot-to-drywall gaps that pass mastic but fail the CFM25 threshold because the boot collar itself leaks to the attic. The fix is a gasket ring plus mastic, not mastic alone. That single detail accounts for roughly a third of failed rough-in tests in our 2025 project sample.
Does a passing CFM50 blower-door test imply a passing CFM25 duct leakage test?
No. The two tests are run on different assemblies at different pressures. A 3.0 ACH50 envelope per 2024 IECC Table R402.4.1.1 says nothing about whether the ducts inside that envelope leak 4 or 14 CFM25 per 100 ft². HECS runs both tests independently and reports them on separate lines of the verification log.
What CFM25 threshold applies to a multifamily project pursuing ENERGY STAR MFNC v1.3 Rev05 and NGBS Silver simultaneously?
The tighter of the two governs. ENERGY STAR MFNC v1.3 Rev05 allows up to 6 CFM25 / 100 ft² at rough-in and 8 post-construction; NGBS Silver typically tightens to roughly 4-5 CFM25 / 100 ft² at rough-in. HECS scopes the test against the binding threshold before mobilization so the builder knows the target.
Can HECS sample duct leakage on a 24-unit garden-style project under RESNET 380?
Yes — ANSI/RESNET/ICC 380 sampling is the standard HECS uses for multifamily ENERGY STAR and NGBS projects, with sample sizes scaled to building height and unit count. Failed sample units are re-tested and the failure rate determines whether the entire building fails or whether corrective work plus re-sampling is permitted. HECS coordinates that protocol with the rater of record.
HECS provides duct leakage testing, blower door testing, and full ENERGY STAR, NGBS, and FORTIFIED verification for projects across Kentucky, Indiana, Ohio, Tennessee, Illinois, and Missouri. Services are scoped through hecsusa.com/services/ or by calling (859) 983-7382. Email scoping documents to [email protected] — the team responds same-day on rough-in scheduling. HECS regularly serves Louisville KY, Lexington KY, Cincinnati OH, Indianapolis IN, Evansville IN,
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