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EPS or XPS for exterior insulation?

user-4572730 | Posted in Green Building Techniques on

My name is Ben.  I am building a home in climate zone 6.  Our plan is to use 3-4” of XPS foam, then WRB, and then sheathing for the exterior.  Dense packed cellulose will be used for our wall cavities.  We are going to frame on 24” centers to increase the interior wall cavity area to be insulated.  I am looking at the option of switching to a type IX EPS for the exterior insulation because I would like to be environmentally responsible.  Is there a concern about how vapor permeable the EPS will be vs. XPS given the fact that we plan to omit the interior polyethylene layer?

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  1. natesc | | #1

    I would check craigslist for used polyiso. Fraction of the cost, same performance, and you're recycling.

    You don't need to worry about permeability at those thicknesses. Also, you can't put sheathing over top of the foam, it's got to be the other way around. The OSB/plywood is what holds your house together when the weather turns bad. Don't want to mess with that.

  2. GBA Editor
    Martin Holladay | | #2

    Nathan is right. EPS is fine, and the vapor permeance of the foam is irrelevant. (This type of wall is designed to dry to the interior, not the exterior.) For more information on the building science concepts behind this type of wall, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

    Nathan is also correct about the fact that the continuous exterior rigid foam belongs on the exterior side of the wall sheathing. For more information on this issue, see "How to Install Rigid Foam Sheathing."

    If you absolutely need a layer of OSB or plywood sheathing on the exterior side of the rigid foam -- as is sometimes the case for a siding like cedar shingle siding -- you can add a second layer of OSB or plywood on the exterior side of the rigid foam. This approach is called a "site-built SIP." (SIP stands for "structural insulated panel.")

    Then again, you can just buy SIPs if you want.

  3. Peter Yost | | #3

    Hi Benjamin -

    Since this wall is designed/specced to dry to the interior, you want to install the most vapor permeable interior components as you can to promote drying while also providing enough restriction to wintertime interior moisture both getting into your building assemblies as well as getting cold enough to condense in there.

    What is "the most vapor permeable interior components that you can" mean? It means at least one layer that is the dedicated vapor retarder and is a Class III retarder (1 - 10 perms) and nothing less vapor permeable to the retarder's interior. In this case "can" can mean what is best practice in your climate and/or what your code allows.


    1. user-4572730 | | #4


      Thank you to everyone for the replies. What are your thoughts on the CertainTeed MemBrain for the interior vapor barrier? Also, something that I am a bit concerned about; two of the showers share exterior walls. We typically use schluter kerdi membrane over the top of cement board for our shower wall prep. This membrane runs approximately 5 feet up the wall. There will still be three feet above the kerdi membrane, which will consist of cement board with tile covering, to breathe. Do you see that detail being a problem? Kerdi has a perm rating of 0.75. We are in a very arid climate being Bozeman, Mt.

      1. GBA Editor
        Martin Holladay | | #6

        While Dana's statement (below) that MemBrain is "fine" is correct -- in the sense that the MemBrain won't cause any harm -- it is also unnecessary if your wall has an adequate thickness of exterior rigid foam.

        1. user-4572730 | | #8


          Do you see an issue with using rockwool batts to insulate the stud bays in this enclosure? I know that cellulose has been most recommended when partnered with the use of exterior rigid foam. I have heard little mention of the use of rockwool batts in conjunction with exterior rigid foam.

          1. GBA Editor
            Martin Holladay | | #9

            Q. "Do you see an issue with using rockwool batts to insulate the stud bays in this enclosure?"

            A. No.

    2. Deleted | | #7


    3. Jon_R | | #10

      See below for a different view from the best in the business. Exterior perms do matter - you may want Class II on the interior (even though code allows or requires Class III). Solutions to the conflict: use MemBrain or avoid foam with foil facing.

      "Zone 5, Zone 6 and Zone 7 require a Class II (or lower) vapor retarder on the interior surface of insulation in insulated wall and floor assemblies where the permeance of the exterior sheathing is 0.1 perm or less..."

  4. Expert Member
    Dana Dorsett | | #5

    >" I am looking at the option of switching to a type IX EPS for the exterior insulation because I would like to be environmentally responsible."

    To be more environmentally responsible start by using lower density Type II (1.5lbs per cubic foot) or Type VIII (1.25lbs per cubic foot) EPS, or better yet, 1lb foil faced polyisocycanurate, which uses even less polymer per R, and a polymer that has less overall environmental impact. Polyiso is also slightly less flammable than polystyrene, and chars in place when burning rather than running and dripping of flaming liquid polymer once it gets started.

    XPS is among the least-green insulating materials in common use today, primarily due to it's HFC blowing agents. Also, it's labeled R5/inch is only warranteed to R4.5/inch, since it loses performance over decades as the HFCs diffuse out. EPS and polyiso are blown with low impact pentane, and in the case of EPS most of the pentane leaves the foam even before it leaves factory, where it is recaptured (to meet local air pollution requirements), often burned for process heat.

    MemBrain is fine, even with the 0.75 perm Kerdi. Both are under 1 perm when the moisture levels in the wall cavity are low. At 0.75 perms the cavity can still dry toward the interior on a seasonal basis through the Kerdi if it needs to.

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