The 2024 International Energy Conservation Code tightens residential air-barrier thresholds and reshapes the test-and-balance handoff on every project.
The 2024 IECC, as published by the International Code Council, replaces the 5 ACH50 ceiling that governed most Climate Zone 4 and 5 jurisdictions under the 2015 cycle. Under Section R402.4.1.2, the residential blower door test now requires:
Builders carrying forward mental models from 2015 IECC projects will over-permit leakage on every new submittal. Plan reviewers in Kentucky, Ohio, and Indiana are flagging stale compliance notes on the 2021 and 2024 cycles, and the documentation gap costs inspection slots.
ASHRAE 90.1-2019 follows a parallel logic but uses CFM per square feet of envelope area, not ACH50, so designers should not interchange the metrics without re-calculating.
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Call (859) 983-7382 Get a QuoteSection R402.4.1.2(1) allows builders to demonstrate compliance through a CFM50 at 50 pascals value scaled to dwelling volume, but the substitution depends on the building's conditioned floor area, number of stories, and ceiling height. The table is not linear; a 2,400 square foot single-story home and a 2,400 square foot two-story home land in different rows.
HECS measures the conditioned volume on-site during the blower door test, then references Table R402.4.1.2(1) directly rather than relying on a calculator. On recent projects, the CFM50 equivalent has run 800 to 1,100 CFM50 for typical three-bedroom homes in the Ohio Valley — well above the rough "1 CFM per square foot" rule of thumb many trades still apply. The CFM25 path referenced in some software outputs is the RESNET MINHERS derivative used for HERS ratings, not an IECC compliance path.
Practical steps for the field:
The table below is a snapshot based on HECS project work and public ICC tracking. AHJs update adoption mid-cycle, so confirm with the local building department before scheduling a test. The 2024 IRC Appendix AE and Section R402.4.1.2 govern the blower door requirement regardless of which print cycle the state has adopted.
| Jurisdiction | Effective Cycle (Early 2026) | Notes |
|---|---|---|
| Kentucky | 2018 IECC residential | Several jurisdictions enforcing 2021 locally — verify |
| Indiana | 2018 IECC residential | Statewide baseline; overlay ordinances vary |
| Ohio | 2019 residential code (RCO 5323) | Based on 2018 IECC with amendments |
| Tennessee | 2018 IECC residential | Local amendments active in Davidson, Knox |
| Illinois | 2021 IECC statewide | Chicago and Energy Code Appendix CA differ |
| Missouri | 2018 IECC residential | St. Louis and KC may enforce tighter thresholds |
Where local amendments exist, they typically tighten — never loosen — the air leakage ceiling. When in doubt, assume the more stringent value.
IECC Section R403.3.3 requires that ducts be sealed and verified to 4.0 CFM25 per 100 square feet of conditioned floor area when the air handler is inside the building envelope, or 8.0 CFM25 per 100 square feet when the air handler is outside the envelope. The test must occur at rough-in or post-construction; rough-in gives the builder a chance to seal accessible leaks before drywall closes them.
HECS runs duct leakage to total and to outside standards in a single appointment, using a Duct Blaster under CALIBER/PG&E protocol compatible with RESNET MINHERS Chapter 8. Results are reported per the ENERGY STAR v3.1 HVAC Checklist when the project is targeting that label, and the same data feeds a 45L Tax Credit submission under IRC §45L for eligible energy-efficient homes placed in service through December 31, 2032.
The blower door and duct leakage tests share the same calibration set and the same pressure conditions (25 Pa for ducts, 50 Pa for the envelope). Scheduling them together eliminates a second mobilization and shortens the path to certificate of occupancy.
NFPA 54 fuel-gas inspection now cross-references envelope tightness on many jurisdictions' checklists, so a failed blower door can stall the gas tag as well.
What is the actual 2024 IECC blower door requirement for Climate Zone 4 and 5 homes? Section R402.4.1.2 sets 3.0 ACH50 for Climate Zone 4 and 2.5 ACH50 for Climate Zone 5. The CFM50 equivalent from Table R402.4.1.2(1) is a permitted alternative, not a replacement, and the CFM50 value is a function of conditioned volume rather than a free-floating number. HECS reports both values on every HECS blower door test report.
How does duct leakage to outside interact with the blower door result? Section R403.3.3 requires 4.0 CFM25 per 100 square feet of conditioned floor area when the air handler is inside the envelope, and 8.0 CFM25 per 100 when it is outside. HECS performs both the blower door and duct leakage tests on the same visit, and the deliverable is a single PDF report that satisfies code officials and ENERGY STAR v3.1 reviewers.
Can a builder use 2021 IECC documentation on a project permitted under 2018 IECC? No. The code in effect at permit issuance governs, and the ACH50 ceiling under 2018 IECC is 5.0 in Climate Zones 4 and 5. HECS confirms the effective cycle with the local AHJ before testing, which prevents the most common cause of failed final inspections on air-tightness items.
HECS is an NCI-certified testing and balancing firm headquartered in Louisville, Kentucky, performing blower door, duct leakage, HVAC test and balance, radon, and decibel testing across Kentucky, Indiana, Ohio, Tennessee, Illinois, and Missouri. For builders, developers, and code officials, the engagement starts with a scope email to hecs@hecsusa.com or a call to (859) 983-7382. HECS schedules single-visit envelope and duct packages, issues same-day reports, and supports ENERGY STAR, NGBS-2023, EarthCraft, FORTIFIED Home 2025, and 45L Tax Credit certification paths. Service descriptions, sample reports, and the request form live at hecsusa.com/services/ and hecsusa.com/contact/.
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