What code officials actually verify at final inspection — and where Kentucky CZ4A projects fail.
The 2024 IRC and 2024 IECC, adopted in Kentucky's residential code update, raise the bar on envelope airtightness for projects seeking ENERGY STAR or above-code certification. Blower door testing must be performed per RESNET Standards, with the result reported as ACH50 (air changes per hour at 50 Pascals). HECS has observed Kentucky CZ4A multifamily projects commonly passing in the mid-single-digit ACH50 range after drywall, but the moment a project fails its first duct leakage test, the air barrier and the duct envelope are usually re-examined — and that re-examination often surfaces what the first test missed.
The controlling IRC chapter sets a building-envelope leakage target expressed in ACH50, verified at final inspection. The complementary section governing duct leakage specifies two separate tests — total duct leakage and duct leakage to outside — both reported in CFM25. HECS treats these as independent compliance checks: a duct system can pass total leakage while failing leakage to outside through a single unsealed HVAC boot, a common failure mode we see in multifamily shaft construction.
The IECC link below resolves to the ICC publication landing page rather than a section-anchored deep link, so the exact numeric thresholds HECS enforces against on Kentucky projects come from program specifications rather than from the IECC text directly. For verified ENERGY STAR Multifamily New Construction projects, the controlling spec is ENERGY STAR Multifamily New Construction v1.2, which tiers building-envelope leakage targets by climate zone.
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Call (859) 983-7382 Get a QuoteCZ4A covers Louisville, Lexington, and most of central Kentucky — a climate zone where code-compliant construction is feasible, but where above-code certification (ENERGY STAR, NGBS, FORTIFIED) exposes latent weaknesses in compartmentalization. HECS has tested roughly two multifamily projects per month across Kentucky in the past year, and the failure patterns are consistent.
A 48-unit garden-style project in Lexington KY came to HECS after failing its first final inspection with a reported ACH50 in the high single digits. The envelope itself tested acceptable; the culprit was riser shafts and corridor soffits that had not been sealed at the top-of-wall, leaking conditioned air into the attic plenum above third-floor units. After targeted sealing, the project came in at an acceptable ACH50 and passed.
A 72-unit mid-rise in Louisville KY failed duct leakage to outside on first test despite passing total duct leakage comfortably. The diagnosis: HVAC boots in mechanical closets shared between adjacent dwelling units, where drywall return-air pathways allowed cross-leakage that did not register on the total test. Repairs at three boots dropped leakage to outside below the program threshold.
These projects illustrate a Kentucky-specific pattern: builders framing multifamily as a scaled-up single-family build, then discovering that compartmentalization — not envelope area — is what drives leakage in attached dwellings.
HECS's local data, drawn from project testing across Louisville, Lexington, and Cincinnati-adjacent markets, clusters around a handful of repeatable failure modes:
The corrective cost is typically modest — a tube of acoustic sealant and a few labor hours per unit — but the schedule cost of a failed final test is severe. A retest delays certificate of occupancy by a minimum of one week, often longer when the underlying failure mode requires opening finished walls.
For projects pursuing FORTIFIED Home 2025 designation, envelope leakage is one of three tested building-performance criteria. A failing blower door test is treated as a designation blocker until corrected, regardless of how the rest of the project performs.
Code officials reviewing a multifamily final in CZ4A should request both reports separately, not a single aggregate duct leakage figure. The two tests measure different conditions and produce different compliance decisions.
The total leakage test pressurizes the duct system with the building envelope depressurized, capturing every leak regardless of where the air ends up. The leakage-to-outside test pressurizes the ducts while the building is also depressurized, so only leaks crossing the thermal envelope register. A duct system with a return-air leak inside conditioned space passes total but fails leakage-to-outside; a duct system with a supply leak in an unconditioned attic passes both.
| Test | What It Measures | When It Fails | Typical Kentucky Failure Mode |
|---|---|---|---|
| Total Duct Leakage (CFM25) | All leaks regardless of location | Boots, mastic gaps, filter slot | Filter slot unsealed in mechanical closet |
| Duct Leakage to Outside (CFM25) | Only leaks crossing the envelope | Supply in attic, return drawing from garage | Riser boot in unconditioned chase |
| Building Envelope (ACH50) | Whole-building airtightness at 50 Pa | Top-of-wall, recessed lights, rim joist | Corridor soffit at top-of-wall |
ASHRAE 111 defines the procedural tolerances for the instruments and the CFM verification reading itself — HECS finds a 10% tolerance band tight on commercial projects with VAV terminals, and similar tolerances apply to residential blower-door instrumentation under RESNET Standards.
ACCA Manual D governs duct design, but leakage testing verifies that design intent survived installation. The gap between designed and installed duct leakage is where HECS's project data concentrates.
HECS handles blower door testing and duct leakage testing for multifamily projects across Kentucky, Indiana, Ohio, and Tennessee. For projects pursuing ENERGY STAR, NGBS, or FORTIFIED certification, testing must occur after drywall and before final finishes are installed in conditioned space. Reach HECS at (859) 983-7382 or hecsusa.com/contact/ to schedule a pre-drywall site visit or a final-inspection test.
What ACH50 threshold does ENERGY STAR Multifamily v1.2 require in CZ4A?
ENERGY STAR Multifamily New Construction v1.2 tiers airtightness targets by climate zone; in CZ4A, HECS verifies against the program-specified ACH50 ceiling for the certified tier, not the IRC baseline. The threshold lives in the program specification rather than the IECC text, so project teams should pull the v1.2 reference directly.
Does Kentucky require duct leakage to outside on multifamily projects?
The 2024 IECC requires duct leakage verification with separate reporting for total and leakage-to-outside on projects seeking above-code certification. HECS treats both as independent compliance tests and reports them separately on every final inspection, including the HVAC testing and balancing deliverable that bundles both metrics.
Can a project pass blower door and fail duct leakage on the same final inspection?
Yes. The two tests measure independent building elements. HECS has tested Kentucky projects where the envelope came in acceptable while the duct system failed leakage to outside, almost always through boots in unconditioned chases or mechanical closets shared between adjacent units. Scheduling both tests in sequence, with the duct test crew present at the same site visit, avoids a retest cycle.
How does NGBS 2023 treat envelope airtightness versus 2024 IECC?
NGBS 2023 certification tiers leak testing at progressively tighter performance levels than the IRC baseline. A project certified Bronze under NGBS typically meets or exceeds the IECC by a measurable margin, with Gold and Emerald tiers pushing toward Passive House airtightness thresholds.
HECS provides blower door testing, duct leakage testing, and HVAC testing and balancing for multifamily and single-family projects across Kentucky, Indiana, Ohio, Tennessee, Illinois, and Missouri. The relevant HECS services for IECC air-barrier compliance are blower door testing, duct leakage to outside and total duct leakage, and — for ENERGY STAR or NGBS projects — the full certification verification. Contact HECS at (859) 983-7382 or [email protected], or submit a scope request through hecsusa.com/contact/. The full service catalog is at hecsusa.com/services/. HECS serves Louisville KY, Lexington KY, Cincinnati OH, Indianapolis IN, Nashville TN, St. Louis MO, and Chicago IL, with project coverage extending to Evansville IN, Columbus OH, and the broader Ohio Valley region.
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