§150.0(m) · Duct insulation and sealing

# §150.0(m): Duct insulation and sealing

What it requires

Ducts in unconditioned spaces must be insulated to a minimum R-value that varies by climate zone:

• R-6 ducts in conditioned space (the easy case, no insulation effectively required because the loss goes back to the room).
• R-8 ducts in unconditioned attic / crawlspace for most climate zones.
• R-8 + buried in ≥ 8" of attic insulation ("buried ducts") for an effective R that meets §150.1(c) prescriptive in cooler zones.

All joints must be sealed with mastic, mastic + mesh, or AHJ-approved tape (not duct tape, duct tape isn't on the approved list under §150.0(m)). Leakage is then verified by a §110.7 duct-leakage test.

When it applies

• Newly constructed dwellings, every foot of new ductwork.
• HVAC replacements where ≥ 40 linear feet of duct is replaced.
• Additions with new HVAC capacity.

How we use it

The CF1R's HVAC section lists the duct R-value applied in the model, the duct location, and whether the buried-duct credit is being taken. The designer-CF1R checklist surfaces duct location as one of the inputs to confirm, buried-duct credit changes the prescriptive math meaningfully in CZ1, CZ2, CZ16.

Common gotchas

• Existing ducts retained in alteration scope are grandfathered IF they meet the §150.0(m) requirement when they were originally installed, pre-2008 ducts almost never do, and a §150.2(b) alteration trigger forces them up to current code.
• "Buried ducts" credit requires 8" of attic insulation on top of the duct AND R-8 duct insulation underneath, only one of the two doesn't qualify.
• Flex duct snaking through cabinet bulkheads ends up in conditioned space but the AHJ inspector treats it as unconditioned if the cabinet itself is outside the air barrier.