Duct leakage: total leakage ≤ 4 cfm/100 sqft conditioned floor area (post-construction); leakage-to-outside ≤ 4 cfm/100 sqft (when ducts in unconditioned space)
IECC R-value (DOE climate-zone) — Duct Leakage — air sealed + leakage limitDescription
IECC 2021/2024 sets the duct leakage limit at ≤ 4 cfm per 100 sqft of conditioned floor area, measured via a duct-leakage tester after rough-in (with the system pressurized to 25 Pa). Where ducts are located in unconditioned spaces (typical attic / crawl), leakage to outside must be ≤ 4 cfm/100 sqft. Ducts entirely within conditioned space have a relaxed leakage-to-outside requirement (the leak is to inside, less of a penalty).
Why this exists
Duct leakage is HVAC's single biggest performance penalty. Typical un-sealed residential duct systems leak 20-30%; sealed-to-IECC ducts leak 4-8%. Mastic-sealing every joint + boot-to-drywall connection is the labor that gets you there. Architects should require the duct leakage test as a contract deliverable.
Measurements
| Property | Operator | Value | Unit | Note |
|---|---|---|---|---|
totalLeak | max | 4 | cfm/100 sqft | Total duct leakage |
outsideLeak | max | 4 | cfm/100 sqft | Leakage to outside (when ducts in unconditioned space) |
Categories
Source
Solver enforcement
Browsable only — the solver does not currently enforce this directive (no spec-level data to check against). This entry exists so the architect personas can cite it in conversation and the user can read what the rule says.
Related directives
- Natural light in habitable rooms · IRC R303.1
- Carbon monoxide alarms in sleeping areas · IRC R315
- Garage HVAC duct prohibition · IRC R302.5.2
- Natural ventilation in habitable rooms · IRC R303.1 (vent)
- Bathroom ventilation · IRC R303.3
Last reviewed 2026-05-15.