The 2024 IECC, ENERGY STAR MFNC v1.3 (Rev05), and NGBS-2023 collide in mid-2026 — here's what documentation your project actually needs.
The DOE's 2026 final rule locks multifamily energy code compliance into the 2024 IECC residential provisions for any project seeking federal funding or tax-credit adjacency. For Kentucky builders, that means Climate Zone 4A (Louisville, Lexington, most of the state) and CZ 5A in the northeastern counties carry a binding 3.0 ACH50 envelope limit — not 5.0. HECS field data shows roughly 3-5 ACH50 is the realistic construction range before diagnostic sealing, and most projects fail first-pass blower door testing on fireplace/chase penetrations and attic hatch gaskets.
The documentation gap is worse than the air leakage gap. Code officials enforcing 2024 IECC R402.4.1.2 want a test report tied to ASTM E779, the tester credentials, and the unit-level result — not a project summary. Multi-family sampling per Table R405.4.2(1) is permitted, but the sample size scales with building height and unit count, and undersampling is the single most common rejection trigger HECS sees on third-party plan review.
Ducts in unconditioned space trigger a separate verification under 2024 IECC R402.4.2 / R403.3.5 — verified total leakage to outside ≤ 4 CFM25 per 100 ft² of conditioned floor area. That number is per 100 ft², not per unit, and the denominator trips up contractors who test before drywall without conditioned-area calculations finalized.
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Call (859) 983-7382 Get a QuoteThe 2024 IECC residential envelope applies in full to dwelling units in multifamily buildings three stories or less above grade plane. Above that height, 2024 IECC commercial provisions govern with ASHRAE 90.1-2019 as the parallel compliance target. Either way, the documentation chain runs through R402.4.1.2 (blower door), R402.4.2 (duct tightness), and R405 (Simulated Performance Alternative / ERI).
ENERGY STAR MFNC v1.3 (Rev05) — the National Program Requirements Version 1.3, January 2025 — layers on top of IECC. ERI target ≤ ASHRAE 90.1-2019 reference, common areas meeting ASHRAE 90.1-2019, and the HVAC Functional Testing Checklist Version 1.1/1.2/1.3 completed for every participating unit. Duct leakage for MFNC v1.3 Rev05 runs ≤ 6 CFM25 per 100 ft² conditioned area at rough-in and ≤ 8 CFM25 post-construction, verified via RESNET sampling for multi-family.
ASHRAE 62.2-2022 §4.1 drives ventilation. The whole-building equation Q_total = 0.03·A_floor + 7.5·(N_br+1) cfm sets the mechanical ventilation minimum. Section 4.1.4 (updated by the February 2024 addendum) allows MERV-rated filtration credits that reduce the required mechanical rate. Section 4.4 caps infiltration credit at the lesser of 2 ACH50 or 0.04·A_floor / 7.6 cfm — important in CZ 4A/5A Kentucky where builders chase the 3.0 ACH50 ceiling and inadvertently starve the §4.4 credit.
NGBS-2023 §702.1.1 mandates third-party verification at every tier — Bronze, Silver, Gold, Emerald. Verifier credentials under §702.1.1.1 include NGBS Green Certified Verifier, RESNET Certified HERS Rater, or BPI Building Analyst, with specific compliance pathways requiring HERS credentials regardless of the NGBS verifier on file.
HECS scopes every Kentucky, Indiana, or Ohio multifamily job in three phases: pre-drywall envelope and duct verification, final blower door and HVAC functional testing, and certification submission. The pre-drywall visit catches framing and air-barrier issues while access remains — the visible air-barrier inspection required by R402.4.1.1, plus duct leakage at rough-in against the MFNC Rev05 ≤ 6 CFM25 per 100 ft² threshold.
Final testing uses calibrated NCI Commercial Air Balancer and NCI Residential Air Balancer procedures for airflow, plus RESNET/ICC Standard 380 sampling where the project elects it. For ENERGY STAR MFNC, HECS registers all HERS ratings with an ANSI-accredited HERS Provider and completes the HVAC Functional Testing Checklist per unit.
When NGBS-2023 certification is in scope, HECS verifies under its NGBS Green Certified Verifier credential, coordinating with the RESNET HERS Rater pathway when a specific compliance section requires it. FORTIFIED Home evaluations are handled under the IBHS FORTIFIED Home Evaluator credential, which becomes binding for any inland Kentucky high-wind project permitted on or after 2025-11-01.
HECS handles multifamily code-compliance documentation — blower door, duct leakage, HVAC functional testing, ENERGY STAR MFNC, NGBS-2023, and 45L certification — for projects across Kentucky, Indiana, Ohio, Tennessee, Illinois, and Missouri. For a scope call: (859) 983-7382 or hecs@hecsusa.com. See the full service list at hecsusa.com/services/.
Every HECS multifamily deliverable includes: the test method standard (ASTM E779 for blower door, ASTM E1554 or RESNET/ICC Standard 380 for duct leakage), the calibrated equipment serial numbers and last-calibration date, the unit identifier and conditioned floor area, the raw CFM/ACH50/CFM25 reading, the pass/fail threshold pulled directly from the controlling code section, and the tester credential reference.
The HERS rating side includes the ERI target, the rated ERI, the registered provider name, and confirmation of registration in the ANSI-accredited HERS Provider database. For NGBS-2023, the report cross-references the specific §702.1.1 verification tier and the verifier credential under §702.1.1.1. For ENERGY STAR MFNC v1.3 (Rev05), the Functional Testing Checklist Version 1.1/1.2/1.3 completion record attaches by unit.
A common pitfall HECS catches on plan review: builders confuse R402.4.2 (duct tightness) with R402.4.1.2 (blower door) on the code summary sheet. They are different sections, different tests, different pass criteria, and code officials reject packages that conflate them. Another recurring error is the climate zone mapping — projects in Louisville are quoted as CZ 4 with a 5 ACH50 limit, but 2024 IECC Table R402.4.1.1 reads CZ 3-8 = 3.0 ACH50. Louisville is CZ 4A and the binding limit is 3.0.
| Test | Code Section | Threshold | Method |
|---|---|---|---|
| Blower door (CZ 4A/5A KY/IN) | 2024 IECC R402.4.1.2 | ≤ 3.0 ACH50 | ASTM E779 |
| Duct leakage to outside | 2024 IECC R402.4.2 / R403.3.5 | ≤ 4 CFM25 per 100 ft² CFA | ASTM E1554 / RESNET 380 |
| ENERGY STAR MFNC rough-in duct | MFNC v1.3 (Rev05) | ≤ 6 CFM25 per 100 ft² CFA | RESNET sampling |
| Whole-building ventilation | ASHRAE 62.2-2022 §4.1 | Q_total = 0.03·A + 7.5·(N_br+1) cfm | Per §4.1 equation |
Does ENERGY STAR MFNC v1.3 (Rev05) replace 2024 IECC blower door testing, or do I need both?
ENERGY STAR MFNC v1.3 (Rev05) does not replace the 2024 IECC R402.4.1.2 blower door requirement — it layers on top. If your jurisdiction has adopted the 2024 IECC, you must meet the 3.0 ACH50 envelope limit (CZ 4A/5A Kentucky/Indiana) and submit an ASTM E779 test report. The MFNC checklist verifies additional items — HVAC functional testing, ERI target, common-area lighting — that IECC does not cover. HECS delivers both reports in a single site visit.
My project is sampling under Table R405.4.2(1). Does each unit still need duct leakage testing?
Duct tightness under 2024 IECC R402.4.2 / R403.3.5 is verified per the sampling protocol in Table R405.4.2(1) for blower door, but ENERGY STAR MFNC v1.3 (Rev05) and NGBS-2023 §704.1 apply their own sampling rules. In practice, HECS tests every unit at rough-in to lock in the ≤ 6 CFM25 per 100 ft² MFNC Rev05 threshold, then applies RESNET/ICC 380 sampling for post-construction confirmation. Code officials reject packages that sample duct leakage without a documented sampling protocol on file.
We're pursuing IRC §45L — does multifamily qualify, and what's the June 30, 2026 deadline?
Homes acquired AFTER 2026-06-30 do not qualify for the §45L credit. Public Law 119-21 (One Big Beautiful Bill, signed 2025-07-04) accelerated the §45L sunset to 2026-06-30. Multifamily can qualify when each unit meets ENERGY STAR or comparable certification thresholds. The credit formula pre-OBBB was $2,500 base plus a $1,000 prevailing-wage bonus — HECS recommends confirming the current-year basis with tax counsel before closing.
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