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Community and Q&A

Vapor sandwich with foamboard on both sides of framing?

michaelbluejay | Posted in Green Building Techniques on

I have a 2×4-framed wall with very little cavity space for the cellulose insulation I plan to install, because windows comprise a lot of the wall, and because headers above the window take a lot of what’s left. My plan to increase the insulation was:

– Install 3/4″ foamboard, sheathing, and siding to the exterior.
– Add 2×2’s to the interior framing, converting it to 2×6 framing.
– Install cellulose into the cavities.
– Glue 2″ of foamboard to the headers.

But now i’m worried that putting foamboard on both sides of the headers creates a vapor sandwich which could promote moisture accumulation, mold, and rot. What say the experts? Thank you for your help!

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  1. Expert Member
    Dana Dorsett | | #1

    Not all foam board is created equal. Unfaced Type-II EPS at 1.5" (the nominal thickness of a 2x2) would have a vapor permeance between 1.5 - 2 perms, and is not a major concern.

    1.5" of XPS would run about 0.8 perms, a minimal class-II vapor retarder, still not of much concern.

    Foil faced polyiso is a Class-I vapor retarder at any thickness but if the 3/4' goods on the exterior is not foil-faced it's not a big deal.

    Adding a 2x2 to a 2x4 results in a 5.0" cavity depth, not the 5.5" nominal cavity depth of a 2x6. Installing them perpendicular to the 2x4 rather than simply adding depth results in much lower thermal bridging. (Sometimes called a "Mooney wall", named after a proponent of this approach.)

  2. michaelbluejay | | #2

    Thank you so much, this is extremely helpful!

    I was planning on ripping the lumber to get add true 2" of framing, to match the 2" of foamboard I'd be adding to the header.

    Let's say I installed the foamboard only on the inside. If I installed housewrap like Tyvek over the ply/OSB sheathing, does that create a vapor sandwich problem? Is it okay to not use housewrap?

  3. Expert Member
    Dana Dorsett | | #3

    Tyvke is 30 perms or higher.

    3/4" XPS is 1.5 perms

    3/4" Type II EPS is 3.5-4 perms.

    3/4" foil faces polyiso is less than 0.5 perms

    The effect on vapor permeance of the stackup exterior to the sheathing is unmeasurable, and within the manufacturing variance of the foam sheathing.

    BTW: Unless you live in US climate zone 4 or lower, you will need more than 3/4" of foam on the exterior of the structural sheathing for dew point control on 5.5" of fiber insulation without a Class-II or lower vapor retarder on the interior side of the assembly. See:

    Where is this house located?

  4. GBA Editor
    Martin Holladay | | #4

    Here is a link to an article that discusses the installation of rigid foam on the interior side of walls: Walls with Interior Rigid Foam.

  5. michaelbluejay | | #5

    Thank you both again for the continued help.

    I'm in Austin, 78722, climate zone 2.

    Martin, your article said, "Will there be any problems if your walls can’t dry to the interior?
    The answer is no — as long as your walls can dry to the exterior. So if you’re planning to install interior rigid foam, you shouldn’t install any exterior rigid foam." That suggests I shouldn't go with my original idea of 3/4" on the exterior, and 2" on the interior headers, but if I read Dana's first reply, he seems to think that it would be okay. So I guess I'm confused.

    Dana, I'm afraid I didn't understand at all what you were trying to tell me about vapor permeance in reply #3. If I used foamboard on the interior headers, is housewrap then contraindicated on the outside?

  6. michaelbluejay | | #6

    Dana, I think I understand your reply #3 now. I had the scale backwards, so now I understand that higher perms = more permeability. So I think you're saying i don't have vapor sandwich issues by using housewrap, because it lets moisture through so well. Which seems to defeat the idea of housewrap to me in the first place, but I'm off to read up on it now.

    I'm still unsure whether I can put polyiso or EPS on both the whole exterior wall and on the headers (which are maybe 1' or more high and run the entire length of the ~13' room).

  7. GBA Editor
    Martin Holladay | | #7

    Sandwiching a wall with foam on both sides is unusual, so you have to be careful. This approach is less risky in mild climate zones like yours than in extremely cold climate zones, so you have that in your favor.

    Dana is right. You can proceed with your plan, as long as:

    (a) You use EPS, not XPS or foil-faced polyiso, and

    (b) The wall shows no signs of water-entry problems.

  8. Dana1 | | #8

    Spelling it out in the simplest terms possible:

    You absolutely can NOT install foil faced goods on BOTH sides of the assembly.

    With 2" of any type of foam on the interior you can't safely put foil faced goods on the exterior.

    In Austin you don't need to worry about dew point control from interior moisture drives, but you still need a drying path. With 2" of foam (any type) on the interior the sheathing will have to dry toward the exterior, thus unfaced 3/4" EPS is safer than 3/4" XPS.

  9. michaelbluejay | | #9

    Thank you again both for your help! I have "innie" windows, so I think I install, from inside to outside, asphalt paper, foamboard, OSB/ply, then siding. Is that right?

  10. Expert Member
    Dana Dorsett | | #10

    An innie stackup goes

    inside air

    interior paint


    studs & fluff

    OSB or ply

    asphalted felt or housewrap (crinkle type housewrap preferred)


    furring (or siding, long nailed)

    siding & paint

    outside air

  11. michaelbluejay | | #11

    Thanks again guys, this is exceptionally helpful! I'm excited to get more insulation on my walls than I would have gotten had I just let a contractor do a typical rebuild of the wall.

    I think I'm going to skip the "outside air" layer though because they don't stock it an my local Home Depot.

  12. GBA Editor
    Martin Holladay | | #12

    You can order it online. (See below).


  13. michaelbluejay | | #13

    Two changes: first, the insulation installer says that 2" is deep enough for him to install cellulose, so I can put 2" nailers onto the headers and get cellulose there, so I don't have to have any rigid insulation on the interior, so I can use greener materials and I won't have vapor sandwich worries.

    For the 3/4" exterior, it turns out that Home Depot doesn't stock EPS and Lowe's has only foil-faced EPS. Is the foil a no-go for my application (which is now for 3/4" rigid on the outside only), or is the problem only with foil-faced polyiso?

  14. Expert Member
    Dana Dorsett | | #14

    There's no problem with foil facers in your climate as long as it's only on one side of the assembly. In Austin having the foil facer on the exterior with a bit of air space to the siding would give it a modest boost in performance, and would make it easy to air seal with a decent quality foil tape.

  15. michaelbluejay | | #15

    The wall is built, and now I've found this in another article:

    "Some experts (including Joseph Lstiburek) note that a wall sheathed with OSB, which is more vulnerable to rot than plywood or board sheathing, should almost always include a rainscreen gap. Lstiburek has written, 'We learned through trial and error (mostly error) that if you use OSB, really good cavity insulation and a housewrap, make sure you have an air gap between the cladding and housewrap/OSB interface.'”

    Does this not apply to my case because I'm in a mild climate and am using only 3/4" foamboard? If it does apply to me, I wish reply #10 above had recommended ply instead of OSB, and a rainscreen.

  16. GBA Editor
    Martin Holladay | | #16

    It's hard to provide an encyclopedia of building advice every time that a reader posts a question.

    The answer to the question, "What's better: plywood or OSB?" is "Plywood," but that doesn't mean that your wall will have any problems. For more information on this issue, see Wall Sheathing Options.

    The answer to the question, "What's better: a wall with a rainscreen gap or a wall with no rainscreen gap?" is "A wall with a rainscreen gap." That doesn't mean that your wall will necessarily have problems. For more information on this issue, see All About Rainscreens.

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