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Musings of an Energy Nerd

Two Views of Double-Stud Walls

John Straube and Jesse Thompson talk about double-stud walls and risk

Double-stud walls are easy to explain. Jesse Thompson tells builders, “Let’s build a house, and then we’ll put an extra wall on the inside.”
Image Credit: Image #1: Peter Amerongen / Riverdale Net Zero

At the recent BuildingEnergy 17 conference in Boston, there were at least two presentations that touched on double-stud walls. John Straube, a professor of building envelope science at the University of Waterloo in Ontario, used his presentation to raise a warning flag, noting that “these walls will work if everything works — if there aren’t any defects — but they don’t work if there is something wrong.”

Jesse Thompson, an architect from Portland, Maine, was one of several presenters at a session called “Evolving Assemblies.” Thompson is clearly more of a double-stud fan than Straube. On many of his projects, Thompson said, discussions “keep coming back to the double-stud wall. We tell builders, ‘Let’s build a house, and then we’ll put an extra wall on the inside.’ It’s easy to explain.”

So who’s right? Is the double-stud wall risky or robust?

Measuring the moisture content of OSB sheathing

John Straube’s comments were made in a presentation called “Moisture Safe? The Writing on the Wall.” His co-presenter was Kohta Ueno, an engineer at Building Science Corporation.

Straube told the audience, “There has been a rise in interest in double-stud walls in the last decade. If labor is cheap, this is an inexpensive wall.”

Over the years, Straube has been involved in many research projects that show that the sheathing on a double-stud wall has a higher moisture content in late winter than the sheathing on a wall with continuous exterior rigid foam.

At the Boston conference, Straube described one of these research studies. (A report on the study cited by Straube can be found on pages 55 through 117 of The Hygrothermal Performance of Exterior Insulated…

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66 Comments

  1. Kaplan Thompson Architects | | #1

    John is right
    Just to be clear (never hurts on the internet...), I agree with John Straube.

    The double stud wall certainly isn't for all project types and John sees a far wider range of climate zones and building types than we do. And to top it all off, he's an actual real live scientist doing monitoring and research, which we certainly are not.

    Double stud has a great history in New England custom single family home building so far. 1 - 2 story buildings with small stack effect at work, careful builders who can quickly learn to install their air barrier as carefully as they do their finish work, and a wide spread network of skilled dense-pack cellulose installers seem to be a good mix for this construction system to be built successfully.

    Remove any of these characteristics and your risk certainly rises. If I was John and had to give recommendations to Code officials and builders all across the country, I would certainly say different things from what I said at the Building Energy Conference.

    Jesse Thompson
    Kaplan Thompson Architects

  2. User avater GBA Editor
    Martin Holladay | | #2

    Reply to Jesse Thompson
    Jesse,
    Thanks for your comments. And thanks for your contributions, here in New England and elsewhere up and down the Atlantic coast, to improved details for superinsulated homes.

    -- Martin Holladay

  3. Skip Harris | | #3

    So, what ARE the most forgiving optimally-insulated systems?
    I'd love to see a consensus (or even a discussion) for roof and wall systems, for each climate.

  4. Charlie Sullivan | | #4

    I-studs with OSB are better than the ones in the test
    To me, the most surprising thing in this article was the analysis that seemed to indicate that I-studs have about as much thermal bridging as regular studs.

    The paper ( available at https://www.brikbase.org/sites/default/files/EE6-1_haavi_final.pdf ) includes more details: They compared 36-mm thick wood studs (1.42") to I-studs with 8-mm thick (5/16") fiberboard webs. The thermal conductivity they measured in the fiberboard is 0.38 W/(mK). And they estimated 0.1 W/(mK) in the studs, based on moisture content. Based on those numbers, the thermal bridging is about 18% worse in the wood studs than in the fiberboard webbing, and the difference gets a little smaller when you consider the 3-dimsional multi-material I-stud.

    However, the typical value most manufacturers publish for OSB thermal conductivity is 0.13 W/(mK). So it appears that this problem with I-studs is only really true of fiberboard I-studs and not ones you can buy in North America made with OSB. I'm not sure whether you can buy I-studs in Europe made with fiberboard; the ones used in the paper were assembled by the authors. But I'm not sure how carefully the thermal conductivity of OSB has been measured--it would be good to know that better for an assembly where it matters significantly.

  5. Jon R | | #5

    In a cold climate, it's going
    In a cold climate, it's going to be warm sheathing and breathable to the exterior (eg, rock wool).

  6. User avater
    Dana Dorsett | | #6

    Fiberboard sheathing for double-studwalls in cold climates,
    With 16" o.c. framing 3/4" asphalted fiberboard is sufficiently structural for most wind loading, and strapped by 1x4 girts 16" o.c. to provide the back-ventilation for siding it should stand up pretty well to dense packing too. (Don't try this with half-inch fiberboard, wider stud spacing, or without girts.) The insulating properties of fiberboard add another R1.5 to the whole-wall performance too, but the real reason to use it is the high vapor permeance and high moisture tolerance.

    In Europe 2" low density fiberboard sheathing is available good for about R5-R5.5, but it's not adequate as structural sheathing (the walls would need let-in bracing or shear panels), and it's pretty pricey stuff even in Europe, let alone the US. One version is described in this GBA blog:

    http://www.greenbuildingadvisor.com/product-guide/prod/agepan-functional-wood

    also here:

    http://c.ymcdn.com/sites/www.polyiso.org/resource/resmgr/cptg_2013/agepan__a_vapor-permeable,_w.pdf

    I've seen one example of a UK level 6 (= ultra-sustainable) house covered in popular media using double-studwall /cellulose with 2" fiberboard sheathing, but in the video of the Grand Designs television episode covering that project it was clearly not dense-packed. (That episode currently available on Netflix, but frustratingly thin on some details.)

    On this side of the puddle 3/4" asphalted fiberboard isn't necessarily more expensive than better grades of OSB, and can be structural if the fastener schedule is followed (though you might choose to still use shear panels or let-in bracing, and fewer nails on the fiberboard), eg:

    http://www.homedepot.com/catalog/pdfImages/95/958dd23e-8bdf-4340-a701-61b855d9c5df.pdf

  7. Brian Croston | | #7

    should I be worried?
    I've been lurking here for a long time, but still no clear answers regarding double stud walls. I am an owner-builder, starting my project in a few months.

    I am in southern Montana (7000 HDD, -10*F & +91*F design temps) and had planned a wall construction similar to Jesse Thompson's design (1/2" air gap + Tyvek Drainwrap + taped CDX + cellulose + Intello Plus + 1.5" utility chase w/Rockwool infill + GWB), but want to make sure it will work in my climate. Most of the discussion and testing seems to have occurred in New England, with significantly different levels of humidity.

    The simplicity of my proposed wall design and significantly reduced construction cost are why I'm wanting to go in this direction. Here in Montana, items like Rockwool, asphalt fiberboard, and gypsum board are still considered exotic and expensive and I want to steer away from foam where possible.

  8. User avater
    Jim Baerg | | #8

    Alternative to ext foam
    We built double stud houses in the 80's. Based on many years of discussions at GBA and BSC, I thought that the consensus was to stay away from fat walls with exterior sheathing. I'm surprised at the continued popularity of traditional double stud walls. Ease of construction seems to be one argument.
    I'm also wondering why there is no mention of using exterior D.P. cellulose. If exterior mineral wool works so well, why wouldn't cellulose? We've done a number of houses this way in Montana and builders adapt well to it.

  9. User avater
    Dana Dorsett | | #9

    The drier air of Montana makes it easier @ Brian
    The drying capacity toward the exterior during the late winter / early spring is going to be more favorable in high-dry MT climate than New England. As temps warm up in MT the outdoor dew points and outdoor relative humidity remain low, whereas in New England the outdoor dew points & RH track a bit higher in the early part of the drying season, which slows the drying rate of the cold sheathing. f you're still nervous, run a WUFI simulation for your location but it should be fine to clone a Jesse Thompson double-studwall stackup.

    Intello works, but it has to be made air tight. An OSB (the cheap stuff is fine here) or CDX air barrier at that location would be more robust, at a similar varying vapor retardency. OSB will be somewhat more vapor-tight than CDX, and may be the better choice for the interior side air-barrier & vapor retarder if you go that route. (An OSB air barrier/vapor retarder in a layer near the interior is found in a number of PassiveHouse designs.)

    Fiberboard can be ordered through box stores (HD, Menards, etc) , if nobody else carries it, but if you're dense packing you'll need at least 1x girts (horizontal furring, not vertical, which limits some siding options) for a 3/4" rather than 1/2" air gap. IIRC half-inch fiberboard has better structural specs than 3/4" stuff (better fastener retention?) and it's cheap- under $10/sheet (half the price of ZIP OSB.)

  10. Brian Croston | | #10

    response to Jim
    Jim, I've also considered the wall construction you propose, using OSB/plywood on the interior stud wall and reinforced exterior WRB such as Mento Plus on the outside. Based on the few houses I've built, I'd think it's more difficult (dense pack from exterior, additional strapping, installing both insulweb + housewrap, etc.) to build.

    I'll let others with more experience comment on other reasons why builders continue to choose exterior sheathing.

  11. David Powers | | #11

    Appropriate timing for this
    Appropriate timing for this article. Just yesterday I did some moisture testing in a deep energy retrofit/large addition my company did in Sutton Vt in 2013-2014.

    Wall system is GWB, 12" double stud, dense pack cellulose, Zip sheathing, 6mm home slicker and cedar clapboards. Sheathing MC was in the high 20s on the north and south sides. Higher up the wall the higher the MC. Unvented 18" roof assembly with dense pack cellulose was surprisingly in the low teens MC. I am tempted to open up a wall for a more thorough inspection.

    Since this project, we have built two other double stud homes but used an interior smart vapor retarder and service cavity on each. Plan to test these homes in the near future. Sure hope to see dryer results.

  12. Kye Ford | | #12

    Martin So if....
    So if cold sheathing is in fact ok for double stud walls as many contend, where does this leave us for recommendations for continuos exterior insulation values for other wall assemblies?

  13. User avater GBA Editor
    Martin Holladay | | #13

    Response to Kye Ford
    Kye,
    The recommendations for minimum R-values for exterior rigid foam apply to walls that can't dry to the exterior.

    Double-stud walls can dry to the exterior.

    -- Martin Holladay

  14. TIM MCCARTHY | | #14

    Free double stud wall.
    We are building double stud walls, floor and roof like a truss, all as one frame, no plates. Essentially taking the labor equation out of the double stud, reducing cost to almost zero and the part I like: no hairy eyeball from contractor. I order the frames from truss company and say here build this. Its patent pending. WE have sheathing both sides of wall for strength then a finish of gyp or just use nice plywood. We spray foam from inside of outer sheathing layer to approx 2 inches from inner sheathing leaving a zero cost electrical chase, electric boxes are inside the heat envelope. Exterior wall has slight pitch to help break the box look.

    Wall is 12 inches thick at bottom and 8 or 9 at top but can be wider. . We are building small modular s with them. With walls, floor and roof framed at once its very fast.

  15. User avater GBA Editor
    Martin Holladay | | #15

    Response to Tim McCarthy
    Tim,
    That's a truss-framed wall, not a double-stud wall.

    It looks like you give your clients two choices: either have an exterior wall that isn't plumb, or an interior wall that isn't plumb. If I were offered that choice, I'd say, "No, thanks."

    -- Martin Holladay

  16. User avater GBA Editor
    Martin Holladay | | #16

    Response to Skip Harris (Comment #3)
    Skip,
    Q. "So, what ARE the most forgiving optimally-insulated systems? I'd love to see a consensus (or even a discussion) for roof and wall systems, for each climate."

    A. When it comes to walls, my advice (tailored to different climate zones) is provided in this article: How to Design a Wall.

    For ceilings, I'm a firm believer in vented unconditioned attics with a thick layer of cellulose on the attic floor. This approach requires raised-heel trusses or rafters installed above the joists.

    For sloped roof assemblies, the best assemblies require an adequate layer of rigid foam above the sheathing, as explained in this article: How to Install Rigid Foam On Top of Roof Sheathing.

    -- Martin Holladay

  17. User avater GBA Editor
    Martin Holladay | | #17

    Response to Charlie Sullivan (Comment #4)
    Charlie,
    I have emailed your comment on the U-factor of I-studs to John Straube. I hope he responds, because this is certainly a question that interests me as well.

    -- Martin Holladay

  18. User avater GBA Editor
    Martin Holladay | | #18

    Response to Dana Dorsett (Comment #6)
    Dana,
    GBA has published several articles about walls with fiberboard sheathing that have been insulated with dense-packed cellulose or dense-packed fiberglass. Many of the builders who have tried this technique have had problems with bulging fiberboard, so builders interested in fiberboard sheathing should (a) choose their product with care, and (b) research the bulging issue, and solutions to the bulging issue, before proceeding.

    For more information on this topic, see Wall Sheathing Options.

    -- Martin Holladay

  19. User avater GBA Editor
    Martin Holladay | | #19

    Response to Brian Croston (Comment #7)
    Brian,
    Q. "Should I be worried?"

    A. In my opinion, no. I agree with Jesse Thompson's statement: "The rainscreen is the savior of the double-stud wall.”

    Other important details that help keep these walls out of trouble are:

    1. The use of plywood sheathing instead of OSB sheathing.

    2. Airtight construction techniques, verified with a blower door.

    3. The use of a smart vapor retarder on the interior side of the wall.

    -- Martin Holladay

  20. User avater GBA Editor
    Martin Holladay | | #20

    Response to Jim Baerg (Comment #8)
    Jim,
    Q. "I'm also wondering why there is no mention of using exterior dense-packed cellulose. If exterior mineral wool works so well, why wouldn't cellulose?"

    A. GBA has published lots of articles on walls that include dense-packed cellulose on the exterior side of the wall sheathing. Here are two examples:

    All About Larsen Trusses

    The Klingenberg Wall

    The second article includes many links to more GBA articles on this topic.

    -- Martin Holladay

  21. User avater GBA Editor
    Martin Holladay | | #21

    Response to Dana Dorsett (Comment #9)
    Dana,
    You wrote, "Intello works, but it has to be made airtight."

    That statement is only true if the builder is depending on the Intello to be an air barrier. If the builder chooses to install Intello as a smart vapor retarder, it will perform very well in that capacity even if it isn't installed in an airtight manner.

    Of course, every wall needs at least one (and sometimes two) air barriers, but a variety of materials can be used for this purpose, including the exterior sheathing and the interior drywall.

    (I suspect that you know all these facts, Dana, but it's important to clarify them for GBA readers.)

    -- Martin Holladay

  22. User avater GBA Editor
    Martin Holladay | | #22

    Response to David Powers (Comment #11)
    David,
    First of all -- hi, neighbor. (It's always good to hear from GBA readers from the Northeast Kingdom).

    Thanks for sharing your moisture content data. The readings are consistent with the findings of academic researchers. In general, sheathing with a high moisture content in February or March tends to start drying out in April or May. This type of annual moisture cycling is probably more concerning in OSB sheathing than plywood sheathing.

    While the long-term effects of this type of annual moisture cycling are unknown, double stud walls that experience this type of cycling appear to be doing OK, as long as the drying that occurs in April and May can occur rapidly. A ventilated rainscreen gap (with openings at the bottom of the wall and the top of the wall) really help.

    -- Martin Holladay

  23. TIM MCCARTHY | | #23

    new systems defy categorization
    Martin The sloped wall is an option. The current model Im working on neither wall is sloped or what you call "not plumb". Art does play a part in successful buildings. New systems transcend definition or categorization. My system has full thermal break between the inner frame member and the outer frame member, the most important aspect of the double stud wall and its done WITHOUT THE COST. High Performance housing has to think out of the box, reduce cost, otherwise it will remain as it is, a very fringe percentage whats being built.

  24. Malcolm Taylor | | #24

    Batts rather than Cellulose
    John Straub raises concerns about substituting batts for the more common cellulose in double walls. Does anyone have a feeling for how much more vulnerability this represents if the other precautions (rain screen gap, plywood sheathing, good air-sealing) are taken?

  25. Malcolm Taylor | | #25

    Basic Question
    Does eliminating or moving the sheathing inwards on a double-stud wall completely remove the risk? Is the remaining exterior wall framing a condensing surface subject to smaller but still possible damage, or at that point am I asking how many angels can fit on the head of a pin?

  26. User avater GBA Editor
    Martin Holladay | | #26

    Response to Malcolm Taylor (Comment #24)
    Malcolm,
    It was Jesse Thompson, not John Straube, who emphasized that cellulose is preferred to fiberglass batts for a double-stud wall. I imagine that the main reason to use cellulose is that it cuts down on air leakage, making the wall less susceptible to moisture damage.

    That said, there are other ways to cut down on air leakage, and fiberglass batts can work well if perfectly installed.

    -- Martin Holladay

  27. user-6717844 | | #27

    These sort of systems are discouraged through taxes.
    Double stud walls take large amounts of floor space, and building footprints grow along with wall thickness. When the building footprint grows, taxes grow along side. Cities are discouraging sustainable construction by punishing owners with higher taxes.

  28. Dan Kolbert | | #28

    Labor
    I remember Jesse saying that at the conference, but I don't remember compared to what.

    We stick pretty much entirely to double stud walls. For me at least (and we keep framing in-house, and I'm paying a lot more than $15/hr), double stud is cheaper than the alternatives. Exterior foam is expensive and fussy to install. It requires additional labor and material to prepare for siding. And you still need to insulate the walls. The biggest expense with dense pack cellulose is the prep - filling a 12" thick wall is a lot cheaper per unit than a 6" wall.

    Exterior mineral wool has the same problems, in spades.

    I've priced out exterior i-joists but never done it. Between the price of the material, and the difficulty of working vertically on the exterior of a building, my budget was much higher than for double stud. Perhaps in a factory setting like EcoCor's, the numbers are better, but not for a site built house, in my opinion.

    As for the cold sheathing problem, I just haven't seen it. As Jesse pointed out, the very first double stud house we built (with his firm, back in 2008) was recently renovated. We turned a relatively unfinished 3rd floor into a studio, and in the process replaced a window with a door. It was the north side of the house, facing the ocean. We saw zero evidence of any wetting or deterioration of the sheathing.

    Of course the stakes are higher with well-insulated buildings. Caution and careful detailing is critical. But even Straube seems to admit (and I wasn't at his session) that the dangers aren't as grave as BSC's documents would lead you to believe.

    Martin & I have discussed the perils of the "expert-ization" of green building. One way to resist it is precisely thru sessions like the one Chris Briley moderated at NESEA - to compare real-world experiences of various wall sections.

  29. User avater
    Stephen Sheehy | | #29

    Air barrier in double stud wall
    Our exterior wall was sheathed with Advantech, with all joints taped. But our primary air barrier was Siga Majpel, applied to the outside of the inner stud wall. By running the membrane a foot or so over the top plate and outside the ends of the framed walls, it was easy to lap the ceiling membrane (stapled to the bottom truss chord) over the wall membrane and to overlap at the wall corners.

    Our contractor had not done a double stud wall before, but found it pretty easy to execute. By using a membrane on the outer side of the interior wall, we were able to run plumbing and wiring inside the inner stud space with minimal penetration of the membrane. Insulating the space between the sheathing and membrane, about 8 1 /2" , was simple. We just cut holes in the membrane, blew the cellulose in and taped the holes.
    Jesse and his colleague, Jamie Broadbent, did the design.

  30. Malcolm Taylor | | #30

    Taxes
    That argument has come up before and I'm not sure the impact is large enough to have any effect. Increasing the width of the exterior walls from 6" to 1'-0" would add about 2.5% to the floor area of a 1600sf house. And given that property taxes are based on the assessed value of a house, not just its floor area, I can't see there being much of a noticeable change.

  31. Malcolm Taylor | | #31

    Martin
    Sorry, you are quite right. Is it just the ability of the cellulose to reduce air leakage, or does it's ability to absorb moisture help too?

  32. Malcolm Taylor | | #32

    Dan Kolbert
    The sheathing is dry. That is very reassuring to hear - and it's what you had been predicting all along.

    What do you see as a viable alternative to the "expert-ization"? The counteracting force is I suppose market-driven short-cuts which here in the PNW lead to widespread building failures. Do you think more robust building codes and the use of a few trusted assemblies for each climate are the answer?

  33. User avater
    John Straube | | #33

    Response to Charlie Sullivan (Comment #4)
    Hi Charlie,
    I think there is just a confusion about language. They use the term "fibreboard" for the web. Fibreboard in the US would often be low-density fiberboard (an slightly insulating product) that could never be used as a web because it is too weak.

    OSB is what we would use in the US, and that is also very common for I-joist in Sweden today, although tempered hardboard (what I assume they made for the paper, as that is more what the photo seemed to show) is also very common, more in the past.

    See http://byggma.com/brands/masonite/ByggmaMerkevare.aspx#2 for the largest I-joist manufacturer in Sweden (who use 10 mm OSB webs whereas 9.5 mm is more common here) For solid wood, thermal conductivity across the grain (what is normally measured) is in the 0.1 W/mKrange but routinely varies from 0.08 to 0.12 : this is why people who do hotbox testing routinely test the materials used. That is what our lab does. Along the grain, the thermal conductivity is higher.

    For OSB, across the panel, a value of 0.13 is reasonable (we have measured this in our lab), because the OSB is compressed and hence higher density. But higher density products have higher thermal conductivity.
    The thermal conductivity of wood along the grain is about twice that across the grain (a value of 0.22 is often used) The thermal conductivity produced by the manufacturer is in the direction perpendicular to pressing direction: the web in an I-joist is parallel to the direction of pressing. This makes a big difference to the conductivity.

    Hence, while initially surprised by the results, when I dug into their results I found them both plausible and relevant to North American practice.

    There is a reason researchers need to measure real walls: we often don't know as much about this stuff as people think, and there are a lot of variables.

    It would be great if we could get funding to measure this stuff… but really it does not change much, because both physics and measurements show that I-joist walls have other limitations, and they are not cheap!
    The quote from the paper, describing the “fibreboard plates” may useful for others.

    "It is common to measure the thermal conductivity through the thickness of fibreboard plates, but the thermal conductivity in the longitudinal direction is not well known. The thermal resistance in the longitudinal direction, i.e. the direction of the heat flow through the I-stud and the U-stud, was therefore measured in a heat flow meter apparatus according to the governing standard (NS-EN 12667 2001). The corresponding thermal conductivity λfb║ = 0.38 W/(mK) were used in the numerical simulations.”

    -- John Straube

  34. User avater GBA Editor
    Martin Holladay | | #34

    Response to Malcolm Taylor (Comment #31)
    Malcolm,
    Q. "Is it [the advantage of cellulose over fiberglass] just the ability of the cellulose to reduce air leakage, or does its ability to absorb moisture help too?"

    A. That's a complicated question. After researching the purported value of hygric buffering and hygric resistribution, I'm somewhat of a skeptic when it comes to the purported benefits of hygric buffering. If there is a mechanism that allows the moisture content of a building assembly to ratchet up, the building assembly will eventually fail unless there is a periodic opportunity for the building assembly to dry. On an annual basis, it is always essential for the drying rate to exceed the wetting rate.

    Here is a link to my article on the topic: Hygric Buffering and Hygric Redistribution.

    The paragraphs I quote below came from that article.

    Most building scientists agree that some wall assemblies that seem risky are made safer by hygric redistribution. The classic example is a double-stud wall insulated with dense-packed cellulose. In a cold climate, moisture tends to accumulate on the cold side of this type of wall during the winter months, leading to damp wall sheathing. Yet when these walls are disassembled and inspected, the wall sheathing is almost always in good shape. The lack of mold or rot is often attributed to two factors: hygric redistribution by the cellulose (which pulls moisture from the sheathing and redistributes it toward the center of the wall) and outward drying in April and May.

    That said, it’s not as if builders can cut corners with water-management details, and glibly announce, “We’re fine — the cellulose provides hygric redistribution, and that will keep us out of trouble.” So hygric redistribution may be one of those phenomena which is interesting and worth studying, but is so hard to model that it is useless as a design principle.

    Moreover, there is no easy way to compare the advantages and disadvantages of blown-in fiberglass insulation with the advantages and disadvantages of cellulose insulation. Water is more likely to drain quickly through fiberglass than cellulose, and that’s good — right? And damp fiberglass dries more quickly than cellulose — also good, right?

    But on the other hand, cellulose provides both a hygric buffer and hygric redistribution — characteristics that are absent from fiberglass insulation — so the cellulose must be preferable, right?

    Assessing the effects of these pluses and minuses is difficult even for building scientists. To weigh all the relevant factors, field observations may be just as valuable as, or more valuable than, hygrothermal modeling.

    In our recent phone conversation, [Joseph] Lstiburek noted that cellulose insulation can't perform miracles. “A mass wall built out of several wythes of brick is a spectacular example of hygric distribution,” Lsitubrek told me. “The rainwater penetrates, is wicked away, stored, and redistributed in a material that is not water-sensitive. But hygric redistribution doesn’t work that way with cellulose. Yes, the cellulose wicks and it stores moisture, but the moisture all ends up on one side of the wall because of the thermal gradient. The hygric redistribution buys you something, but it doesn’t buy you enough to keep you out of trouble. It you have a window that leaks, cellulose will not save you compared to fiberglass insulation.”

    -- Martin Holladay

  35. Skip Harris | | #35

    "Water is more likely to drain quickly through fiberglass"
    Really? This might be more akin to saying a bigger hole in the bottom of my boat will allow the water to drain out faster when I bring it ashore after it sinks. If my wall ever needs to actually drain water, something is SERIOUSLY wrong.

  36. Charlie Sullivan | | #36

    John Straube (I-joist webs and anisotropic thermal condutivity)
    Re: comment 33 by John Straube: Thanks for sorting out some more of those details. Putting the numbers together I would estimate that the advantage of a typical commercial I-joist over a regular stud is probably significantly more the 15% found in the paper, and would be worthwhile if those were the only two options on the table, but as you say, those are rarely the two best options.

    The higher thermal conductivity along the grain is a useful thing to keep in mind, in considering, for example, why rafter tails can be such severe thermal bridges.

  37. Andy Kosick | | #37

    Wrong Question?
    Really appreciate hearing about the research and recommendations here but I can't help but think when reading about wall assemblies these days that what we're really talking about is OSB. If it's wet, were it's at, etc. Is the elephant in the room actually an industry wide obsession with an inferior building material, something that's too risky to be in modern high R wall and and unvented roof assemblies. Many solutions seam to involve CDX, or Advantech, or eliminating OSB form the assembly all together. Despite that last one being a personal favorite, I don't mean getting rid of structural sheathing. My question is does the standard for what constitutes the minimum properties for structural sheathing in a residential wall assembly need to change. Are market prices made possible by using an inferior material making it overcomplicated to move forward with high R assemblies. I'm just thinking out loud here, any thoughts?

  38. Malcolm Taylor | | #38

    Andy
    It would be interesting to have some idea of how much more resilient to continued high moisture cycling plywood is than OSB. We know it's better, but from my experiences with rotten sheathing it isn't immune to the same problems when wetted repeatedly. High moisture levels in exterior walls have other effects too. Fasteners, electrical connections, framing and other components also suffer. Is a wet wall where the sheathing maintains its structural properties a good idea? I'm also just thinking out loud.

  39. Jon R | | #39

    If one is going to add
    If one is going to add furring strips, is there a reason that Densglass (or maybe DensElement) isn't the standard for double walls?

  40. User avater
    Ethan ; Climate Zone 5A ; ~6000HDD | | #40

    ZIP and vapor retarder
    Quick question... The diagrams suggest that ZIP sheathing eliminates the need for a vapor retarder. Am I reading this correctly?

  41. Malcolm Taylor | | #41

    Jon
    Once you move from sheathing with plywood or OSB that can provide shear resistance and a solid backing for trim and flashing, is there any real advantage to having sheathing at all? Does fibreboard or something like Densglass offer anything that house wrap alone wouldn't?

  42. User avater GBA Editor
    Martin Holladay | | #42

    Response to Andy Kosick (Comment #37)
    Andy,
    You wrote, "What we're really talking about is OSB."

    Every building material has appropriate uses and inappropriate uses. Sometimes, asphalt felt makes sense as a roofing underlayment; other times, you really need Grace Ice & Water Shield. That doesn't mean that asphalt felt is a bad product.

    Designers have to specify the right product for the assembly they are building.

    OSB performs well as long as it stays dry. GBA does not recommend the use of OSB on the exterior side of double-stud walls. For example, you can find this GBA recommendation in my article, How to Design a Wall.

    For more information on this topic, see Wall Sheathing Options.

    -- Martin Holladay

  43. User avater GBA Editor
    Martin Holladay | | #43

    Response to Ethan T (Comment #39)
    Ethan,
    Q. "The diagrams suggest that Zip sheathing eliminates the need for a vapor retarder. Am I reading this correctly?"

    A. The diagrams show wall assemblies used by Kaplan Thompson Architects (the company founded by Jesse Thompson and Phil Kaplan). GBA isn't endorsing these drawings, although they look fine to me.

    An interior vapor retarder is required by most building codes. You can satisfy this requirement with a variety of approaches, including vapor retarder paint or an interior "smart" membrane like Intello or CertainTeed MemBrain. In my opinion, including a smart vapor retarder (one with variable vapor permeance) makes a double-stud wall more robust.

    In my article on walls (How to Design a Wall), I recommended the use of a smart vapor retarder on the interior side of a double-stud wall.

    -- Martin Holladay

  44. User avater GBA Editor
    Martin Holladay | | #44

    Response to Jon R (Comment #40)
    Jon,
    Q. "If one is going to add furring strips, is there a reason that Densglass (or maybe DensElement) isn't the standard for double walls?"

    A. As far as I know, there is no standard for double-stud walls. But GBA recommendations for sheathing choices for double stud walls have always included DensGlass or other brands of fiberglass-faced gypsum panels.

    For example, in my article on walls (How to Design a Wall), I recommended that double-stud walls should have "any type of exterior sheathing material other than OSB — for example, plywood, diagonal boards, fiberboard, or fiberglass-faced gypsum panels."

    In my article titled How Risky Is Cold OSB Wall Sheathing?, I wrote, "OSB is more susceptible to rot than plywood. So if you’re worried about the durability of your sheathing, choose plywood, DensGlass Gold, or diagonal board sheathing over OSB. One other possible (vapor-permeable) sheathing choice is structural fiberboard sheathing."

    -- Martin Holladay

  45. Jon R | | #45

    > Does fibreboard or
    > Does fibreboard or something like Densglass offer anything that house wrap alone wouldn't?

    My understanding is that Densglass provides structural support for racking and is stiff enough for dense pack cellulose (unlike fiberboard). And can't rot like OSB/Zip or even plywood. I see 5/8" sold for $18/sheet.

    So when I see "use CDX, Advantech, or Zip", I think "but which is best" and "where does Densglass fit in".

    "These walls are more prone to failure... but double-stud walls can be made low risk” - this comment makes me wonder "can it be made lower than a insulation-outside-the-sheathing wall".

  46. User avater GBA Editor
    Martin Holladay | | #46

    Response to Malcolm Taylor (Comment #41)
    Malcolm,
    Q. "Does fibreboard or something like Densglass offer anything that housewrap alone wouldn't?"

    A. Yes. Structural fiberboard and DensGlass can both be used as structural wall bracing. For more information, see Wall Sheathing Options.

    -- Martin Holladay

  47. Malcolm Taylor | | #47

    Martin
    Looking more closely at the spec sheets for various brands of fibreboard and Densglass they can't be used as a fastening surface for siding or trim, and the shear strength values they use to call them "structural" are too low to meet our code.

    They are used primarily on commercial projects where the cladding systems and structures don't require these attributes. and their shortcomings are why they haven't penetrated the wood frame residential market.

  48. User avater GBA Editor
    Martin Holladay | | #48

    Response to Malcolm Taylor
    Malcolm,
    As the article I linked to (Wall Sheathing Options) clearly noted, neither fiberboard nor DensGlass is designed or intended to hold siding fasteners. All I said in my comment is that these sheathing products can be used to brace walls. Housewrap can't.

    While structural fiberboard and DensGlass are routinely used to brace residential walls in the U.S., any bracing plan should be checked by an engineer. In high wind areas, earthquake zones, or for use on a multi-story building, a different type of sheathing might well be called for.

    If these products can't be used to brace residential walls in Canada, that is news to me. Thanks for letting us know.

    -- Martin Holladay

  49. Malcolm Taylor | | #49

    Martin
    No, there are regions where both insulated fibreboard and gypsum based sheathing can be used to resist shear (although on where I build). My point was a bit different.

    I see switching to these materials with marginal structural values in much the same way I look at Advanced Framing. You are taking a robust, established way of building and reducing the structure as much as possible - removing the redundancy that makes it resilient - and this is done in the name of efficiency.

    This seems like looking at building assemblies as a zero-sum game. I can't see how it makes sense for high performance houses to have less structural integrity simply because they are designed to use less energy. Surely they should have at least as robust a structure as their mass market cousins. If the wall assembly can't safely support plywood or OSB, any alternative needs to perform the roles we traditionally have asked sheathing to do. Otherwise why have it?

  50. William Murray | | #50

    higher perm sheathing
    We're just wrapping up construction of a PHIUS house here in southern Quebec. We are not keen on OSB. Having seen it in soggy shreds in countless renovations, it doesn't inspire a lot of confidence in our team. "Performs well when it's dry" says it all. We sheathed the house in Eco4, a fibreboard made here in Quebec. At 1-1/2", it was more than rigid enough to resist the bulge of 17" of dense pack cellulose. It is essentially three sheets of 1/2" fibreboard glued together with a wax emulsion. It's a far cry from Agepan — no sexy tongue & groove and the edges don't always match up but with a perm rating of 26 and its little extra R-4, we were more than willing to put up with it. I don't know what kind of racking rating Malcolm needs in BC but it definitely fit the bill here in frosty Quebec.

  51. Malcolm Taylor | | #51

    William
    Sounds like a nice product that would meet my objections. What's the price like? I remember you had a thread here a while ago asking whether it would need additional bracing. Presumably the answer you got from your inspector was no?

  52. User avater GBA Editor
    Martin Holladay | | #52

    Response to Malcolm Taylor (Comment #49)
    Malcolm,
    Q. "You are taking a robust, established way of building and reducing the structure as much as possible - removing the redundancy that makes it resilient - and this is done in the name of efficiency."

    A. No, this is done in the name of durability. When it comes to structure, here's my guideline: If the engineer says it is good, then it's good. I'm not going to second-guess the engineer. Choosing a material with a higher structural rating than is necessary (according to an engineer) does no one any good -- not the client, and not you.

    We're not specifying fiberboard or DensGlass for a trivial reason. We're specifying one of these products because these sheathing materials are less likely to rot.

    -- Martin Holladay

  53. User avater GBA Editor
    Martin Holladay | | #53

    Response to William Murray (Comment #50)
    William,
    Thanks for letting GBA readers know about SonoClimat Eco4 fiberboard manufactured by MSL Fiberboard.

    For those interested in more information, here is the contact information for the manufacturer:

    MSL Fiberboard
    (Manufacturer of SonoClimat Eco4)
    161 St-Paul Street
    C.P. 38
    Louiseville, Quebec J5V 2G9
    Canada
    800-561-4279 or 403-532-8700

    -- Martin Holladay

  54. Jon R | | #54

    Malcom, if something is truly
    Malcom, if something is truly overbuilt, with the excess not serving a useful purpose, it makes sense to trade it for more robustness in some other area. Eg, trading the ability to withstand a wind that will never occur (some areas) for reduced moisture damage risk.

    One could put OSB at an inner layer and Densglass or thick fiberboard on the outside, providing even more structural integrity than OSB alone. But that's less "robust" in terms of cost.

    It's nice to see more discussion of the trade-offs.

  55. Malcolm Taylor | | #55

    Jon
    I agree entirely. I'm not sure that is the case with Densglass and will have to agree to disagree with Martin on that point. The old wood frame buildings I have renovated benefited immensely from the "redundancy" of their structures. Shear walls end up taking loads from shifting foundations, doubled top plates keep walls true and straight, etc.

    Although it has generated some interesting discussion, I probably could have forestalled a lot of it by being clearer in my response to you. The two double-wall approaches that hold the most promise to me are either:
    - Moving the sheathing inwards until it is safe. If as you said you are strapping anyway, why not substitute a WRB for the outer layer.
    - Substituting a material that has the same attributes as plywood but is more resilient to moisture. Hopefully these are being developed, and something like the product William used will prove to be a good choice.

    And of course all this may just be so much talk. Jesse and Dan may well be right. With a few precautions there may be no reason to switch from plywood at all.

  56. William Murray | | #56

    Malcolm
    The Eco4 cost us $31.25 (CDN) a sheet last fall. The ASTM E72-13 numbers are 5.84 kN / m as opposed to OSB's 6,72. I don't know how that would fare in seismic zones. To be honest, the only inspections we tend to get in Quebec is someone coming to make sure everyone on the worksite has their competency cards and to levy hefty fines if they don't. Zero quality control. Just fyi, our labour costs are quadruple Jesse's. Even an apprentice carpenter working on a new construction is paid 56$/hour although most of it goes right back to the union... Don't get me started.

  57. Malcolm Taylor | | #57

    William
    Thanks. Labour costs like that sure make for a different perspective on material costs. I grew up in Montreal. I know what you are having to deal with.

  58. User avater
    Ethan ; Climate Zone 5A ; ~6000HDD | | #58

    I will check on Eco4 availability in the USA
    In the mean time... Malcolm, you write:

    ...Moving the sheathing inwards until it is safe. If as you said you are strapping anyway, why not substitute a WRB for the outer layer.

    I am trying to wrap my head around the idea of just having a wrap on the outside of a wall... (no pun intended). It just seems crazy... and when I have seen it done, is on most of the Passive House projects around here, I get very nervous.

  59. William Murray | | #59

    eco4 in the us
    Martin,
    I just found out there is a US rep for the eco 4 if anyone's interested.
    Ben Flanagan
    Director of Sales
    403-479-1189
    Toll Free : 1-800-561-4279
    bflanagan@mslfibre.com

  60. User avater GBA Editor
    Martin Holladay | | #60

    Response to William Murray
    William,
    Thanks.

  61. User avater
    Ethan ; Climate Zone 5A ; ~6000HDD | | #61

    If there is going to be a Yeti, it will be David Powers' wall
    ...since the wall he mentions in comment 11 kind of violates the main principles often discussed here unvented cathedral ceiling with dense pack and no rainscreen.

  62. User avater GBA Editor
    Martin Holladay | | #62

    Response to Ethan T (Comment #61)
    Ethan,
    I guess you are worried about the wall that David Powers described this way: "Wall system is GWB, 12" double stud, dense pack cellulose, Zip sheathing, 6mm home slicker and cedar clapboards."

    David also mentioned a roof assembly, but provided few details, simply describing it as "Unvented 18" roof assembly with dense pack cellulose."

    Ethan, your comment is cryptic. It's hard to tell whether you are worried about the wall that David built or the roof that David built. Your comment "no rainscreen" appears to refer to the wall, but the comment is inaccurate, since Home Slicker provides a rainscreen gap.

  63. User avater GBA Editor
    Martin Holladay | | #63

    Response to Malcolm Taylor (Comment #62)
    Malcolm,
    I'm faced with the task of deciphering two cryptic comments in a row. Ethan was apparently worried about the lack of a rainscreen gap, when in fact the wall in question had one. You are apparently worried about (or at least commenting on) the lack of exterior sheathing, when in fact the wall in question had exterior sheathing.

    Just to clarify: Are you and Ethan talking about the wall described by David Powers this way: "Wall system is GWB, 12" double stud, dense pack cellulose, Zip sheathing, 6mm home slicker and cedar clapboards"?

  64. Malcolm Taylor | | #64

    Reply to Ethan (comment # 58)
    Instinctively I agree with you. I feel the same way about getting rid of the slab as part of a basement floor assembly. But if you step back and look at both situations there is a good case to be made.

    Most of the main functions of sheathing - structural support and air-sealing - are carried out in much the same way when it is moved to the inner wall. The only thing that really differs is that the sheathing isn't available as a backer for the siding. But since the advent of rain-screens, sheathing isn't usually used for that anymore. A robust WRB doesn't require sheathing behind as long as there is backing where it is needed at penetrations and to support flashing.

    Remember Riversong? He used to build without any sheathing at all, or a WRB. That would keep me up at night.

  65. Malcolm Taylor | | #65

    Martin
    No sorry. I was responding to Ethan's response (58), which I just found in this thread, to my earlier comment to Jon (55), that I think it can be a good approach to move the sheathing inward on double-stud walls. Ethan is uncomfortable with the idea of not having sheathing directly under the siding.

    I should probably adopt your format of specifying which comment I'm replying to.
    Edit: There, I've fixed it.

  66. User avater
    Ethan ; Climate Zone 5A ; ~6000HDD | | #66

    Sorry Malcolm and Martin
    ...I believe I am coming around to the idea of using only a fancy wrap on the outside of a larsen truss/klingenberg type wall... ...I also should have looked into what Rainslicker was before I commented erroneously...

    As far as I can tell, the continuum that exists spanning from double stud to larsen truss begins as you begin to move (or eliminate) the sheathing...

    A "regular" double stud wall has interior GWB and exterior plywood or ZIP sheathing... To eliminate the cold sheathing problem you move the ZIP in to the outside face of the interior 2x4 stud on what was your double stud... and *voila!* you've got what is slowly becoming a larsen truss...

    In my head (and on napkins) I've been developing a wall that uses appearance grade plywood as sheathing and finish on the interior face of the double stud, placing the air barrier and WRB on the exterior face, held on with battens, and then a sturdy siding (or, as Riversong does, just apply the housewrap and then use 3/4" siding over that.) *Voila!* I've eliminated one step (oh, yeah, this will require all electric to run in conduit, which I am fine with).

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