Insulation on ductwork in a residence: When running flex duct (i.e., Ys, boots, Ts, or short pieces of hard pipe) Does the insulation have to be foil back or have any kind of vapor barrier?
The Dwelling Specialty Code (ODSC) does not require duct insulation to be foil back or to have any kind of vapor barrier when installed in unconditioned spaces. Proof that the duct insulation meets the R-8 requirement may be required. OSSC, Section 1308.1.1 requires a vapor barrier to be installed on supply and return ducts in spaces which are vented to the outside. Additionally the OSSC, Division III (Other buildings), Section 1317.7 requires duct- work conveying both heated and cooled air that is outside the insulated building envelope to have a vapor barrier having a perm rating not exceeding .05 perm. The requirements found in the OSSC cover all work not regulated by the ODSC. (Tri-County Code Forum, April 22, 2004).
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