A field reference for code officials, custom builders, and developers navigating the 2024 IECC Residential air-sealing and duct leakage requirements in Climate Zone 4.

What the 2024 IECC Residential Actually Requires in Climate Zone 4

The 2024 International Energy Conservation Code Residential provisions tightened air-leakage and duct-leakage thresholds relative to the 2021 cycle, and HECS verifies these on every new-construction project in Kentucky, southern Indiana, and southwest Ohio. The two sections that drive blower door and duct leakage scope on a custom home are 2024 IRC R402.4.1.2 and R403.3.3 — they are independent tests, conducted at different project phases, with different allowable limits.

Per R402.4.1.2, the building thermal envelope must be tested with a blower door and verified at an ACH50 rate not exceeding the value in IRC Table R402.4.1.2. For Climate Zone 4, that table sets the ceiling at 5.0 ACH50. The test is conducted after the building is fully enclosed, with all penetrations sealed and the HVAC system balanced. HECS logs CFM50, fan pressure, building volume, and conditioned floor area so the code official receives traceable documentation.

Per R403.3.3, duct leakage to outside must be verified by either a rough-in test (with the air handler installed and all registers sealed) or a post-construction test, against the limits in IRC Table R403.3.3. For Climate Zone 4, the rough-in ceiling is typically 5.0 CFM25 per 100 sqft of conditioned floor area, and the post-construction ceiling is typically 8.0 CFM25 per 100 sqft. R403.3.5 also permits a third compliance path where total duct leakage, not just leakage to outside, is verified against a higher threshold. The IECC R403.3.5 option is rarely the best economic choice for a custom home; HECS generally recommends the leakage-to-outside test for tighter, simpler documentation.

How the IECC Thresholds Compare to ENERGY STAR, NGBS, and FORTIFIED

Code is the floor, not the ceiling. Most certified programs run tighter than the 2024 IECC, and builders pursuing certification should design and test to the strictest program they intend to claim. HECS verifies all four programs in-house.

Verify all program thresholds against the current program document before bidding. Program tiers and option menus change, and HECS project data should not be substituted for a current program read.

Why FORTIFIED Roof Projects Cannot Rely on Old ENERGY STAR Versions

The §ENERGY STAR v3.0/v3.1 under the Inflation Reduction Act permits builders and developers to claim up to $5,000 per qualified new energy-efficient home, with the eligible basis tied to certification under a recognized program. For single-family, the eligible baseline since 2023 has been ENERGY STAR SFNH v3.2 (the v3.1 reference window closed for new construction in 2023). Multifamily follows the ENERGY STAR MFNC v3.2 program requirements.

This version detail matters because HECS regularly sees builders in Kentucky submit 45L packages tied to SFNH v3.1 checklists. IRS examiners and third-party QA reviewers are flagging v3.1 references as obsolete, and credit claims are being kicked back. The 45L submission requires a completed checklist dated to the program version in effect on the certificate date, paired with a RESNET HERS rating, and HECS delivers both through our FORTIFIED Roof Evaluation service.

Working with HECS: HECS handles blower door testing, duct leakage testing, and 45L certification for projects across Kentucky, North Carolina, Alabama, Georgia, West Virginia, Kansas, and Missouri. We bundle the ENERGY STAR SFNH v3.2 or MFNC v3.2 inspection, the HERS rating through a RESNET-certified Rater, and the certified verifier signature into one submittal package. For a scope call: (859) 983-7382 or hecsusa.com/contact/.

Field Pitfalls HECS Sees on Kentucky and Ohio Projects

A few items consistently drive failed first tests in this region, and builders can avoid them with sequencing discipline:

• Blower door tests conducted before the attic hatch is fully weatherstripped. The IECC envelope includes the attic access, and an unsealed hatch alone can add 0.5-1.0 ACH50.
• Duct leakage rough-in tests run with the air handler energized. The air handler must be off and all supply and return registers sealed at the grille, not just at the boot.
• Recessed lights in the envelope ceiling specified as ICAT but shipped as IC. The non-airtight canisters leak directly into the attic and rarely close the gap on a 5.0 ACH50 ceiling.
• Garage-to-house walls not detailed as part of the thermal envelope. Code officials increasingly call this out under R402.4.5.
• Missing third-party test documentation on the certificate of occupancy. HECS provides a sealed, code-official-ready report that satisfies RESNET ANSI 380 documentation requirements.

For Kentucky and Ohio projects, HECS also coordinates with the local code official on scheduling and witnessing, since some jurisdictions in the region require the test to be observed in person.

Frequently Asked Questions

Does 2024 IRC R402.4.1.2 apply to additions, or only new construction?

R402.4.1.2 in the 2024 IECC Residential applies to dwelling units where the building thermal envelope is being verified for code compliance. Additions under 50% of the existing conditioned floor area typically follow the prescriptive path without mandatory testing, but local amendments can override. Verify with the local jurisdiction before scheduling HECS.

Can a builder claim 45L using an ENERGY STAR SFNH v3.1 certificate?

Generally no, for new construction certificates dated after January 1, 2023. The current eligible baseline for single-family 45L claims is ENERGY STAR SFNH v3.2. HECS flags obsolete version references in 45L packets before they reach a reviewer. Refer to the current §45L instructions for the applicable version window.

Is blower door testing required under FORTIFIED Home 2025?

FORTIFIED Home 2025 requires envelope air-leakage testing for all three roof designations (Hurricane, High Wind, Severe Wind), with the specific ACH50 ceiling set by the program path. Builders should request the current FORTIFIED evaluator checklist from HECS at scoping, since thresholds are program-path dependent. Visit our FORTIFIED evaluation services page for path details.

How to Engage HECS

HECS delivers blower door testing, duct leakage to outside testing, RESNET HERS ratings, ENERGY STAR v3.2 inspections, NGBS-2023 verification, FORTIFIED Home evaluation, and 45L certification across Kentucky, North Carolina, Alabama, Georgia, West Virginia, Kansas, and Missouri. A typical 45L or ENERGY STAR engagement begins with a scoping call where we confirm program version, climate zone, and test sequencing, followed by a single site visit and a sealed report package within five business days. Reach us at (859) 983-7382 or [email protected], or submit a scope through hecsusa.com/contact/. A full list of services is available at hecsusa.com/services/. HECS serves Louisville KY, Lexington KY, Cincinnati OH, Indianapolis IN, Nashville TN, and Evansville IN as primary city clusters for envelope and 45L work.