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Building Code Reference

Got a CMC 603.1 correction?

Duct sealing mastic

For builders in California, Greater Seattle, the Las Vegas area, the Denver area and Kansas City · CrossBeam Permits

What this code requires, how to fix it, and how cities and counties review it

What this code requires

CMC 603.1 is the California Mechanical Code section that requires all duct joints and seams to be sealed with mastic, mastic combined with fabric mesh, or UL 181A/181B-listed tape. Standard cloth duct tape is not acceptable unless used with mastic and drawbands. The requirement applies to both supply and return ducts, wherever they run (CMC 603.1.1 requires all ducts sealed to Seal Class A). HVAC, forced-air heating, and most new construction mechanical permits are the most common permit types flagged under 603.1.

How to fix it

  1. Add a duct sealing specification to the mechanical notes

    On the mechanical plan sheet, add a note that reads something like: "All duct joints, seams, and connections shall be sealed to Seal Class A with UL 181A/181B-listed mastic or tape. Cloth-back duct tape is not permitted unless used with mastic and drawbands." Place this note in the general mechanical notes block, not buried in a spec sheet that will not get reviewed at plan check. This single note clears the most common 603.1 correction.

  2. Mark sealing locations on the duct layout diagram

    A general note is often not enough if the plan-check reviewer wants to see where sealing applies. On your duct layout or floor plan, add callout arrows or a symbol legend identifying the air handler collar, all flex duct couplings, boot connections at registers, and any duct-to-plenum transitions. A single bubble note pointing to all transitions with "seal w/ mastic or UL 181A/181B tape, typ." satisfies most reviewers and gives the field inspector a clear reference point during rough inspection.

  3. Specify the listing standard, not just a brand or tape type

    The code references UL 181A (closures for rigid metal or rigid fiberglass duct) and UL 181B (closures for flexible duct) as the listing standards (CMC 603.9.1). Naming a brand without the UL listing (e.g., "3M tape" or "foil tape") does not satisfy the plan checker because not all foil tapes carry a UL 181A/181B listing. Either call out "UL 181A-listed tape" or "UL 181B-FX listed tape" by standard, or call out "mastic sealant per CMC 603.1" generically. If you name a specific product, include the UL listing number alongside the brand name in the note.

  4. Call out mastic plus mesh at flex duct connections and large gaps

    Mastic alone can crack on connections where the gap between surfaces exceeds 1/8 inch. At flex duct collars, boot connections, and any transition where surfaces are not flush, specify mastic applied over fabric mesh reinforcement. A cross-section detail showing the mastic-plus-mesh application at a typical air handler collar connection is optional on small jobs but appreciated by reviewers on larger forced-air systems with long attic duct runs.

  5. Cover any ducts added during a non-HVAC permit scope

    If you repositioned, extended, or added duct runs as part of a re-roofing, attic access, or remodel permit where HVAC was not the primary scope, those duct connections still fall under 603.1. Add a note to the relevant sheet identifying those specific duct transitions and calling out the sealing requirement. Plan-check reviewers will look for this if they see any duct work shown on a non-mechanical sheet. Skipping this note is a reliable second-round correction.

  6. Check return duct connections, not just supply

    Contractors who have seen 603.1 before sometimes only note sealing on supply ducts, leaving return connections un-called-out. The code covers both. Return duct runs in unconditioned attics or crawl spaces need the same mastic or listed tape specification as supply runs. If your duct layout shows a return air plenum or return chase in an attic, add the sealing callout to those connections as well. Missing the return side is a common cause of a second 603.1 correction on resubmittal.

  7. Resubmit with a response letter citing the corrected sheets

    When you resubmit, include a corrections response letter that lists: sheet number, note or detail number, and a one-line summary of what was added. Example: "CMC 603.1: Added duct sealing spec to mechanical general notes (Sheet M1.1, Note 7) and callout to duct layout (Sheet M1.0, Detail A). Return duct connections in attic also called out (Sheet M1.0, Detail B)." This saves the plan-check reviewer time and cuts the chance of a third round because the fix got buried.

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Why you got flagged

Plan-checkers flag CMC 603.1 most often when the mechanical notes on a set of plans specify duct tape without naming the listing standard. A note that says "all ducts sealed per manufacturer" or "duct tape at all joints" is not enough. The correction is that cloth duct tape is not a listed material under UL 181A or 181B, and the plan reviewer has no way to confirm compliance from an unlisted spec. This comes up constantly on basic forced-air furnace replacements and whole-house AC installs where the mechanical contractor's notes come off the same boilerplate they've used for years.

A second scenario is omission. The plans have no duct sealing spec at all. This happens with design-build contractors who carry the mechanical scope in-house and submit their own plans. The plan-check reviewer sees a duct layout diagram with no callout for sealing materials, and the correction letter cites 603.1 requesting a specification for mastic or listed tape on all duct joints and seams in unconditioned spaces.

Attic installations get flagged more than any other location because duct runs in attics are both long and hard to inspect after the fact. When a plan checker sees a flex duct layout in an attic with no sealing note at the air handler collar, boot connections, or flex duct couplings, 603.1 is almost always in the correction letter. Adding specific notes at each transition point rather than a general plan note is what gets the correction cleared.

Some corrections cite 603.1 on re-roofing or attic access projects where new duct work is incidental but still needs to meet current code. If a contractor added or repositioned any duct during a non-HVAC permit scope, the plan-checker applies 603.1 to those connections. The fix is adding a duct sealing schedule to the mechanical sheet, or a specific note on the cross-section detail if the duct work is minimal.

The official code

California Mechanical Code Part 4 — Duct Sealing and Duct Systems

Read the source →

Common questions

Does CMC 603.1 apply to supply ducts only, or return ducts too?
Both. CMC 603.1 requires all ducts to be sealed to Seal Class A, including return ducts. Return duct leakage in an attic or crawl space causes the same efficiency and indoor air quality problems as supply leakage, so the code treats both the same way. If your return runs through the attic to a central air handler, every coupling and connection along that return path needs to be sealed with mastic or listed tape. Skipping the return side is a common cause of a second-round correction on resubmittal.
Can I use regular foil tape on HVAC ducts in California?
Not unless it carries a UL 181A or 181B listing. Generic foil tape sold at hardware stores does not always carry that listing, even if it looks similar to listed tape. The listing means the adhesive and backing were tested for temperature cycling, humidity, and long-term adhesion in duct conditions. Always check the roll for the UL 181A-P or UL 181B-FX marking printed on the tape itself or the product spec sheet. If that mark is not present, it does not meet CMC 603.1 and the plan reviewer or field inspector can reject it.
Is mastic or listed tape better for flex duct connections?
Mastic with fabric mesh is the preferred method for flex duct connections, especially at collars and boot connections where surfaces are not perfectly flush. Flex duct couplings have gaps that tape bridges poorly over time, especially in attic environments with wide temperature swings. Mastic fills those gaps and bonds mechanically once cured. For rigid sheet metal duct joints with tight seams, listed tape works well and is faster to install. On any connection where the gap exceeds 1/8 inch, use mastic-plus-fabric rather than tape alone.
Does CMC 603.1 apply to mini-split systems?
Ductless mini-split systems do not have duct runs, so 603.1 does not apply to the refrigerant line set or the wall-mounted air handler. If you have a ducted mini-split (some multi-zone systems use short duct runs from a concealed ceiling cassette or air handler), those duct connections do fall under 603.1. Check your unit specs: if there are any duct joints or flex duct transitions running through an attic or crawl space, they need mastic or listed tape just like a conventional forced-air system.
What happens if cloth duct tape is shown on the plans?
The plan-check reviewer will flag it with a CMC 603.1 correction and return the plans. Closures must be listed to UL 181A or 181B, and the Energy Code bars cloth-back duct tape unless it is used with mastic and drawbands. If your plans or specs reference cloth tape anywhere in the mechanical notes, change the spec to mastic or UL-listed foil tape before resubmitting. There is no variance path for this requirement. The materials spec is clear, and reviewers do not have discretion to accept non-listed materials.
Do I need to seal ducts inside conditioned walls and floors?
Yes. CMC 603.1.1 requires all ducts, regardless of pressure class, to be sealed to Seal Class A, so the requirement is not limited to attics, crawl spaces, garages, and exterior chases. Your plan notes should apply the sealing spec to every duct joint, wherever it runs. Inspectors check this at rough-in, so call it out on every duct layout sheet.

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