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Vapor Retarder Requirement with Proper Exterior Insulation

user-6445625 | Posted in General Questions on

Dear Colleagues,
We are working on new homes in a few different climate zones. All 2×6 walls with continuous exterior insulation. According to the 2021 energy code – Table R702.7, we are permitted to use a Class III Vapor Retarder (latex or enamel paint) instead of either Class I or Class II in Zones 4-6, provided we meet the following insulation requirements:

Zone 4: Continuous insulation with R-value ≥ 3.75 over 2 x 6 wall.
Zone 5: 
Continuous insulation with R-value ≥ 7.5 over 2 x 6 wall.
Zone 6: 
Continuous insulation with R-value ≥ 11.25 over 2 x 6 wall.

Assuming we meet these R-value requirements, does anyone here have an objection to my using paint on the inside rather than something like Intello Plus, etc? 


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  1. Expert Member
    Akos | | #1

    The insulation R values in code tend to be fairly conservative. If code allows for an assembly, it generally means it will work when built by trades that might not take the best of care to get every detail right.

    Since you have sufficient exterior R value for condensation control, you won't need anything more in your walls.

    I would recommend that the sheathing is taped as your primary air barrier and the drywall installed in an airtight manner. For the drywall this means using vapor tight device boxes (these are plastic boxes with a gasketed flange that seals against the drywall) and caulking behind the drywall around the perimeter of any exterior wall.

    1. user-6445625 | | #2

      Thanks Akos

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