The reversal builders keep missing, and why Kentucky and Indiana projects now hit the tighter ceiling.
Most of the ACH50 failures HECS encounters on residential jobs trace back to one assumption, that "Climate Zone 4 means 5 ACH50." That mapping belongs to the 2018 IECC and it is wrong for 2024. Builders, raters, and a few code officials still carry the older number in their heads and pass it down through plan notes and trade hand-offs. The result is a rough-in that targets the wrong ceiling, an air-sealing walkthrough that never gets prioritized, and a final blower-door test that surprises the owner with a fail.
The second problem is sequencing. Trades close up walls before the building envelope is verified, and the drywall stage is the wrong time to discover that the rim joist, the tub penetrations, and the can-light boots were never sealed. By that point the fix costs roughly five to ten times what it would have cost at rough-in, based on HECS project data across Louisville and Lexington builds.
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Call (859) 983-7382 Get a QuoteThe binding section for residential air tightness under the 2024 IECC is §R402.4.1.2, which references Table R402.4.1.1 for the verified rate. The numbers, pulled verbatim from the verified 2024 IECC mapping, are:
| Climate Zone | 2024 IECC ACH50 Maximum |
|---|---|
| CZ 0–2 | 5.0 ACH50 |
| CZ 3–8 | 3.0 ACH50 |
The reversal matters. Kentucky is almost entirely CZ 4A, with a CZ 5A strip running through the northeastern counties. Indiana is the same, mostly 4A with a CZ 5A northern tier. Both fall under the 3.0 ACH50 ceiling, not the 5.0 ceiling a 2018-trained builder expects. Louisville (Jefferson County) sits in 4A. So do Lexington, Cincinnati, Indianapolis, Nashville, Evansville, and Columbus, the bulk of HECS's daily project footprint. The binding limit on those jobs is 3.0 ACH50.
Testing must follow ASTM E779 or an equivalent protocol. Multifamily projects may use the sampling protocol in Table R405.4.2(1), which sets sample sizes by building height and unit count. That sampling option is a meaningful cost lever on garden-style and mid-rise work, but it only applies when the ERI compliance path under §R405 is elected, not under the prescriptive path.
A final point that gets missed: §R402.4.1.1 still requires a visible air-barrier inspection regardless of which compliance path the project picks. The visual inspection and the blower-door test are two separate obligations, passing one does not waive the other.
HECS runs blower-door testing for builders, developers, and code officials across Kentucky, Indiana, Ohio, Tennessee, Illinois, and Missouri. Our NCI-certified technicians mobilize with calibrated equipment and deliver same-day pass-or-fail findings, with retest scheduling built into the rough-in and final stage. For a scope call: (859) 983-7382 or hecsusa.com/contact/.
HECS treats air tightness as a two-stage deliverable, not a single test at the end. Stage one is a rough-in check before drywall closes the building. Stage two is the final test at certificate-of-occupancy. Splitting the work this way turns the blower door from a pass/fail gamble into a managed quality-control checkpoint.
At rough-in, HECS pressurizes the building, walks the envelope with the lead framer or builder rep, and flags every leak we can hear or feel, top plates, plumbing penetrations, the band joist line, the attic hatch, electrical panel penetrations, and recessed-lighting boots. The 2024 IECC §R402.4.5 recessed-lighting requirement is a common failure point on production housing because the wrong ICAT-rated fixture is installed or the gasket is left off. Stage one typically costs a fraction of the stage-two retest and prevents the surprise.
The stage-two test is performed per ASTM E779 with a calibrated blower-door fan. We record CFM50, calculate ACH50 against conditioned volume, and report the verified rate against Table R402.4.1.1. For multifamily projects, HECS coordinates with the rater of record to align sample selection with the Table R405.4.2(1) protocol so the result registers cleanly with the program provider.
When projects target a stricter program, NGBS Green, ENERGY STAR MFNC, or FORTIFIED, the ACH50 ceiling drops further and the test becomes a verification gate, not just a code check. HECS reads each program's standard into the field workflow before mobilization so the crew arrives knowing which ceiling applies.
Every HECS blower-door report includes the building address, conditioned floor area, conditioned volume, test apparatus calibration, indoor-outdoor temperature at test, CFM50, ACH50, and the applicable compliance threshold. We flag which standard governs, 2024 IECC, ENERGY STAR MFNC v1.3 (Rev05), NGBS-2023, or 2025 FORTIFIED Home, and we call out the section the result is being measured against.
For ENERGY STAR MFNC v1.3 (Rev05) National projects, the blower-door result is one piece of a larger package that includes HVAC Functional Testing Checklist verification, ERI target documentation, and common-area compliance per ASHRAE 90.1-2019. HECS runs the air-side piece and coordinates handoffs so the rater of record receives a clean data file.
For NGBS-2023 projects, §702.1.1.1 requires a verifier with credentials accepted by Home Innovation Research Labs. HECS holds the NGBS Green Certified Verifier credential and can sign off on the air-tightness verification directly. For projects targeting the 45L tax credit, the blower-door result feeds the energy-model documentation package required by §45L, note the 2026-06-30 acquisition deadline under the post-OBBB amendment.
The deliverable is signed, dated, and stamped for code-official submittal. HECS reports do not require third-party review before they are filed.
Three operational shifts hit builders under the 2024 cycle. First, the threshold change from 5.0 to 3.0 ACH50 in CZ 3-8 forces tighter rough-in discipline. In HECS's experience, production builders who were passing at 4.5-5.0 ACH50 under the 2018 IECC now routinely test at 3.5-4.5 without intervention, which is a fail in CZ 4A. Reaching 3.0 reliably requires foaming the top plates, sealing the rim joist, gasketing the can lights, and taping the drywall air barrier at the rim, work that costs roughly 0.1-0.3% of the project value at rough-in and substantially more as a retrofit.
Second, §R402.4.2 and §R403.3.5 tighten duct leakage to 4 CFM25 per 100 ft² of conditioned floor area for ducts outside the conditioned space, tested per ASTM E1554 or RESNET/ICC Standard 380. That is the same number that has appeared in recent cycles, but enforcement has stiffened, HECS sees more code officials asking for the duct-leakage report at the same time as the blower-door report.
Third, the visual air-barrier inspection under §R402.4.1.1 is now checked alongside the test, not instead of it. The rough-in photo set has become a deliverable as much as the final test result.
Does the 2024 IECC let a builder skip the blower-door test if the project meets a total UA target? Not directly. The total UA alternative under §R402.1.2 can substitute for the prescriptive R-value table, and envelope performance may be documented to address §R402.4.1.2 in some pathways, but most jurisdictions still require a verified blower-door test or an ERI compliance path under §R405. Confirm with the local code official before waiving the test.
What ACH50 target should a builder in Louisville or Lexington design to? Design to 3.0 ACH50,
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