Flexible Duct Installation Rules Under Title 24
Flex duct must carry an R-value label at least every 3 feet, be clamped with stainless worm-drive bands or UV-resistant nylon ties rated 150 pounds, be sealed with mastic or UL 181B tape, and stay uncrushed in framing cavities. These are mandatory measures in Section 150.0(m) for new and altered ducts.
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At the label
Insulated flexible duct installed to meet the insulation rules must be labeled at maximum intervals of 3 feet, showing the R-value of the insulation alone, with air films and vapor retarders excluded (Section 150.0(m)6). Label values rest on installed thickness, which for flex is the outside diameter minus the nominal inside diameter, divided by two (150.0(m)5C). An 8-inch nominal duct measuring 10 inches across has 1 inch installed.
At each connection
- The inner core is mechanically fastened to the collar (150.0(m)1C).
- Drawbands are stainless-steel worm-drive clamps or UV-resistant nylon ties, rated at least 150 pounds tensile strength and tightened with an adjustable tensioning tool as the manufacturer recommends (150.0(m)3E).
- Tapes and mastics on flex must meet UL 181 and UL 181B (150.0(m)2C), and cloth-back rubber adhesive tape appears only together with mastic and drawbands.
Tape rules are on the duct tape page.
In the framing
Building cavities and support platforms may contain ducts but may not convey conditioned air unless they are lined with sealed sheet metal, duct board or flexible duct, and ducts in cavities must not be compressed in a way that reduces their cross-section (150.0(m)1E).
Material details
Flex with a porous inner core needs a non-porous layer or air barrier between the core and the outer vapor barrier (150.0(m)10). The floor is R-6.0 installed (150.0(m)1Bi); zone-by-zone levels are on the insulation page.
What the rater looks at
Where the smoke-test route applies, Appendix RA3.1.4.3.7 lists what the visual inspection covers: plenum and coil connections, register boots, and joints at takeoffs, wyes, tees and splitter boxes. Flex runs have many of those joints. The ECC-Rater verifies, and results are explained in duct leakage testing. If more than 25 feet of duct is new, see the 25-foot rule, and for ordering a report, residential Title 24.
Common Title 24 Questions
- When is Title 24 required for a home?
- Does a remodel need Title 24?
- Does a room addition need Title 24?
- What plans are needed for a residential Title 24 report?
- How much does a residential Title 24 report cost?
- How long does a residential Title 24 report take?
Frequently Asked Questions
On the duct itself, repeated at intervals of no more than 3 feet. It states the R-value of the insulation, not of the jacket or air films.
A cavity may contain ducts, but it may not carry the air itself. The duct must be flex, sheet metal or duct board, and not squeezed so its cross-section shrinks.
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