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Exterior Wood Fiberboard Insulation with WRB

thierry19 | Posted in General Questions on

I am planning to build a wall with exterior wood fiberboard (MSL Eco4, R4, 26 perms) over 2×6 filled with Rockwool. The fiberboard will be covered with a regular WRB membrane, double furring to create a rainscreen cavity, and finally covered with vertical metal siding. Drying will be exclusively toward the exterior since I will be using a foil-faced polyiso board as my vapor-barrier on the warm side. Construction zone 6, Prov. of Quebec

I recently saw the 2021 Sweet 16 wall design contest and saw comments from Christine Williamson on two walls with exterior wood fiberboard covered with WRB housewrap. Her comments were that she found those assembly risky and that “…it is not practical to detail an unsupported membrane that is continuous enough in this application”. She suggested in one case to cover the WRB with exterior gypsum sheathing and in the other case using a drainage mat or textured WRB to allow some drying between the furring and the wood fiberboard. Both walls have rainscreen cavities.

I’m wondering if I should be concerned with my wall as detailed above, despite having a generous rainscreen cavity. Are we talking about a purely theoretical risk or are there documented issues with fiberboard insulation covered with a standard WRB membrane (beside sloppy installation that is). If I’m confident my builder will do a good job installing the WRB, should I change something to my wall to add another layer of protection? Adding 2000sq ft of drainage mat would be very costly. A textured wrap would be more feasible.

I realize that wood fiberboard used on the exterior side might not be as resilient as plywood if exposed to water but exterior wood fiberboard is apparently common in Europe and likely to become more popular in North America as well. Should these walls be protected differently than walls with plywood or OSB?

Thank you for our guidance


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  1. Expert Member


    Can you tell me which of the assemblies Christine was talking about? There are a lot to wade though.

    I don't understand her remark. What does she mean by the WRB being "unsupported"?

    1. strausjw | | #2

      I think "unsupported" refers to the fiberboard not holding staples well and therefore making it difficult to attach the WRB. Of course, once applied, the rainscreen furring should be sufficient to clamp it down. Personally I do not know if the fiberboard can hold staples or cap nails well enough (at least tempoarily) to get the WRB on. May be worth testing out or exploring an adhered WRB. Have you asked MLS what they suggest?

  2. maine_tyler | | #3

    I know nothing about the MSL Eco4 product you've linked, but I did glance at the Sweet 16 comments you are referring to. It's confusing that the author's of the comments are not labeled, but assuming the fuchsia color is Christine, my take is that she is nervous about wood-fiber in general or even has something against it (or at the least doesn't see its benefits).

    She states that "I do not view the carbon savings from one material or another as significant enough to influence my design decisions, but other people do. If you're going to use something more moisture sensitive, then you'll want to take the added step of protecting it. Note: the manufacture of this material surely disagrees with me and believes this material to be water resistant enough for this application."

    So I guess we can choose whatever materials we want, regardless of carbon. It's not as if a building's sum embodied carbon is a collection of its individual components. ;) To be clear, I'm not saying wood-fiber MUST be used on a basis of carbon, but to write off the benefits it has in this regard seem suspect.

    Then the other assembly where the WRB is outboard of the wood-fiber sheathing, she brings up the 'unsupported membrane' issue. And again, even with an outboard WRB, cites concerns with wood-fiber moisture sensitivity. Huh? It really doesn't make a lot of sense to me.

    The use really does depend on the product, but some wood fiber products are actually designed to be used solely as the WRB. They are impregnated with (I believe) paraffin and some other stuff, and have been tested as well as used in those applications. This may not be true of all wood-fiber insulation products. I would check with the manufacturer. If concerned either way, I would say use an outboard WRB. You know, like how we do on all our wood walls. How is it different?

    Note, that 'fiberboard sheathing' describes a different set of products than the more European (and coming to U.S. market) wood-fiber insulations like Gutex and Steico (and soon to be TimberHP).

    1. andyfrog | | #24

      it should be noted that Straube and Lstiburek are similarly adverse to exterior wood fiberboard and exterior cellulose insulation due to moisture sensitivity concerns. I haven't seen any of the three explain in detail why, but it'd be interesting to hear from them in more depth.

      1. maine_tyler | | #25

        Do you have a link to where they cite their concerns?

        What is exterior cellulose?

        1. andyfrog | | #26

          I am personally in favor of switching to wood fiberboard despite its relatively lower vapor permeance and potential moisture sensitivity compared to mineral wool because it seems like a sensible product supply chain. So that's why I'm interested in figuring out why these three relatively notable building scientists are not in favor of it.

          An example of exterior cellulose would be cellulose that is dense packed into bays made of TJIs projecting off of exterior sheathing wrapped in a WRB. I suppose the terminology might be more semantic, but using "exterior" here to refer to the position of the insulation relative to the sheathing condensation plane.

          Straube talks about WRBs being exposed to temperature extremes when outboard of insulation, and mentions moisture sensitive exterior insulation briefly:
 (keep watching through 15:05)

          Straube responds to a question about how insulation is protected from absorbing water by saying the continuous insulations typically used are foam and mineral wool and are all able to tolerate moisture by design. He specifically calls out batt insulation and cellulose as things he can't use:

          Williamson talks about moisture sensitive materials outboard of the water control layer in two separate instances:

          Lstiburek also indirectly mentions moisture sensitive exterior insulation briefly:

  3. maine_tyler | | #4

    Oh, and she also cites concern with wood-fiber insulation because 'it burns.'

    She must hate foam then. (to be fair, it seems like she might prefer mineral wool, which is probably the bees knees in fire resistance).

  4. Expert Member
    AKOS TOTH | | #5

    The only issue with fiberboard sheathing is that it is hard (impossible?) to detail as an air barrier. I haven't use the ECO4 but it looks very close to their Sonopan which is a fairly loose fiber, so air will blow through it. The fibers are also soft so taping the seams is questionable.

    This means for an air tight assembly you are relying on an interior air barrier which is harder to detail, test and fix. You could also detail your house wrap as an air barrier, I find this hard to do as it can easily "balloon" from positive pressure which creates a lot of 3d airflow paths.

    In terms of durability, provided as you have a sheet WRB over the fiberboard and your rough openings are flashed properly, there is no issues with using it. Since the material is vapor open any moisture from the inside can easily move through it and be vented into the rain screen cavity.

    Since you are installing a rain screen, textured WRB won't do anything, a standard heavy duty house wrap (ie Tyvek Commercial) is all you need.

    One option is to see if the ECO4 is a suitable substrate for one of the fluid applied WRB products. Since these form a solid film, they could be used to air seal at the sheathing layer.

  5. Expert Member
    Josh Salinger | | #6

    Hi Thierry,

    I couldn't find Christine's comments in the link, but I wanted to comment nonetheless as we use a similar wall assembly often here in Portland, OR (CZ4C). I tend to agree with Christine in that I don't have the risk tolerance to use the unprotected wood fiberboard as a WRB as some of the product manufacturer's claim one can. One of the issues is that when it is cut it leaves an exposed edge that isn't as protected and the wood fiberboard has a low 'storage capacity of materials' and can potentially lead to durability problems. Admittedly, different brands of these products are made differently and some such as the Steico have a waxy covering on the exterior (and likely some in the interior), but when it is cut and exposed the waxy surface isn't there to protect the cut area. The way we deal with this is to place the WRB to the outside of the wood fiber insulation to protect it and use is as our thermal control layer and not as our bulk water control or air control layer. We do choose this product based on its low embodied carbon as we believe this is a critical choice one should make when choosing a product so long as the product is robust and installed in a durable way (the first and most important thing, I'm sure Christine would concur). The difference in our wall assembly is that we use a layer of plywood sheathing on the framing which acts as our main vapor control layer and air control layer. When the wood fiberboard is attached to the sheathing (with the sheathing seams taped for the AB) it gives it positive support. We then temporarily tack on the WRB with cap staples (it holds fine) and then install the rainscreen battens through the assembly which is what permanently holds the WRB in place. We are in a seismic zone, so the sheathing is required, but this may not be the case where you are and is why, I'm guessing, you aren't using any sheathing. I suppose I would question where in your assembly the air barrier is, maybe you have it on the interior with the foil faced polyiso? I could see an issue with the temporary attachment of the WRB on the wood fiberboard with no sheathing behind it as you could damage the wood fiberboard when temporarily tacking on the WRB. One would also want to make sure that the air barrier is protected and continuous througout the assembly (floors to walls to roof transitions). I don't know that a textured WRB would be necessary, but it certainly wouldn't hurt anything. If you were to switch to a hydrogap or similar mechanically attached WRB that could be cheap insurance. I've attached a recent install where we did this exact thing and covered it with metal siding (you can see the temp cap staples in the pic)

    1. Expert Member
      MALCOLM TAYLOR | | #7


      Thanks for weighing in. I agree that Christine's remarks make sense read as "unprotected" rather than "unsupported", but I'm not sure that's what she was getting at.

      One thing still puzzles me in your rely. If the WRB is only in contact with the vertical furring strips, what advantage does a dimpled product like Hydrogap give you?

    2. maine_tyler | | #9

      "I tend to agree with Christine in that I don't have the risk tolerance to use the unprotected wood fiberboard as a WRB as some of the product manufacturer's claim one can."

      That seems a reasonable concern (though is it scientifically justified, I don't know-- do Europeans have enough experience with it yet?), but Christine's concerns go quite beyond that into some odd territory, imo.

      The textured drain wrap, she says, is so the wood insulation can dry where the vertical strapping is. Really?
      Gypsum sheathing is recommended because the insulation isn't rigid enough to be a backer for taping the WRB. Is that true with the Eco4? I have not used rigid wood fiber, but I have touched Steico, and my understanding with both Steico and Gutex is that they are pretty darn rigid. It seems like an odd concern.

      Nice to hear from those that have worked with it.

  6. Expert Member
    Josh Salinger | | #8


    Re: my comment on the dimpled WRB, It is really splitting hairs... The dimples on the WRB behind the furring strips would keep the furring strips from contacting the WRB directly and it would reduce hydrostatic pressure at this contact point. I was trying to address the comment about the dimpled WRB and that was the only benefit I could ascertain. To be clear, I don't think it is necessary-- rather a cheap insurance that wouldn't hurt anything if done.

  7. thierry19 | | #10

    Thank you all for your comments.

    Josh, this is very useful, thank you.

    I've included print screens of the comments for those who couldn't find them.

    So both walls that she commented on have a WRB membrane over the wood fiberboard and both have a rainscreen. One of the walls also uses the Eco4 panels that I plan on using.

    Her comment about the WRB: "The first issue is that the water control membrane over the wood fiberboard insulation is unsupported. The manufacturer no doubt tells you that this is okay.... but in my experience, it's very very difficult to detail these types of sheet membranes to be watertight unless they're fully supported by something rigid." 

    She recommends exterior gypsum over the fiberboard to support the water control membrane.

    I'm a little puzzled by this. Gutex and Steico wood fiberboards are pretty rigid. So is the Eco4. It actually doesn't need additional bracing when used in a non-seismic zone. I don't see why installing a WRB over wood fiberboard would be more challenging than over OSB/plywood, especially when the WRB is held in place by the battens.

    My primary air barrier will be the interior foil-faced polyiso board which is also my vapor-barrier. I'm adding a 2x3 service cavity to limit the number of holes in my air barrier. I don't think I can tape the Eco4 as Akos mentioned, however I will try to do a good job at taping the WRB as a secondary air barrier. The Eco4's main use is a thermal break over the studs.

    I also looked at switching to the Hydrogap WRB to provide some drying behind the battens, just for added protection.

    Anyway, I'm still not too sure what her comment was about but your feedback is great and I'm confident my wall will be fine.

    Thank you.

    1. Expert Member
      MALCOLM TAYLOR | | #11


      GBA has featured quite a few assemblies - both walls and roofs - with no sheathing at all, which relied on taped sheet WRBs. I'm not convinced it's more than a theoretical concern.

  8. AlexPoi | | #12


    I'm in Quebec as well. Eco4 is fine with a WRB on top of it. It's a pretty ridgid material not to be confused with the cardboard that some builders use in the US. I have seen plenty of passive houses built here and in Ontario with this material. I would upgrade to a peel and stick membrane though if you can afford it, that would make the wall more robust.

    In your wall assembly, I would be more concerned about the polyiso vapor barrier. Summer can be pretty warm here and if you plan to have an AC, you could have moist accumulation in your wall. Your rain screen will mitigate that concern though.

    1. Expert Member
      MALCOLM TAYLOR | | #14


      Does the increased drying capacity to the outside a rain-screen adds really mitigate concerns about summertime moisture accumulation on a wrong side vapour-barrier?

      1. AlexPoi | | #15

        Good question Malcolm. I'm not sure but in my opinion the rain screen will add a bit of drying since the metal siding is almost a perfect vapor barrier otherwise and it will act as a capillary break in place where his siding is leaking.

        I'm not a fan of having a class I vapor barrier on the inside so I wouldn't take the chance.

  9. thierry19 | | #13

    Hi Alex,

    Thanks for your advice. Will a peel-and-stick WRB stick well to the Eco4 board? I also want to know which tapes would work well. Akos mentioned above that the texture of the Eco4 would be challenging for sticky membranes/tapes.

    I would like to talk to someone at MSL but they are not returning my calls or messages. Very frustrating.

    Regarding the polyiso board, from what I understand this should be fine. The exterior surface of an interior polyiso board will always be much warmer that the inside air with A/C. So summertime condenstion shouldn't happen.

    This assembly is actually one of the recommended assemblies by Quebec's Novoclimat program.
    See page 6.


    1. AlexPoi | | #16

      From what I've read, it will work as long as you apply a primer. Wood fiber being a bit rough and quite porous, it can take quite a lot of primer though. You'll want to use good quality acrylic tapes (3M acrylic tape, Siga, Proclima, Delta) which are more expensive but stick to almost anything (the red tuck tape that everyone use here is crap). Same thing with the membrane. You could always test it on one sheet to see how it goes before commiting to that assembly.

      About the vapor barrier, it's never a good idea to have a perfect interior vapor barrier unless you live in the arctic circle. When it's hot outside and cold inside the vapor drive will be toward the inside of the house not outside. So if you have a leak and water get in your wall, it won't be able to dry to the inside and you could end up with mold.

      I think in Quebec (and eastern Canada in general), we are a bit obsessed with vapor barriers but it's just a terrible idea to put a perfect vapor barrier on the inside. I think it's because most people confuse a vapor barrier with an air barrier. See this article (they have a french version as well).

      What you want is an interor air barrier and a Class II vapor retarder so you'll reduce the vapor transfer in winter but your wall will still be able to dry in summer if it get wet. The code says your vapor barrier should have a dry perm rating of 1.0 or less. Smart vapor barrier, osb + vapor paint barrier or 3/4 Osb alone would qualify. In my opinion, it's a bit too strict and I would not be afraid to use 1/2 Osb with your assembly but the code is the code...

      1. thierry19 | | #19

        Hi Alex,
        We are veering off-topic a bit but interior foam can work well too. See Martin's article below which summarizes this very well.  With the current cost of OSB/plywood, choosing polyiso is actually cheaper and allows me to add R5 to my wall.  I will detail my interior polyiso boards as a primary air barrier and a service cavity will help protect it from damage.

        I know that the current thinking is toward smart membranes to allow some drying to the inside. I agree that poly sheets may not be the best material, but it has been / is being installed in millions of houses in cold climates and those are not all rotting away.  Foil-faced fiberboard is also being used frequently as an interior rigid vapor-barrier. If you are very conservative with your choice of material, you can argue that smart membranes have only been around for 15 years and that we don't know what they will look like in 50 or 100 years.  Managing the outward movement of moisture in cold climate is critical but it also needs to be as durable as the rest of the building.

        In any case, with a very permeable Eco4 and WRB behind a rainscreen, my wall should easily dry to the outside. Also my metal siding is not a "reservoir" cladding so there should be no inward vapor drive on hot sunny days after a rain.



        Relevant parts:

        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. Most other types of sheathing — including plywood, OSB, fiberboard, DensGlas Gold, and boards — are all vapor-permeable enough to work well on this type of wall.

        Most wet-wall problems are caused by moisture that enters the wall from the exterior, not the interior. (In other words, the big problem is rain, not wintertime condensation — especially if the wall was built with attention to airtightness.) So walls that dry to the exterior work well, as long as the builder has installed adequate flashing. A ventilated rainscreen gap between the sheathing or WRB and the back of the siding isn’t mandatory, but it’s an excellent detail to encourage walls to dry to the exterior.

        What about inward solar vapor drive during the summer? Interior rigid foam prevents any problems from inward solar vapor drive, because the exterior face of the rigid foam never gets cold enough to permit condensation. Even if the homeowners like to crank up their air conditioner until the indoor temperature is down to 66°F, the R-value of the foam keeps the exterior face of the foam warm, so that the foam panel doesn’t present a condensing surface to any moisture lurking between the studs."

    2. Expert Member
      Josh Salinger | | #17


      Tapes don't stick to the wood fiberboard products. Even the good European ones. The self adhered membrane wouldn't work either so I wouldn't recommend it as it would just act like a mechanically attached membrane. We use the liquid applied products if we need to adhere something to the wood fiberboards such as the top leg of flashing (see pic). In this case we used the Prosoco R-Guard product.

      I can speak from experience that the wood fiberboard is plenty rigid to install a membrane over. I think maybe there is confusion on Christine's part. Maybe she is thinking of the beaverboard type wood paneling products?? I'm not sure.

      1. thierry19 | | #18

        Thanks Josh. I'll look at the Prosoco stuff for the fiberboard and keep the tapes for the WRB.

      2. andyfrog | | #28

        Hi Josh,

        Can you comment on this video where they're applying Adhero on top of Gutex?


    3. andyfrog | | #27

      I'm not sure about Eco4, but apparently Solitex Adhero sticks to Gutex: .,

  10. Expert Member
    AKOS TOTH | | #20

    I would not do both exterior and interior insulation. Pick one or the other and use that.

    Interior insulation on a roof is not an issue but is a challenge for walls if you are looking to hang anything heavy down the road. Strapping out the foam or covering with OSB/CDX underneath the drywall would avoid this issue but adds a lot of cost.

    If you want more R value than your 2x6 wall with R5 fiberboard would give, I would look at bumping up the wall to 2x8. About the same assembly R value, about the same material cost as your wall with interior rigid but much less labour.

    If you do go with interior rigid, make sure to talk to your electrician when they are doing the rough in to set the devices boxes at the right depth.

  11. thierry19 | | #21

    Hi Akos

    Well, the interior strapping is needed since we'll be putting shiplap or something else that's not drywall. I also like how my vapor barrier will be protected this way

    You said : "I would not do both exterior and interior insulation. Pick one or the other and use that."

    Would you elaborate on why you think this is not a good idea? My builder doesn't see a problem with this. I was originally going to use a thinner foil-faced fiberboard as my interior vapor barrier, but I thought I might as well add some R value to my wall by using polyiso foamboard instead.


    1. Expert Member
      AKOS TOTH | | #22

      Simply cost.

      Otherwise the wall will work great, foil faced rigid is very easy to tape and detail as a vapor barrier.

      1. thierry19 | | #23

        Thanks for your feedback Akos. Adding insulation should provide a return on investment eventually.... I'm also considering adding insulation in the 1.5'' service cavity, making sure I follow the 1/3 - 2/3 rule.

        thanks again

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