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Continuous insulation outside of the framing

user-1140531 | Posted in Building Code Questions on

Do building codes require this type of wall design in the U.S.?  If so, what is the reasoning behind the requirement if the desired insulation performance can be achieved without continuous exterior insulation?

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  1. Expert Member
    BILL WICHERS | | #1

    Code does not REQUIRE you to install continuous exterior insulation, but if you DO install continuous exterior insulation you ARE required by code to use certain minimum thicknesses depending on climate zone and wall construction.

    The big advantages to continuous exterior insulation is that it helps a LOT with thermal bridging of studs and other framing members, and it also provides another air barrier (usually, if it’s taped and sealed properly).

    I wouldn’t build a home without exterior rigid foam myself. It just offers too many advantages.


  2. Jon_R | | #2

    Some codes (like the 2018 IECC and most codes derived from it) list only cavity+continuous (exterior or interior is fine) R-values for the coldest zones - BUT an alternate prescriptive U-factor method is also listed and it allows the use of cavity insulation alone (eg, double walls or foam strips). Meet (or exceed) this spec and you are free to use ANY amount of continuous insulation (including none).

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Ron.

    I just wrote a bit about this, in this article: The Four Control Layers of a Wall. Scroll down to the last control layer, temperature.

    Also, you can read what the 2018 International Residential Code has to say about insulation, cavity and continuous here:

    Of course, what the IRC has to say and what any local jurisdiction requires is often not the same. I just read a comment by Mike Maines (to my article listed above) in which he said that VT does and Maine is soon to require continuous exterior insulation. I haven't verified that, but Mike is a reliable source. I'm sure he'll clarify if he sees this.

    1. Expert Member
      Michael Maines | | #6

      Brian, my wording was a bit sloppy and my understanding was incomplete. Starting on July 1, 2020, adherence to Maine's building code will be required in all jurisdictions, as opposed to only being required for towns over 4,000 people as it is now ( We (I'm in Maine) are currently using the 2015 IRC except for energy, where we use the 2009 IRC/IECC. I thought we were upgrading to newer IRC also in 2020 but I can't find a reference for that at the moment.

      As for Vermont, based on what I've heard from others, they require either exterior insulation or equivalent U-factor for the entire assembly.

  4. user-1140531 | | #4

    Thanks Brian, Jon, and Zephyr7. I did read the comments by Michael Maines about continuous insulation being required by code, and that is why I am asking. I would have thought the code would govern insulation performance, but not design details that create the performance. I know that a lot of people really like the concept of continuous insulation, but I prefer to use double studs with all of the insulation in the cavity. So I am curious about any plans to require continuous insulation by code, and if that is happening, I would be more than curious to hear the rationale behind it.

    1. GBA Editor
      Brian Pontolilo | | #5

      Hi Ron.

      The codes that we're discussing are prescriptive, which means that they are for builders who aren't going to take a performance path or do any energy modeling, etc. This way, they can follow what the code says, satisfy their building official, and end up with a house that performs to the levels that the code writers saw as appropriate. A builder who wants to do things differently can often satisfy their building inspector through a performance path.

      1. user-1140531 | | #7

        Hi Brian,

        Okay, I understand that, but what about the path of using much more insulation than code minimum in a stud wall without any additional prescription mandate or performance testing? Is that path not available?

        If that is the case, in which states does this new requirement apply? How likely is it that other states are going to follow?

        1. GBA Editor
          Brian Pontolilo | | #9

          Hi Ron.

          I don't know the answer to your question second question, no one does. There is at least one state (Maryland, I think) that adopts each iteration of the IRC as a state law. Some states are still on the 2006 IRC and some don't do statewide adoptions. Some local areas amend the IRC or the state code.

          Typically, you must take the prescriptive path or a performance-based path, which is not really different than structural codes where you either have to do what the code says, or have your design engineered. However, if you plan is to exceed the prescriptive path, why not do the modeling to be sure and get buy in from your building official?

  5. user-6184358 | | #8

    In California a energy calculation for a residences is about $200.00 ( search online for title 24 energy calc.) this is the performance path referenced above- This method allows you to deviate from the prescribed assemblies.

  6. Expert Member
    Peter Engle | | #10


    Your questions about the prescriptive path and performance testing as sort of apples and oranges. The path you choose is a design choice/requirement. Testing is done during or after construction to verify that the design requirements were met.

    If you choose to design based on the performance path, you can generally use one of the recognized software modeling programs to show compliance with the overall building energy use requirements. The allowable software programs vary by jurisdiction. Some jurisdictions also allow a HERS rating in lieu of the prescribed software. If your HERS rating shows performance at or better than code, you're OK.

    under the 2018 IRC and IECC, blower door testing is now mandatory to verify that certain air leakage targets are met. Again, this varies by jurisdiction. In NJ, they eliminated this requirement (again), and still allow verification by inspection, as if it is possible to measure air leakage by looking at the caulking details. But I digress.

    So far, your questions have all been generic. The real answer to all of them is, "it depends." It depends on your location, jurisdiction, code enforcement agencies (or lack thereof), etc.

  7. Expert Member
    MALCOLM TAYLOR | | #11

    I don't think that's how codes should be written. A very narrow prescriptive path and the onus on the designer to show performance testing to comply.

    Ours lays out various alternatives, as it also does for fire-rated assemblies and acoustical separations:

  8. user-1140531 | | #12

    In the strictest of these new codes, is there a prescriptive path that allows the following?

    An insulated stud wall with an amount and type of cavity-only insulation having a thermal rating sufficient to meet the code minimum thermal performance required.

    Or must the acceptable prescriptive path also include a prescribed amount of continuous insulation on the exterior side of the studs?

    If such a path does not require some amount of continuous insulation on the exterior side of the studs, and if the cavity insulation exceeds the code minimum thermal rating, does it have to be tested to verify thermal performance?

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