For California residential projects permitted under the 2025 Energy Code—generally permits applied for on or after January 1, 2026—the HVAC duct-insulation requirement depends on duct location and the compliance method.
| Duct location/system | Minimum requirement |
|---|---|
| Ducts or plenums in unconditioned space | R-6 mandatory minimum |
| Prescriptive compliance—Climate Zones 3 and 5–7 | R-6 |
| Prescriptive compliance—Climate Zones 1, 2, 4 and 8–16 | R-8 |
| Entirely within conditioned space | No duct insulation required, subject to qualifying conditions |
| Qualifying unvented attic | R-4.2 minimum, subject to specific conditions |
| Performance-compliance project | Follow the R-value modeled and shown on the approved CF1R; never below applicable mandatory requirements |
The unvented-attic R-4.2 exception requires all of the following:
- At least R-30 insulation between the roof rafters and in contact with the roof deck.
- Properly insulated gable ends.
- Verified building-envelope leakage of no more than 3.0 ACH50, tested with the attic hatch open.
The California Energy Commission confirms that R-6 is the general mandatory minimum for supply and return ducts and plenums outside conditioned space. The prescriptive requirement may increase to R-8 depending on the climate zone and selected roof/attic compliance option. 2025 Single-Family Residential Compliance Manual
Important field notes:
- Check the approved CF1R and mechanical plans; a project modeled with R-8 must install R-8 even where R-6 would otherwise be the mandatory minimum.
- The insulation must carry a visible installed R-value label.
- Crushed or compressed insulation may not deliver its labeled installed R-value.
- Ducts must also meet sealing and leakage-testing requirements; insulation alone does not establish compliance.
- Existing projects permitted under the 2022 or earlier code must follow the code cycle identified on the permit.
- Nonresidential and central multifamily systems follow different requirements under §120.4 and the approved mechanical design.
For most Southern California residential projects: Climate Zones 6–7 generally use R-6 prescriptively, while Climate Zones 8–16 generally use R-8. Always verify the exact climate zone and CF1R before installation.
Here is the QueryTool to query the duct R-Value according the ZIP code.
