Kentucky has no statewide energy code, but a builder cannot ignore federal pressure. The DOE final rule for multifamily buildings (effective 2026) effectively forces adoption of the 2024 IECC Residential Provisions for new covered multifamily construction receiving federal financing or HUD oversight, regardless of state adoption. Louisville (Jefferson County) and Lexington (Fayette County) already enforce the 2024 IECC locally. Builders working across the I-64 and I-75 corridors now juggle three simultaneous compliance tracks: the 2024 IECC prescriptive or simulated performance path, ENERGY STAR Multifamily New Construction (MFNC) Version 1.3 (Rev05), January 2025 when the owner wants efficiency branding, and NGBS-2023 (ICC 700-2023) for projects pursuing third-party green certification. The most common documentation failure is conflating these tracks, a builder passes a 5 ACH50 blower door on a project assumed to be CZ 4A, but 2024 IECC Table R402.4.1.1 sets 3.0 ACH50 for CZ 3-8, so every Kentucky and Indiana county HECS serves is in the 3.0 band.

Builders also misread the sampling rules. Multi-family projects can use 2024 IECC Table R405.4.2(1) sampling protocols for blower door and duct leakage, but sampling only applies when the same builder repeats a tested design across multiple units, and the verifier must register each sampled result with the rating provider.

The Code Path: 2024 IECC, ASHRAE 62.2-2022, and MFNC Functional Testing

For envelope airtightness, the binding Kentucky and Indiana limit is 3.0 ACH50 per 2024 IECC Table R402.4.1.1. Testing follows ASTM E779, with the building thermal envelope verified by visual air-barrier inspection under 2024 IECC R402.4.1.1. Ducts outside the conditioned space must show total leakage to outside ≤ 4 CFM25 per 100 ft² of conditioned floor area under 2024 IECC R403.3.5, tested per RESNET Standard 380 or ASTM E1554. If the project is also pursuing ENERGY STAR MFNC v1.3 (Rev05), the rough-in duct leakage limit tightens to 6 CFM25 per 100 ft² and post-construction to 8 CFM25 per 100 ft², both stricter than the IECC minimum when measured at the same reference.

Ventilation falls under ASHRAE 62.2-2022 §4.1 for whole-building mechanical ventilation: Q_total = 0.03·A_floor + 7.5·(N_br+1) cfm, where A_floor is conditioned floor area in ft² and N_br is bedroom count. Local exhaust follows §4.3, kitchens ≥100 cfm intermittent (range hood) or ≥25 cfm continuous, bathrooms ≥50 cfm intermittent. The §4.4 infiltration credit is capped at the lesser of 2 ACH50 or 0.04·A_floor / 7.6 cfm, which means a tighter blower door envelope directly reduces required mechanical ventilation runtime.

For projects pursuing green certification, NGBS-2023 Chapter 7 duplicates the duct and envelope tests, and §903 references ASHRAE 62.2-2022 directly for indoor environmental quality. NGBS Bronze, Silver, Gold, and Emerald all require third-party verification under §702.1.1.

The HECS Recommendation: One Verifier, Three Passes

HECS recommends builders pick the strictest of the three pathways as the design target, then document against whichever program the owner is funding. In practice, that means designing to ENERGY STAR MFNC v1.3 (Rev05) duct leakage thresholds while testing to 2024 IECC R402.4.1.2 ASTM E779 protocols. The verifier should be registered with an ANSI-accredited HERS Provider so the sampling per ANSI/RESNET/ICC 301-2022 carries weight on the project's final registry record.

Working with HECS

HECS handles blower door, duct leakage, and ventilation verification for multifamily projects across Kentucky, Indiana, Ohio, Tennessee, Illinois, and Missouri. For a scope call on a 2026-code multifamily project: (859) 983-7382 or hecsusa.com/contact/.

What HECS Verifies on a 2026 Multifamily Project

HECS delivers three reports per project, each tied to a specific code reference. The blower door report cites 2024 IECC R402.4.1.2 and Table R402.4.1.1, lists tested ACH50 against the 3.0 CZ 4A/5A target, and includes the multi-family sampling declaration when applicable per Table R405.4.2(1). The duct leakage report references 2024 IECC R403.3.5 with CFM25 per 100 ft² of conditioned area, and a parallel line item showing the tighter ENERGY STAR MFNC v1.3 (Rev05) rough-in and post-construction thresholds. The ventilation report documents whole-building CFM per the ASHRAE 62.2-2022 §4.1 equation and notes the §4.4 infiltration credit when the ACH50 is below 2.0. Each report is delivered as a PDF with serial-numbered test points, calibration logs, and the verifier's NCI credential, HECS is NCI-certified for residential air balancing.

A common pitfall HECS catches on Kentucky multifamily projects: a builder routes the HVAC return through a corridor soffit, then tests the corridor as conditioned space. The IECC treats corridors on a multi-family project as conditioned only when the supply and return are inside the envelope; a soffit return in an unconditioned attic or garage pushes duct leakage testing to the stricter rough-in limit and often fails the 2024 IECC R403.3.5 threshold on first test.

Frequently Asked Questions

Does the DOE 2026 final rule apply to a four-plex in Louisville? Yes. The rule covers any new covered multifamily building four stories or fewer above grade, which includes most Kentucky and Indiana garden-style apartments. Builders in Jefferson County were already under the 2024 IECC locally; the federal rule extends the same documentation expectations to projects in non-adopting counties when federal financing is in the chain.

What blower door target should a Kentucky builder design to under the 2024 IECC and ENERGY STAR MFNC v1.3 (Rev05)? Design to 3.0 ACH50 per 2024 IECC Table R402.4.1.1, then target roughly 2.0-2.5 ACH50 to preserve the [ASHRAE 62.2-2022 §4.4](https://www.ashrae.org/technical-resources/bookstore/standards-62-