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Community and Q&A

Zone 4B Minimum Continuous Insulation Thickness

abutton | Posted in General Questions on

I want to put a bit of rigid foam insulation on the exterior of my new construction build. I am thinking 1 inch EPS will create enough of a thermal break. My wall assembly is 2×6 with fiberglass insulation, 1/2 OSB and hardie plank. I am not able to find the table with prescriptive code thicknesses. Please help this moderately qualified home owner/builder!! Aaron

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  1. Expert Member
    ARMANDO COBO | | #1

    IRC Chapter 7

  2. abutton | | #2

    Please, further detail would be most appreciated, I have looked and relooked and did not see the information that I am looking for. All I can see is the class three vapor retarder for Zone 4 Marine.

  3. Expert Member
    ARMANDO COBO | | #3

    I'm not at my computer, but if you have a code book, look at TABLE R702.7(3) Class III Vapor Retarder... or use Google.

  4. abutton | | #4

    It is not clear to me what the minimum is, I have looked at the above referenced table:

    1. abutton | | #9

      Hey Ken, thank you for the response. I do not see the minimum continuous insulation that is required. I am concerned with my sheathing temp. Any help would be greatly appreciated.

  5. brendanalbano | | #6

    What climate zone are you in?

    When you say "I am thinking 1 inch EPS will create enough of a thermal break", what do you mean? Enough of a thermal break for what?

    Energy Code:

    Do you mean enough of a thermal break to meet energy code? That's the table that Kyle Bentley posted in comment #5, although your local code may differ from the 2021 IRC that they posted.

    For example, in climate zone 5, the IRC 2021 code minimum is "20&5ci". The R-20 part comes from your 2x6 wall. The R-5ci part is the continuous EPS insulation. That's about 1.5" of regular EPS or 1" of GPS (Graphite infused EPS).

    Warm Sheathing:

    Do you mean enough of a thermal break to keep your sheathing warm and safe from interior moisture? That is the table you posted in comment #4.

    Again, for the sake of example, assuming that you are in zone 5, you look on that table and see that with a 2x6 wall you need R-7.5. That's 2" of EPS or 1.5" of GPS. (Caveat here is that with a rainscreen, in climate zone 5 you don't actually need the insulation per the building code, but folks on GBA seem to generally recommend that you use the insulation recommendations anyways).

    If you are in climate zones 1, 2 or 3, the IRC doesn't have any minimum values for continuous insulation and adding 1" of EPS insulation certainly isn't likely to cause any problems.

    Does that help at all?

    1. abutton | | #7

      I have updated the title to be more specific, I am in Climate zone 4B. My worry is I will not have enough insulation to keep the sheathing warm to avoid any increased moisture issues. I will review the additional resources provided and get back to you all if I have not addressed my issue, thanks for the help so far!

    2. abutton | | #11

      I think based on the fact that there is no data for Zone 4B, maybe there is no minimum value. You are correct that I want to avoid increasing moisture in my sheathing. The "enough" should not have been added in my question. I want 1" EPS. I just want to make sure I am not doing something that is not ok by code or by physics. Again, thank you for your help.

  6. abutton | | #8

    Ok, I still do not have an answer. I swear I am not stupid, please let me know if there is a minimum number for my zone. I am looking to see if 1" EPS will be ok continuously installed on the outside of my building.

    1. brendanalbano | | #10

      There isn't a minimum in the IRC for 4B.

      Joe Lstiburek does provide a minimum for 4B in this article:

      The minimum in the above article is R-5 for a 2x6 wall, so you'd need 1" of GPS or 1.5" of EPS to meet that requirement.

      I'm not sure why Lstiburek recommends more continuous insulation in 4B than 4C, but going for the full R-5 is the most conservative choice.

      1. abutton | | #12

        Brendan, my man! Thank you for this. I will read ASAP!

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