What ANSI/RESNET/ICC 301 updates mean for HERS ratings, 45L documentation, and multifamily compliance.

1. The problem: “RESNET 2026” is not a new HERS Index framework

The 2026 discussion is often described as a new RESNET HERS Index, but the current reference remains ANSI / RESNET / ICC 301-2022 with addenda through 2025. The HERS Index remains an inverse-rated score: lower scores indicate better modeled performance.

For multifamily projects, the practical issue is not a new scoring formula. It is whether the energy model, field results, rating documentation, and registry submission form a defensible compliance record.

Builders commonly make four documentation errors:

• They treat a modeled HERS score as a substitute for required field testing.
• They mix the RESNET rating process with the 45L eligibility process.
• They submit blower-door or duct-leakage results without a project-specific sampling basis.
• They overlook the applicable acquisition deadline for the federal tax credit.

The federal credit is particularly time-sensitive. Under IRC §45L, as amended by Public Law 119-21, homes acquired after June 30, 2026 do not qualify. The applicable 45L ceiling is $2,500 base plus a $1,000 prevailing-wage bonus, producing a $3,500 maximum per qualifying unit, subject to the current-year basis.

2. The code and standard: what changed and what remains controlling

The governing HERS calculation standard is ANSI / RESNET / ICC 301-2022 with addenda through 2025. Most changes described as “RESNET 2026” are addenda and implementation developments, not a complete replacement of the 301-2022 framework.

Two scoring references remain useful for interpreting project results:

• Net-Zero HERS equals 0.
• HERS 50 means 50% better than the 2006 IECC reference home.

Multifamily projects may use ANSI/RESNET/ICC 380 sampling protocols for energy ratings, with sample sizes determined by building height and unit count. Every HERS rating must also be registered with an ANSI-accredited HERS provider.

This distinction matters because a multifamily energy model can be internally consistent while the supporting envelope, duct, and ventilation documentation remains incomplete. A compliant HERS score does not independently establish every requirement of the 45L pathway.

The code and program records still supporting the project file include:

For Kentucky and Indiana projects, the climate-zone distinction affects the envelope test target. The 2024 IECC climate-zone information identifies most of Kentucky and Indiana as Zone 4A or Zone 5A. 2024 IECC Table R402.4.1.1 establishes 5.0 ACH50 for Zones 0-2 and 3.0 ACH50 for Zones 3-8.

Accordingly, the applicable limit for HECS projects in Kentucky and Indiana is 3.0 ACH50. This is distinct from duct leakage, which 2024 IECC R403.3.5 limits to 4 CFM25 per 100 ft² of conditioned floor area.

Working with HECS

HECS handles 45L documentation support for projects in Kentucky, North Carolina, Alabama, Georgia, West Virginia, Kansas, and Missouri. For a scope call, use(502) 643-7160 or hecsusa.com/contact/.

3. The HECS approach: build one traceable multifamily record

HECS treats the HERS rating and 45L file as connected evidence rather than separate deliverables. The review begins with the energy model, proposed construction specifications, unit mix, building geometry, and multifamily sampling plan.

Next, HECS evaluates the testing that supports the model. This can include:

• Blower-door testing under ASTM E779 or an equivalent method.
• Duct-leakage testing under ASTM E1554 or RESNET/ICC Standard 380.
• HVAC functional testing under the applicable certification checklist.
• Airflow and system performance verification where required.
• A review of installation records affecting modeled energy performance.

The 2024 IECC R402.1.2 total building UA alternative can bypass the prescriptive R-value and U-factor table. It can also bypass R402.4.1.2 testing when envelope performance is properly documented. That alternative requires early coordination because testing, cost allocation, and evidence retention differ materially from the prescriptive path.

For ENERGY STAR participants, HECS also reviews the MFNC v1.3 (Rev05) National requirements. Participating units require the applicable HVAC Functional Testing Checklist. Common areas must meet ASHRAE 62.1-2019 under that program.

The 45L review should not be postponed until final inspections. By then, model assumptions, unit selections, and field results may already conflict. HECS reviews documentation while design decisions remain adjustable.

4. What HECS verifies in the final report

A useful multifamily report identifies what was tested, where it was tested, and how the result supports the compliance path. The file should include:

• Project address, unit count, building configuration, and applicable climate zone.
• Blower-door test results, including corrected leakage and the tested dwelling-unit identifier.
• The sampling basis used for the multifamily project.
• Duct-leakage results, including test configuration and leakage normalization.
• HVAC functional-testing records for the applicable certification pathway.
• Modeled HERS or ERI results, clearly separated from field measurements.
• Corrections made after failed testing and the corresponding retest.
• Registration information for the HERS rating and the responsible HERS provider.

Builders also need a clear distinction between HERS registration, ENERGY STAR certification, NGBS verification, and the 45L claim. These records overlap, but they are not interchangeable.

For example, NGBS 2023 §702.1.1 requires third-party verification at every certification tier. Verifier credentials and sampling depend on the selected tier and compliance pathway. NGBS verification does not replace the HERS registration or establish 45L eligibility by itself.

Similarly, an ENERGY STAR MFNC v1.3 (Rev05) certification does not automatically prove that every federal 45L requirement has been satisfied. The tax-credit file must reflect the applicable standard, qualifying unit, acquisition timing, and prevailing-wage documentation.

HECS provides independent testing, verification, and certification documentation. We do not perform full architectural design or mechanical installation. That separation helps preserve the independence required for third-party verification.

Frequently Asked Questions

Did RESNET replace ANSI/RESNET/ICC 301-2022 with a complete 2026 edition? No separate 2026 edition is identified in the verified source. The controlling standard remains ANSI / RESNET / ICC 301-2022 with addenda through 2025, and “RESNET 2026” generally describes current addenda and implementation activity.

Does a compliant HERS Index prove that a multifamily building qualifies for 45L? No. The HERS Index is one component of the rating record, while IRC §45L imposes separate qualifying requirements. The acquisition deadline, certification pathway, unit eligibility, and prevailing-wage documentation must also be established.

Which field tests should appear beside the multifamily HERS rating? The file commonly includes blower-door and duct-leakage results, plus HVAC functional-testing records when required by the selected certification program. The applicable code sections and sampling basis must accompany those results.

How to Engage HECS

HECS provides 45L certification support, HERS rating documentation, ENERGY STAR multifamily review, NGBS Green verification, blower-door testing, and duct-leakage testing for multifamily projects. Our engineers support projects throughout Kentucky, North Carolina, Alabama, Georgia, West Virginia, Kansas, and Missouri. Request a scope review through HECS services or the FORTIFIED Roof evaluation service for Louisville multifamily developments. HECS also serves Cincinnati, Indianapolis, Nashville, and Chicago through its city-cluster coverage. Call(502) 643-7160 or email [email protected] to schedule a project review.