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EPS Foam on Exterior with interior vapor barrier?

U7XY49xWye | Posted in Green Building Techniques on

I am gong to be re-siding my house (cedar) and am considering adding 1″ or 1.5″ of EPS foam on the exterior. My concern is that the current R13 insulation (2×4 walls) has a kraft faced vapor barrier. I know that I can’t use XPS foam on the exterior because it is impermeable, but I know that EPS has a higher permeability, so I was hopring this would work and allow drying to the outside. I will be using a rainscreen over the foam. I am in Portland, OR which is in zone 4.

Would this work? Is it even worth adding the foam to an R13 wall?

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  1. jklingel | | #1

    This may help you decide. These are the perm ratings of each.

    Extruded Polystyrene 1” 1.2
    Expanded Polystyrene 1” 2-5.8
    Polyisocyanurate (?) or polyisocynene (sp?) is another option, but I don't have any specs on either.
    Is insulating worth it? Absolutely. Your R13 wall is more like R10 over the whole wall (excluding windows and doors) and rigid foam sheathing is a good, continuous thermal break.

  2. GBA Editor
    Martin Holladay | | #2

    Q. "My concern is that the current R13 insulation (2x4 walls) has a kraft faced vapor barrier."

    A. Your concern is groundless. Kraft paper is not a vapor barrier, only a vapor retarder. Such walls dry to the interior if damp.

    Q. "I know that I can't use XPS foam on the exterior."

    A. Yes you can -- if you want to.

    Q. "I know that EPS has a higher permeability."

    A. That's true, but in this case it may not matter.

    Q. "Would this work?"

    A. Yes.

    Q. "Is it even worth adding the foam to an R13 wall?"

    A. Absolutely.

  3. U7XY49xWye | | #3

    That's good to hear. That gives me a lot more flexibility of what foam I can use.

    I had read on the Western Red Cedar Lumber Association's website that foam sheathing can cause mositure to accumulate on the back side of cedar siding ( Is this true? If it is, wil it even be a concern since I will have a rainscreen?


  4. GBA Editor
    Martin Holladay | | #4

    If you read the page you linked to, you'll see that the Western Red Cedar Lumber Association has several recommendations on steps to take when installing red cedar siding over foam sheathing.

    My own advice is: never install wood siding over rigid foam sheathing unless you have an air gap between the siding and the foam. The WRCLA notes, "In severe climates, an air space between siding and rigid foam sheathing can be created by fastening furring strips to the sheathing before installing siding (see Wall Construction). Air spaces allow for the venting of accumulated moisture." I think that advice is appropriate for ALL climates.

  5. albertrooks | | #5


    Not to doubt you, but the reference that you quote for the extruded and expanded polystyrene are from the Spray Polyurethane Association. The styrene boards are not really in their field and the values seem a little higher than I expected. The document notes at the bottom of the page say:

    "Note: The above figures represent approximations from a variety of published sources. When determining moisture vapor drives for a particular system, use thermal resistance and perm ratings provided by the manufacturer for each specific product."

    Regardless, both permeability "per inch" numbers get "halved" as you increase thickness by an inch, so as the thickness increases, the permeability quickly decreases.

    Note to David: I'm only questioning the quoted permeability of these materials. This should not dissuade you from adding exterior insulation to your project. The only question for your application is if you add enough foam. Or add mineral wool which would increase your walls drying ability along with resisting pest infiltration.

    Btw... If you add 1"-2" mineral wool, use 8lb. density and a fully threaded screw (threaded from tip to top).

  6. U7XY49xWye | | #6

    I think I have decide to install 1" XPS foam over the existing plywood with the WRB over the foam. My question now is how to flash the windows.. The windows are all wood with 5/4x6 casing attached to the wood frame. I looked at the flashing details on this site, but it looks like all of the details have the cap flashing between the head casing and the window frame. I will not be able to do this unless I remove the casing from the windows (which I would rather not do).

    I have drawn up a few flashing details that I think will work with the flashing over the casing, but I would like to get any opinions on them.


  7. U7XY49xWye | | #7

    Just checking if anyone would have any comments on my flashing details in the post above?

    Thanks in advance!

  8. user-1083543 | | #8


    I think a far safer approach (in terms of vapour management) to using foam on your exterior would be to use an insulating product with a very high vapour permeability. Roxul is now manufacturing a product for just such a use, and it is called "Roxul Comfortboard IS." (The "IS" stands for Insulated Sheathing.) You can spec it in a variety of thicknesses, and it comes in 2'x4' pieces. The vapour permeability of Roxul is about 300 imperial perms (extremely vapour open), while EPS foam is only about 13 perms. So, there is no comparison in terms of breathability. Roxul is also unaffected by moisture, cannot sustain mold growth, will not harbour insects the way that foam can (and does), and it is fireproof. Not to mention it is made of natural materials, rather than petrochemicals. And it also cuts beautifully with a plain old bread knife!! Simply use large heading roofing nails to attach it until you get your rainscreen on over top. The one disadvantage is that it is kind of itchy, but if you are cutting it with a tablesaw, you won't be covered head to toe with small beads of insulation! Here is a link to the Roxul website's Comfortboard IS page:

    Hope that helps!

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