Air-tightness thresholds tightened, climate-zone mapping reversed, here's what changed for CZ 4A and 5A projects.
The single most common error HECS sees on residential plan review is a code citation copied from a 2018 IECC cheat sheet. Under the 2024 IECC, the air-leakage table was reversed. Per Table R402.4.1.1 of the 2024 IECC, the binding limit is 5.0 ACH50 in climate zones 0-2 and 3.0 ACH50 in climate zones 3-8. Louisville, Lexington, Indianapolis, Cincinnati, Nashville, and St. Louis all sit in CZ 4A, Chicago and northern Ohio border CZ 5A, so the operative ceiling on virtually every HECS project is 3.0 ACH50, not 5.0.
This is not a subtle distinction. A 2,400 ft² home at 3.0 ACH50 leaks roughly 1,440 CFM50; the same home at 5.0 leaks 2,400 CFM50. That delta is the difference between passing and failing a final inspection, and between meeting and missing the 45L tax-credit performance bar. Builders who quote "5 ACH50 in climate zone 4" to their code official are quoting dead text.
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Call (859) 983-7382 Get a QuoteThe 2024 IECC testing path is laid out in R402.4.1.2. Buildings must be tested per ASTM E779 or equivalent, with the verified leakage rate not exceeding the maximum in Table R402.4.1.1. Multi-family projects may use sampling per Table R405.4.2(1). The air-barrier inspection requirement lives in R402.4.1.1, a visual inspection, distinct from the blower-door test itself.
The alternative compliance paths matter for builders trying to avoid a blower-door test entirely. Under R402.1.2, the Total UA alternative lets a project bypass the prescriptive R-value table when documented envelope performance is met, but the air-leakage verification requirement of R402.4.1.2 still applies unless the energy rating index path of R405 is invoked. There is no "ignore the blower door" loophole in 2024 IECC for residential new construction.
HECS handles blower-door testing, duct leakage verification, and air-barrier inspection for projects across Kentucky, Indiana, Ohio, Tennessee, Illinois, and Missouri. Schedules tighten in spring, book the rough-in test the same week as the drywall close. For a scope call: (859) 983-7382 or hecsusa.com/contact/.
Code officials who accept a passing blower-door number as proof of duct tightness are missing the point. Per R402.4.2 and R403.3.5, ducts outside the conditioned space must be verified at 4 CFM25 per 100 ft² of conditioned floor area, tested per ASTM E1554 or RESNET/ICC Standard 380. ENERGY STAR Multifamily New Construction v1.3 Rev05 holds tighter, 6 CFM25 per 100 ft² at rough-in, 8 CFM25 post-construction (ENERGY STAR MFNC National Program Requirements v1.3 Rev05). NGBS-2023 §704.1 references the same ANSI/RESNET/ICC 850 test methodology (NGBS-2023).
These are two separate tests on two separate days. The blower-door test (CFM50, whole-building) is typically run after drywall and before insulation cover-up on the attic floor. The duct leakage test (CFM25, register-sealed) is run at rough-in before drywall and again at final if requested by the rater. Conflating them is the second most expensive mistake HECS encounters.
The HECS report is built around five data points, each tied to a code section so the code official can cross-reference without a phone call. The deliverable includes the ACH50 result with the ASTM E779 reference, climate-zone compliance per the 2024 IECC Climate Zone Map, the CFM25 duct leakage result with the RESNET/ICC 380 reference, the air-barrier inspection checklist signed at rough-in, and a sampling-protocol declaration when multi-family units are tested under ANSI/RESNET/ICC 301-2022.
ASHRAE 62.2-2022 §4.4 also matters here: the infiltration credit is capped at the lesser of 2 ACH50 or a calculated opening-area fraction (ASHRAE 62.2-2022). A home that tests at 2.5 ACH50 is fine for IECC compliance at 3.0 ACH50 in CZ 4A, but it does not earn the full ASHRAE 62.2 §4.4 credit for ventilation. That downstream effect shows up in HVAC sizing, not blower-door pass/fail.
Three variables drive the cost of a tight envelope. First, climate zone: at 3.0 ACH50 the cost of air-sealing detail work is roughly 20-30% higher than at 5.0 ACH50, based on HECS project data across 90+ builds. Second, building size: larger enclosures reach the CFM50 limit with less effort, so a 4,000 ft² custom home has more headroom than a 1,400 ft² cottage. Third, certification pathway: a project pursuing both ENERGY STAR MFNC and 45L has tighter thresholds, more sampling, and additional documentation, see the HECS blower-door service page for project-specific scope.
Does the 2024 IECC require 5 ACH50 in climate zone 4? No. Per Table R402.4.1.1 of the 2024 IECC, the limit is 3.0 ACH50 in climate zones 3-8, which includes all of Kentucky (CZ 4A and CZ 5A), most of Indiana, Ohio, Tennessee, Illinois, and Missouri. A "5 ACH50 in CZ 4" citation traces back to the 2018 IECC and is no longer compliant.
Can a builder skip the blower-door test under the Total UA alternative? Partially. Under R402.1.2 of the 2024 IECC, the Total UA alternative can substitute for prescriptive R-value and U-factor requirements, but the air-leakage verification of R402.4.1.2 still applies unless the project uses the simulated performance path under R405. In practice, every jurisdiction HECS works in still requires a blower-door number on the certificate.
Is the 45L tax credit still available past the IECC compliance date? No. Per IRC §45L, qualifying homes must be acquired (sale closing) on or before June 30, 2026, the OBBB amendment of July 4, 2025 accelerated the prior 2032 sunset. HECS provides 45L certification documentation; book early because the deadline is firm.
HECS delivers blower-door testing, duct leakage verification, and the air-barrier inspection checklist for residential and multifamily projects across Kentucky, Indiana, Ohio, Tennessee, Illinois, and Missouri. The deliverable pairs each test result with the 2024 IECC section reference so code officials can sign off without follow-up. To scope a project, call (859) 983-7382, email [email protected], or submit a request through hecsusa.com/contact/. Service descriptions and the full testing menu live at hecsusa.com/services/. HECS also serves Louisville KY, Lexington KY, Cincinnati OH, and Indianapolis IN directly, with mobile teams covering Nashville TN, St. Louis MO, Chicago IL, Evansville IN, and Columbus OH on a scheduled route.
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