Question from an HVAC supplier:

Has the State of Utah adopted 2015 IRC requiring R8 on the returns at this time? Doesn’t the code allow R-4.2 for return ducts in the attic? If R-8, when did the code change?

A couple of important items to consider includes the differences between the residential and commercial energy code (IECC) in Utah. First, the current adopted residential code is the 2015 IECC, Residential provisions with numerous amendments; however, none of the amendments affect the required R-values for duct insulation. Second, the current adopted commercial code is the 2018 IECC Commercial provisions, adopted virtually without amendments.

The 2009 IECC did for that one code cycle allow R-6 for return in attics, with R-8 for supply ducts; however, Utah did not adopt the 2009 IECC.

R-4.2 insulation for typical sizes of return air ducts in attics has not been allowed by the code since January 1, 2007, with the adoption of the 2006 IECC.

Residential Duct Insulation – The 2015 residential code states:

R403.3.1 Insulation (Prescriptive). Supply and return ducts in attics shall be insulated to an R-value of not less than R-8 for ducts 3 inches (76 mm) in diameter and larger, and not less than R-6 for ducts smaller than 3 inches (76 mm) in diameter. Supply and return ducts in other portions of the building shall be insulated to not less than R-6 for ducts 3 inches (76 mm) in diameter and not less than R-4.2 for ducts smaller than 3 inches (76 mm) in diameter.

Exception: Ducts or portions thereof located completely inside the building thermal envelope.


  • 3” or larger supply and return ducts in attics must be insulated to a minimum R-8.
  • Ducts smaller than 3” in attics must be insulated to R-6. (high velocity systems, new in 2015 IECC)
  • 3” or larger supply and return ducts in crawlspaces or un-insulated basements must be insulated to a minimum R-6, with those smaller than 3” insulated to R-4.2.

Commercial Duct Insulation – The 2018 commercial code states:

C403.11.1 Duct and plenum insulation and sealing (Mandatory). Supply and return air ducts and plenums

shall be insulated with not less than R-6 insulation where located in unconditioned spaces and where located outside

the building with not less than R-8 insulation in Climate Zones 1 through 4 and not less than R-12 insulation in Climate Zones 5 through 8.


  • The commercial code does not give an exception for ducts smaller than 3” ducts
  • In Washington County, Climate Zone 3, the requirements for ducts in attics (or on roofs) is R-8.
  • In all other counties in Utah, Climate Zones 5 or 6, ducts in attic, on roofs, or any other spaces openly communicating with outside, require R-12 insulation.

Please be aware the 2018 residential code, which again we did not adopt in Utah, recognizes the significant heat loss and heat gain reductions realized when ducts in attics are buried in the attic insulation.  Every effort should be made to keep duct systems in attics, low as possible allowing covering with the blown insulation, or even better, bring the duct inside the thermal envelope.

Also remember where at least 25% of a duct system or the air handler/furnace is located outside the thermal envelope, duct blaster testing is required. 

– Dr. Energy