IECC 2021 Duct Insulation Requirements 1

[jdmz]

I am trying to interpret the meaning of a sentence in the C403.12.1 Duct and plenum insulation and sealing section of the 2021 IECC.

"When located within a building envelope assembly, the duct or plenum shall be separated from building exterior or exempt spaces by not less than R-8 insulation in Climate Zones 0 through 4 and not less than R-12 insulation in Climate Zones 5 through 8."

When does the quoted language apply? Unconditioned and exterior ducts and plenums are covered in an earlier section. Is the space above the ceiling in a ducted return system considered a "unconditioned space"? Conditioned spaces where the temp inside and outside of duct are not greater than 15 F is an exception to the insulation requirement.

The only instance where I can think it may apply is a duct that is carrying air more than 15 F in a conditioned space such as 0 F outside air in the winter going to an air handler. Or if you were to design a VAV system with a space design temperature of 70 F in the winter and duct downstream of VAV is designed to carry 90 F air (20 F).

Am I missing something?

Thank you.

 

[HVAC-Novice]

Plenum is unconditioned space and likely more than 15°F warmer than the supply air in a duct. to determine the 15°F you need to use the larger of the dT (in heating or cooling). You don't need to insulate within the space served (below ceiling). You may want to insulate within the space served for condensation reasons, though.

And you need to look at the entire section, not just quote one sentence and then somehow refer to the 15° exception without quoting it. Context is everything. So what we say isn't advice you should trust. Also look at other local codes and ask JHA if in doubt.

 

[nuuvox000]

The IECC commentary has a great figure showing different cases and they go through each one and describe when insulation is required and when it's not. Definitely worth a look at the commentary.

 

[jdmz]

Thanks! I signed up for the ICC premium trial and looked at the commentary and it explained it. Too bad accessing the commentary is not free!

Anyway to summarize insulation in conditioned spaces (other than for condensation purposes) isn't required at all per IECC commentary. The quoted sentence allows the usage of the building envelope itself to act as the insulation. So if you had R8 insulation in your building envelope assembly in climate zones 0-4, you aren't required insulation on the duct.

 

[nuuvox000]

Yeah, it's a little surprising how often you don't need insulation based on this commentary. I've had to explain it to people over and over in some cases because people are so used to supply duct being insulated in all cases.