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IRC on Vapor Retarder for Vented Attic

AugieMarsh | Posted in Building Code Questions on


I know this has already been addressed before, but my local inspector is hammering down that I need a vapor retarder (Class I or II) for a vented attic assembly in climate zone 5. I know that this is misguided. Unfortunately, I cannot find more black and white guidance than that set in section R806.2 “Minimum Ventilation Required”, specifically exception (1), to prove that it’s not required. My inspector is citing R702.7, which I have to admit is a tough thing to argue when R805.1 explicitly states ceilings shall be installed according to R702 standards. I refuse to put a vapor retarder on my ceiling, and it’s disappointing my building department makes no mention of windwashing prevention, air sealing, etc (far more important topics). I would appreciate any recommendations or definitions I’m missing from the 2018 IRC that would put this to rest.

Thank you,

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  1. sayn3ver | | #1

    I looked at both sections.

    Both class I and II are permitted in zone 5. Have you looked at the notes for class II? I would use a listed vapor retarder primer to appease the inspector. Its a listed class II retarder. once on the wall go about your business. I know others make similar products. Sherwin williams has one but its data sheet doesn't specifically call it out as class II, only that is has a WVP less than 1.

    1. AugieMarsh | | #5

      Thank you for the idea. I will look into this.

  2. Expert Member
    BILL WICHERS | | #2

    +1 for a paint-type vapor retarder here. This is the easiest way to go to appease the inspector without really changing your construction plans much. Show the inspector the TDS document then paint the ceiling drywall and you’re done. This will be less hassle than arguing with an adamant inspector.


    1. AugieMarsh | | #6

      Thanks Bill, would you agree that this is an unnecessary step? I'm just curious. That being said, I have already argued more with my building department than I probably should have, and a quick resolution does make sense here.

  3. maine_tyler | | #3

    I'm not a code person, but...
    Would table 702.7.1 where there is an exception for "vented cladding over gypsum" not apply?

    I think this table refers to gypsum sheathing, so in your case you actually have "no cladding at all, over no sheathing at all." In other words, the insulation is completely unfaced on the exterior to the vent space (providing the absolute best possible permeance on the cold side).

    1. AugieMarsh | | #8

      Hi maine_tyler,

      I looked at this as well, believe me! I came to the conclusion that this is tailored towards wall assemblies, especially as it goes on to list 2x4 and 2x6 frame walls with continuous insulation as additional Class III permitted assemblies. Thanks for taking the time to consider another angle.


      1. maine_tyler | | #10

        >"I came to the conclusion that this is tailored towards wall assemblies"

        Right, because it IS. It's section 702—interior wall coverings. But as you said, section 805 references it, and is what your inspector is pointing towards. So one has to interpret a section intended 'for walls' to apply 'to ceilings'. As such, your ceiling is vented beyond even the exemption of "vented cladding over gypsum" assuming the interpreter has a basic grasp on building science.

        I also think exemption 806.2 makes it pretty clear there are assemblies allowed that don't use class I and II vapor retarders.

        For what it's worth, I've seen older posts where Martin claims there is no code requirement for ceiling vapor barriers in vented applications.

        It may also be of interest that the new 2021 code has changed 805.1 from:
        "Ceilings shall be installed in accordance with the requirements for interior wall finishes as provided in Section R702"
        "Ceilings shall be installed in accordance with the requirements for interior wall finishes as provided in Sections R702.1 through R702.6."

        And Section R702.7 (so no longer referenced) is the section on vapor retarder requirements.

        But as you know, there's no harm in just installing the vapor retarder paint or such...
        Good luck!

  4. Expert Member


    Why don't you want an interior vapour-retarder on the ceiling?

    1. AugieMarsh | | #7


      I consider you (and all expert members) to be much more informed on building science principals than I am. I would defer to your argument for why it should be included, but my impression is that the negligible moisture drive by vapor diffusion when compared with air transport is insignificant, and proper venting will easily manage a damaging buildup to materials so long as the ceiling plane is properly air sealed. I understand that it certainly doesn't hurt anything (in this case), but my concern was added cost for a step not even required by the IRC. Again, reading R805.1, I wonder if it's more open to the AHJ than my naivety led me to believe. Following Bill and sayn3ver's advice, I'm going to look at a Class II primer in any case.


      1. Expert Member
        MALCOLM TAYLOR | | #9


        Makes sense!

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