Air sealing, duct leakage, and sampling rules that will decide whether your project closes.

The Problem: Why the 2026 Rule Changes the Math for Kentucky Multifamily

The DOE multifamily final rule effective in 2026 tightens the documentation chain for any project claiming federal compliance. Kentucky sits in climate zones 4A and 5A on the 2024 IECC Climate Zone Map (DOE Building America), so the binding air-leakage limit per 2024 IECC Table R402.4.1.1 is 3.0 ACH50 — not 5.0. Builders accustomed to the older 2018 mapping routinely oversize their envelope target by two full ACH points, then fail at the blower-door test. The 2024 IECC reversed the 2018 threshold: zones 3 through 8 all share the 3.0 cap. Louisville (Jefferson County) is 4A. Northeastern Kentucky counties including Boyd, Carter, Greenup, and Rowan are 5A. The same number applies across both.

The second failure pattern HECS sees on multifamily sites is conflating two different leakage tests. 2024 IECC R402.4.1.2 governs building envelope air leakage via blower door, run per ASTM E779. 2024 IECC R403.3.5 governs duct leakage to outside for ducts in unconditioned space, with a verified limit of 4 CFM25 per 100 ft² of conditioned floor area tested per ASTM E1554 or RESNET/ICC Standard 380. Code officials in Kentucky increasingly require both reports, and many builders submit only one.

What the Codes Actually Require

The compliance pathway determines which sections apply. Builders using the prescriptive path under 2024 IECC must meet R402.4.1.1 (visible air-barrier inspection), R402.4.1.2 (blower-door test at or below the Table R402.4.1.1 limit), and R403.3.5 (duct tightness to 4 CFM25 per 100 ft²). Multifamily projects may use the sampling protocol in Table R405.4.2(1) for blower door — sample sizes vary by building height and unit count. The Total UA alternative under R402.1.2 can bypass prescriptive R-value tables when the calculated envelope UA is documented, but does not waive the duct tightness test.

Projects pursuing ENERGY STAR Multifamily New Construction v1.3 (Rev05) face tighter duct numbers: total leakage ≤ 6 CFM25 per 100 ft² at rough-in, ≤ 8 CFM25 post-construction. Common areas must meet ASHRAE 62.1-2019, and the HVAC Functional Testing Checklist is required on every participating unit. Projects pursuing NGBS 2023 follow §704.1 for duct sealing tested per ANSI/RESNET/ICC 850, and §702.1.1.1 governs verifier credentials — Bronze and Silver tiers accept NGBS Green Certified Verifier; Gold and Emerald pathways typically require RESNET HERS credentials.

Working with HECS

HECS handles blower door, duct leakage, and multifamily sampling per 2024 IECC R405 for projects in Kentucky, North Carolina, Alabama, Georgia, West Virginia, Kansas, and Missouri. For a scope call: (859) 983-7382 or hecsusa.com/contact/. See the full services catalog at hecsusa.com/services/.

The HECS Approach: Sampling Strategy and Field Sequencing

HECS sequences the verification work around rough-in, insulation, and final. Rough-in is when duct leakage is cheapest to fix — a 4 CFM25 per 100 ft² failure traced to a single boot takes minutes; the same boot behind drywall costs a callback. For multifamily projects electing the sampling path, HECS builds the sample frame per 2024 IECC Table R405.4.2(1) and ANSI/RESNET/ICC 380, then runs the blower-door and duct tests in the same visit to compress the schedule.

Where builders want both ENERGY STAR and the IRC §45L credit, the documentation chain has to align. Per the IRS §45L page, qualifying homes must be acquired on or before 2026-06-30 — the One Big Beautiful Bill accelerated the prior 2032 sunset. Certification must be in place at closing. HECS coordinates the HERS rating, the ENERGY STAR checklist, and the blower-door/duct test reports so the verifier signatures land in a single package rather than three disconnected PDFs. For projects pursuing federal tax credit stacking, see HECS FORTIFIED Roof certification for the documentation order HECS recommends.

What HECS Verifies on Site

The deliverable is a report package a code official can read without phone calls. HECS provides:

• Blower-door test results per ASTM E779, ACH50 calculated, with sample identification for multifamily.
• Duct leakage to outside per ASTM E1554 or RESNET/ICC 380, expressed in CFM25 per 100 ft² of conditioned floor area.
• Visible air-barrier inspection notes tied to R402.4.1.1 — every penetration flagged by location.
• Photo log of recessed luminaires sealed to R402.4.5.
• NCI-certified technician signature on the TAB and airflow readings, with the NCI Commercial Air Balancer credential documented.
• For ENERGY STAR and NGBS projects, the Functional Testing Checklist and the verifier credentials under NGBS 2023 §702.1.1.1.

When the ASHRAE 62.2-2022 whole-building ventilation rate is in scope, HECS verifies the Q_total equation inputs and the local exhaust run time so the ventilation credit does not get pulled at final.

Common Pitfall

The most expensive mistake HECS sees: builders sample the blower door and pass, then fail the duct test because the rough-in sealing was never verified. The two tests are independent and both required by R402.4.1.2 and R403.3.5. One passing test does not exempt the other.

Frequently Asked Questions

Does the 2024 IECC apply to my Kentucky multifamily project in 2026? Adoption depends on the state and local jurisdiction. Kentucky has not yet mandated the 2024 IECC statewide, but Louisville Metro and several Kentucky counties enforce it through local ordinance. HECS recommends checking with the local building department before assuming the 2018 cycle still governs. For projects also pursuing ENERGY STAR, the MFNC v1.3 (Rev05) reference standards apply regardless of state code adoption.

Can HECS sample a 40-unit building for the blower-door test under the 2024 IECC? Yes, per the Table R405.4.2(1) sampling protocol and ANSI/RESNET/ICC 380. Sample size depends on building height and unit count. HECS builds the sample frame, runs the tests, and documents the failed units separately so the code official sees a clean compliance record. A failed sampled unit triggers re-test on additional units per the same protocol.

What is the binding ACH50 limit for a Louisville project? 3.0 ACH50. Louisville (Jefferson County) is climate zone 4A per the DOE Building America climate zone map, and 2024 IECC Table R402.4.1.1 sets the limit at 3.0 ACH50 for all of zones 3 through 8. The 5.0 cap applies only to zones 0 through 2 — not Kentucky.

How to Engage HECS

HECS supports code-compliance verification — blower door, duct leakage, ENERGY STAR, NGBS, EarthCraft, and FORTIFIED — for multifamily and single-family projects across Kentucky, North Carolina, Alabama, Georgia, West Virginia, Kansas, and Missouri. Engagements typically begin with a scope call, then a site walk and sampling plan, then field testing at rough-in and final. Reports are issued as PDF packages suitable for code-official submittal, ENERGY STAR registration, and HERS Provider upload. HECS also serves Louisville KY, Lexington KY, Cincinnati OH, Indianapolis IN, Nashville TN, St. Louis MO, Chicago IL, Evansville IN, and Columbus OH.