Q: During plan review, we submitted a 2x6 wall with unfaced R-21 batts and 1-inch continuous polyiso foam on the exterior. This design exceeds the code’s “20 + 5” insulation requirement for climate zone 6, but it was rejected because it doesn’t “meet vapor retarder requirements.” (Because we are using the foam, we are trying to avoid poly on the interior.) To pass, we were told we needed 2 inches of foam on the exterior. Can you explain why the code’s vapor requirements contradict the insulation requirements?

A: Clayton DeKorne, editor of JLC, responds: The building code is written to be as flexible as possible so you have options. For example, you could install a Class I vapor barrier, such as poly, on the interior and stick with “20 + 5” (R-20 cavity insulation + R-5 continuous insulation). Even though the poly and the foam are both vapor barriers, this wall system does work in cold climates.

But I understand your reluctance to add poly given the trend toward warmer climates and the increased use of air conditioning in cooler climates. I wouldn’t say the requirements contradict each other, but the IRC’s minimum insulation requirements are confusing. At the very least, the Chapter 11 insulation requirements listed in Table N1102.1.4 (pre-2021 IRC; in 2021, it’s relabeled as Table N1102.1.2) should have a footnote that specifies which vapor control option is assumed in the table and refer users to the Chapter 7 options. When using a Class III vapor retarder in climate zone 6, Chapter 7 specifies that the continuous insulation layer needs to be greater or equal to R 11.25 over a 2x6 wall (hence the 2" of foam called for by your local building department).

The IRC Chapter 7 vapor control requirements are written to address the potential for condensation to form in building assemblies that don't have an adequate balance of impermeable continuous exterior insulation and permeable interior cavity insulation. More cavity fill (which is usually permeable and will allow moisture laden air to travel through it) requires more continuous insulation. As you add permeable insulation to the cavity, you need to increase the thickness of continuous exterior insulation (on the outside of the sheathing). If you don’t, the increased cavity insulation will result in the sheathing being cooler. This increases the chance that the sheathing temperature will reach dew point and wet the inside face of the sheathing with condensation.
Tim Healey The IRC Chapter 7 vapor control requirements are written to address the potential for condensation to form in building assemblies that don't have an adequate balance of impermeable continuous exterior insulation and permeable interior cavity insulation. More cavity fill (which is usually permeable and will allow moisture laden air to travel through it) requires more continuous insulation. As you add permeable insulation to the cavity, you need to increase the thickness of continuous exterior insulation (on the outside of the sheathing). If you don’t, the increased cavity insulation will result in the sheathing being cooler. This increases the chance that the sheathing temperature will reach dew point and wet the inside face of the sheathing with condensation.

It’s worth noting that the 2021 IRC has added clarifying information to the Chapter 7 discussion of vapor retarders, including guidance on types of vapor retarders allowed in each climate zone and details on using Class II vapor retarders, such as kraft-faced batts. If you use kraft-faced batts, you should be able to use your proposed wall insulation under the new rules, which specify a minimum R-5 continuous insulation over a 2x6 wall.