How much insulation do HVAC ducts in the attic need?
Supply and return ducts in unconditioned attics must be insulated to R-8, and other ducts outside conditioned space to R-6 (IRC N1103.2.1).
Ducts running through an unconditioned attic must be insulated to at least R-8. Ducts located in other unconditioned spaces (crawlspaces, unheated basements, exterior wall cavities) require at least R-6. Ducts located completely inside the conditioned space don't need insulation.
This is section N1103.2.1 in the 2012 IRC, renumbered to N1103.3.1 in the 2015 IRC. The 2015 edition adds a duct-diameter distinction (larger ducts R-8, smaller ducts a reduced value in attics), so check your adopted edition for the fine print.
Separately from insulation, ducts must be sealed at joints and seams, and the duct system may have to pass its own leakage test.
Verified against
Related answers
- How much attic/ceiling insulation does the energy code require by climate zone? — IRC §N1102.1.1
- What blower-door air leakage rate does code require for a new house? — IRC §N1102.4.1.2
- How much attic ventilation do I need? — IRC §R806.2
- What size does an attic access opening need to be? — IRC §R807.1
- Can HVAC return air be pulled from a bathroom, kitchen, or garage? — IRC §M1602.2
- What is the maximum length of a clothes dryer exhaust duct? — IRC §M1502.4.4
- How many CFM does a kitchen exhaust fan need? — IRC §M1507.3
- How much combustion air does a gas furnace or water heater need indoors? — IRC §G2407.5
Need this scoped to your jurisdiction?
Adopted edition and local amendments vary by city and county. Ask Donnie checks your jurisdiction and cites the exact adopted section.
Ask Donnie →General information based on the International Residential Code (a model code). This is AI assistance to verify — not legal advice. Confirm the adopted edition and any local amendments with your local building department (AHJ) and a licensed professional before you build.