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All supply and return ducts and plenums installed as part of an HVAC air distribution system shall be thermally insulated in accordance with Table 6.8.2.
Exceptions to
  1. Factory-installed plenums, casings, or ductwork furnished as a part of HVAC equipment tested and rated in accordance with Section 6.4.1.
  2. Ducts or plenums located in heated spaces, semiheated spaces, or cooled spaces.
  3. For runouts less than 10 ft in length to air terminals or air outlets, the rated R-value of insulation need not exceed R-3.5.
  4. Backs of air outlets and outlet plenums exposed to unconditioned space or indirectly conditioned space with face areas exceeding 5 ft2 need not exceed R-2; those 5 ft2 or smaller need not be insulated.

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