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XPS Between Studs and Plywood Sheathing

Artemisia | Posted in General Questions on

I live in zone 4C and am preparing for a build next spring. One wall assembly that I don’t hear about is putting the rigid foam (XPS) between the plywood sheathing and the studs, instead of on the outside. Also doing a house wrap, rain screen, and siding…probably Hardie, maybe metal. As far as the green concern on XPS, the blower agents have been radically brought down in recent years, and that trend will continue. So, the advantage of having the foam on the inside, is thermal break of course, but there is also a seismic advantage! In testing this wall style, some of the building scene folks in Portland have concluded it improves seismic resilience by 400%…which sounds too good to be true, but that’s the real number. See:
Thermal Break Shear Wall at Sage Green and Plywood over Foam Martha Wall @  (Building Envelope category)
Living in the PNW, seismic strength and fitting in with common building assembly style, as Martha says, is a win/win.
I am a huge fan of so many of the conversations on this site…any thoughts?

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  1. Patrick_OSullivan | | #1

    > One wall assembly that I don’t hear about is putting the rigid foam (XPS) between the plywood sheathing and the studs, instead of on the outside.

    Long story short: not worth it. The main benefit of foam on the outside is that it's dramatically increasing the R value of the locations where you have wood all the way inside to out (i.e. where the studs are). The premise in this article applies to rigid foam between studs:

    Additionally, as often pointed out here, XPS is the worst environmental choice when it comes to foam due to its chemical components.

    Edit: Leaving the above here but it was pointed out that I misunderstood the question.

    That being said, because no one else has done the hard work of making sure this assembly works in your area, it would likely need to be blessed by an engineer. Or, you would need to satisfy other prescriptive measures of the code without structural sheathing, at which point the fact that your sheathing is outboard of the foam does not matter.

    1. Expert Member
      MALCOLM TAYLOR | | #2


      I think Amy is suggesting using foam between the studs and sheathing - a sort of homemade Zip-R.

      1. Patrick_OSullivan | | #3

        Reading is fundamental. Thank you for pointing out my mis-comprehension!

        1. Expert Member
          MALCOLM TAYLOR | | #4


          I made the same mistake. Amy's writing was clear, but it's such a common question I jumped to the same conclusion as to what she was proposing.

  2. Expert Member


    The increased seismic performance in very interesting and it might be worth doing it just for that. The argument about a thermal break misses that no matter where the foam is you get the same benefit.

    I assume the labour is about the same whether the foam goes inside or outside the sheathing. Detailing the flashing and siding would be easier with the foam inside. I don't know which is best for the sheathing: unimpeded drying into the rain-screen cavity with the foam inside, or the added protection it gives keeping the sheathing warmer if placed on the exterior? I'm sure others will chime in.

    1. charlie_sullivan | | #7

      I'm glad you are paying attention to the climate impact concerns with XPS. It is possible to get XPS with reduced climate impact, but please be careful. It is also possible to still buy some that has no reduction in impact whatsoever. And in fact, most of what is being used this year is still just as bad as what was being used 10 years ago. The easiest way to avoid that is to buy EPS or graphite-infused eps ("GPS" or Neopor) which simply does not have that problem, and with gps, has the same r value per inch so it's a drop in replacement.

      The other option is to get Owens Corning ngx foam. Owens Corning is still selling the bad stuff, but if you special order ngx you will avoid about 90% of the problem. That's a lot better than any other kind of xps, but it's a little silly because you can avoid 99% of the problem by getting GPS foam board instead.

      1. Artemisia | | #9

        Thank you, Charlie! I was intending to do the Owens Corning ngx...but will certainly look into the GPS!

    2. Artemisia | | #10

      Yes, Malcolm...the closer I can get to regular procedure, the more likely I can get a builder I can afford. Venturing into any HP with builders can up the build surprisingly. I am being very selective on my deviations. Thank you.

  3. Expert Member
    BILL WICHERS | | #6

    I would think putting the foam between the sheathing and the studs would weaken the assembly due to all the nails in shear through the foam. Maybe the nails passing through the foam provide a bit of springiness though, to absorb energy instead of failing outright? This is interesting. Do you happen to have a link to the testing that was done?


    1. Expert Member
      MALCOLM TAYLOR | | #8


      This gets you a description of the testing, but the link to the code evaluation is broken.

      1. Artemisia | | #12

        Well darn, I have seen the numbers in my endless searching...will bring a link here if I can relocate the site.
        Here is the entire paperwork on it:

    2. Artemisia | | #11

      It is SO interesting, Bill! As a bit of an earthquake, um, awarenesses? person, I am happy to see anything helpful on that particular subject. I am also considering a raft slab foundation...looking at the Legalett systems now. Does anyone have experience with the warm air concept for heating the slab? I had thought I would go with hydro-radiant floors, but have backed off that...was surprised to see this idea. As far as weakening the wall assembly, I actually had a very informative consult with Martha Rose, and she has had great success with it....even in a small community build (she is a spec builder). Thank you for your thoughts.

      1. Expert Member
        BILL WICHERS | | #13

        I understand your interest in earthquake safety technologies. While I'm not in a seismic zone myself, my wife's building collapsed with her family inside (they survived) in a devastating earthquake that destroyed their city when she was young, so I do have some personal interest in these things myself. I'm big on safety, and try to be knowledgeable about things even outside of my own field. It doesn't hurt to have some peripheral knowledge of the "other stuff" when consulting on projects.


        1. Artemisia | | #14

          Absolutely Zephr7...did you see that I brought the data link to the conversation? It is an exciting bit of info...and surprised the designers even. Thank you.

          1. Expert Member
            MALCOLM TAYLOR | | #15


            With an aggressive nailing pattern it makes sense. Shear-walls avoid using adhesives for the same reason. A bit of flexibility is good. It also explains how Zip-R has been able to get approval for its panels.

  4. Artemisia | | #16

    Hello there, I just uploaded the specs on the interior foam study, along with the seismic results.
    Thank you for your comments and interest. See comment #12

  5. Expert Member
    PETER Engle | | #17

    This is an interesting concept. I admit that I also thought that the intermediate foam would weaken the wall. Looking at the test report, the intermediate foam makes the wall more flexible, but the ultimate strength in lateral loading is slightly greater. Pretty cool. Note that the perimeter nailing is done at 3" on center, rather than the code-standard 6" on center for braced wall lines. Considering that there are two lines of nails (one for each panel) where panels are joined on a stud, this is a LOT of nails in the stud. Seems to work, though.

    The flexibility could be an issue for finishes though. The tested wall survived structurally, but deflected almost 5" laterally during the test. With this much deflection, the building might survive but the finishes might not. Good for building safety, but not so good for the budget. I'm thinking now of less-than design events. Moderate wind or seismic events might cause enough deflection to crack walls, pop moldings, even potentially break windows. A building with this sheathing technique in environments where moderate loading is common might experience above-average maintenance costs.

    Seems like this technique has had quite a bit more research done on it since the pilot project, though. Certainly ZIP-R sheathing is a similar design and they've done quite a bit of engineering and testing. I can't recall any cases of increased flexibility causing any problems. If the OP is really headed towards this type of system, I would think that ZIP-R would be the way to go. Foam and sheathing in one pass. Easy, Peasey.

    1. Artemisia | | #18

      The ZIP-R is made with OSB? I am nervous about the wetness issue, plywood can dry out over and over with no problem. On the flexibility aspect, if I can make it through a big shake with only the siding popping, or a window breaking, I would be very pleased :)

      1. Expert Member
        BILL WICHERS | | #19

        Zip-R is made with OSB and their green WRB coating. It's generally thought that Zip is a higher grade of OSB than the regular OSB you'd buy as OSB though, and Zip is generally regarded as a quality product. It has a good history and is used a LOT.

        Plywood CANNOT dry out "over and over" and be fine -- it will eventually delaminate. The "X" in things like "CDX" means they are using an eXterior rated adhesive, and the "rated" sheathings are rated to be exposed for a few months during construction without having major problems. That doesn't mean you won't have ANY problems though, and it certainly doesn't mean it can be cycled wet/dry forever with no problems. Plywood itself should be kept as dry as possible in an assembly, but it does usually hold up better than OSB when something goes wrong.

        I don't have any specific information as to how Zip's OSB compares to plywood in terms of water resistance, but I wouldn't have any problem using Zip on my own home.


  6. Artemisia | | #20

    Hmmm, thank you Bill. I will look into the ZIP system a bit more deeply. Because most issues have such a diverse set of issues and views, I try to narrow things down...endlessly, haha.

    1. Expert Member
      BILL WICHERS | | #21

      There are lots of people on here who have used Zip, and others who prefer plywood. Everyone can give you info based on their expierience with those products, so if you have some specfic concerns there is a good chance someone on here will know the answer. Learn from those who have gone before, basically :-)

      A lot of the pros and cons of any product will depend on the particulars of the specific installation you'll be doing, so the more info you can provide as to exactly what your needs and concerns are, the better the responses you'll get back.


  7. Artemisia | | #22

    I will be interviewing builders this summer to see what their preferred practices are...I will be doing a lot of the finish work, but I am dependent upon a builder/crew to get me to the dried in part. It is a somewhat rural build on the coast, wet and windy for much of the year. Moisture intrusion is a big concern, also wanting a builder to work with a system he might be unfamiliar with seems an unintelligent move. Aiming for something along the lines of a Pretty Good House, a great idea, but certainly not embraced by the general, general contractor :) I will check back in as more questions and judgements arise. The basic envelope is as simple and quiet (architecturally) as I could make it. Thank you!

  8. drewintoledo | | #23

    Amy, it sounds like you’ve been thinking through your build. I’d be interested to hear what you end up with. Do you have any threads on here with your floor plan? I’m betting it’s simple yet interesting.

  9. Artemisia | | #24

    I do not have plans on this site, but here are the basics: It is a 1 and 1/2 story...meaning only a central section has the second floor, The central section downstairs is a kitchen-dining area-living room. No cathedral ceilings, 9'. on each side is one bedroom and one large bathroom is also a laundry room. The upstairs will be my studio, a single, long rectangle. This is not my first building project, but definitely my most comprehensive. I will be a grandmother soon, and am creating the set :) Thank you for being curious, Drew.

    1. Expert Member
      BILL WICHERS | | #25

      When you do the upper level, try to get the roof line such that you have full-height walls on the attic sides of the second story area. This will make air sealing and vent channels easier (assuming you're doing a vented attic). Having shorter knee walls here complicates a lot of things.

      There are some specific air sealing details that are important in houses of this style too. These details have been discussed in the forums here before, and I think there are some articles about them too. Make sure you're familiar with the specifics so that nothing gets missed during your build.

      BTW, I'd plan for a few permanent lights in the attic spaces with a switch near the access hatch to. This will make any future maintenance work a lot easier. It's not a bad idea to have a service outlet in any accesible attic spaces too, since that lets you plug in tools or extra lights when needed.


      1. Artemisia | | #26

        Thank you, again Bill! I have read several articles on the short wall issue but do not have a working model in my head...I will work on that today! :) Not sure I can get to full height, with the silhouette I am aiming for. I have played with it quite a bit, will be meeting with my professional drawing person this week, looking forward to seeing what kinks can be improved upon. I will have no actual attic, it will all be used space. Maybe a 13' peak, down to 4-5 foot short walls...I am making a landing and turn in the stairs to get legal headroom. I think the width will be somewhere around 20', length is 36'... but I could be wrong. I have looked at the "room in the attic" trusses...really not sure what is reasonably possible. I thought Risinger's show on cutting off the rafter tails, sheathing/insulating all the way up, and THEN attaching the overhangs was kind of genius. We shall see, I feel fortunate to be even doing this project.

        1. Artemisia | | #27

          PS I am hoping to do raised heel trusses...I believe that will contain the behind the short walls into the conditioned space? Or is there more to that equation?

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