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Larsen truss questions

user-1105327 | Posted in General Questions on

Hello. I was hoping to get some more feedback from the GBA folk.

We have just framed a small house in an area with a -32 design temperature. our plan is to attach a Larsen truss to a conventionally framed wall. the wall detail is as such: 2×4 stud wall, sheathed with 5/8′ t+g OSB, 8″-ish Larsen truss, cross-strapped with 1x, rainscreen, cedar siding. we have to use batt insulation because dense-pack or wet-pack cellulose is not available (we will use blown cellulose for the attic).

The OSB has acoustic sealant in the t+g joint and has been acoustically sealed to every framing member it touches effectively isolating every stud bay. all the joints were then siga-taped on the exterior side of the panel.

My interpretation of the british columbia building code (bcbc) combined with consulting various people including building scientists and the plywood council of bc, suggests the osb can serve as both the air barrier and vapour retarder. if i insulate the stud wall with r12 and the larsen truss with r28, what and where should i put the vapour retarder?

Have i commited myself to the osb as the vapour retarder and therefore must omit the insulation in the stud cavity due to the 1/3 rule?

What sort of weather barrier should i use and where should i put it? on the exterior of the truss or on the exterior of the 1x cross-strapping?

Is it a horrible idea to skip the 1x cross-strap and rainscreen combination and instead, place the weather barrier directly on the larsen truss and then cross-strap with 1x on a 45 degree angle? this would brace the larsen truss, support the weather barrier, create airflow behind the cedar cladding, and allow moisture to escape via gravity.

Thanks again…

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  1. GBA Editor
    Martin Holladay | | #1

    Don't worry so much about your vapor barrier placement. In case you don't realize it, concerns about vapor diffusion are usually misplaced; these concerns are based on discredited theories that were never supported by research or data.

    More information on vapor barriers:

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

    You concern is particularly mysterious, since your ratio of insulation (R-12 / R-28) shows that less than 1/3 of the insulation is on the warm side of your vapor retarder -- so you pass the test set up by the old rule of thumb.

    The WRB belongs on the exterior side of your last layer of insulation. If you include sheathing, it belongs on the exterior side of the sheathing. If your wall assembly does not include sheathing, it can be either under or over the strapping, as long as you have figured out a way to integrate the window flashing, the door flashing, and the penetration flashing with your WRB.

  2. tirfond | | #2

    hi martin, thanks again for all your help. i'll buy you a bottle of scotch if you're ever in bc!
    my inspector is really not happy with my osb as vapour barrier detail. given my wall assembly, would i be getting myself in trouble if i installed polyethylene to appease the inspector? or a vapour retarding paint? although he doesn't care for the paint idea, either. i know the issues associated with vapour diffusion are mostly misplaced but it seems to be a major issue for him. thanks again...

  3. GBA Editor
    Martin Holladay | | #3

    Does your building inspector have a supervisor you can appeal to?

    The vapor permeance of OSB is about 2 perms, so I can't imagine why the OSB wouldn't satisfy him. In a pinch, you could install a layer of MemBrain. MemBrain is not as dangerous as polyethylene, but it's somewhat expensive.

    If you've already installed OSB as a vapor retarder, installing the MemBrain is just money down the drain... all to satisfy a building inspector who can't look up the vapor permeance of OSB?

  4. tirfond | | #4

    martin, would you suggest using a vapour retarding paint since i have to paint the walls and ceiling anyway?

  5. Expert Member
    Dana Dorsett | | #5

    NO! Do NOT use vapor retardent paints on the interior- it inhibits the ability of the assembly to dry!

    If instead you applied the vapor retardent paint to the OSB rather than the finish wall it would drop the vapor retardency of the OSB from ~2 perms down to ~0.5 perms, which the inspector may find acceptable so long as the OSB it's located at the correct interior/exterior R proportion.

    Living in BC (but apparently in the mountainous interior, not the coast) may find this test project of interest:

  6. user-1140531 | | #6


    Regarding your comment:

    "Don't worry so much about your vapor barrier placement. In case you don't realize it, concerns about vapor diffusion are usually misplaced; these concerns are based on discredited theories that were never supported by research or data."

    This is one area that I am most curious about, and I have asked about it before. How exactly have the theories on vapor diffusion been discredited and shown to never have been supported by research and data? Can you give me a reference to the research and data that shows that the theories about diffusion have been discredited?

  7. GBA Editor
    Martin Holladay | | #7

    Unlike Dana, I'm not too worried about any negative effects from a layer of interior paint. While vapor-retarder paint will change the perm rating of your wall somewhat, it won't be enough to lead to any problems. Your proposed wall assembly is quite forgiving.

    If a single layer of paint were enough to push a wall assembly like yours into failure, we'd all be in big, big trouble. Fortunately, it isn't.

  8. GBA Editor
    Martin Holladay | | #8

    William Rose, a building scientist and professor at the University of Illinois at Urbana-Champaign, has delved into this issue at length. A summary of his findings (in the form of PowerPoint slides) is posted on the web: Vapor barriers do not protect buildings, vapor barriers protect architects.

    In one of my blogs, I alluded to some of the history unearthed by William Rose: "Builders have worried about wintertime vapor diffusion ever since 1938, when Tyler Stewart Rogers published an influential article on condensation in the Architectural Record. Rogers’ article, ‘Preventing Condensation in Insulated Structures,’ included this advice: ‘A vapor barrier undoubtedly should be employed on the warm side of any insulation as the first step in minimizing condensation.’ Rogers’ recommendation, which was eventually incorporated into most model building codes, was established dogma for over 40 years. Eventually, though, building scientists discovered that interior vapor barriers were causing more problems than they were solving. Interior vapor barriers are rarely necessary, since wintertime vapor diffusion rarely leads to problems in walls or ceilings."

    William Rose has written other articles on the topic, but they may not be available on the web.

  9. user-1140531 | | #9


    I looked over that paper by William Rose. I will read it several more times, but initially, it is unclear to me how the conclusions are drawn, or even exactly what those conclusions are. I am referring to conclusions such as this one:

    “As part of this effort, building professionals must recognize that
    building owners are likely to be unsettled by the prospect of
    added wetness, and will respond with the fears, worries and
    concerns that have been planted in them over the previous
    decades. There is a sociology of moisture performance that is
    just as important as the thermodynamics of moisture
    performance. Professionals must be equipped to deal with
    the human side of moisture and its effects. Professionals must
    somehow find the off-button for moisture fears.”

    Professionals must find the off-button for moisture fears???

    Mr. Rose does provide considerable insight into the background of the warm side vapor barrier and the rationale for it. But I do not understand why he asserts that this “dogma” is incorrect. Specifically, what I want to know is why diffusion does not matter. Either this assertion about not needing to control diffusion gets explained in understandable terms or it doesn’t.

    It seems to me that his conclusion is that there will be vapor diffusion into walls without a warm side vapor barrier, but we should not find that unsettling because professionals will show us how to manage that moisture.

    I do not understand the premise of the title, where he asserts that vapor barriers protect architects.

    Regarding the conclusion that vapor barriers cause more problems than they solve:

    How are they causing problems? I get the impression that if a wall was found to be wetted from leaking exterior flashing, and it had a warm side vapor barrier, that vapor barrier would be found to be causing the problem of inadequate drying.

  10. Expert Member
    Dana Dorsett | | #10

    Martin: I agree that putting the paint on the interior is unlikely to create a problem, especially with vented siding. So mayhaps I was a bit histrionic in my presentation (MEA CULPA! :-) ).

    Still, putting it on the interior doesn't contribute to the resiliance of the assembly the way it might if placed on the middle-OSB.

    But the inspector may still balk if that placement doesn't meet Canadian code, which is FAR too stringent regarding exterior R and vapor retarder requirements/placement, to the point that it interferes with better design practices. (The interior poly requirement for insulating foundation studwalls is an actively BAD idea, yet enshrined in code in Canada. But where they got it dead-right in coastal B.C. was the 10mm rainscreen minimum.)

  11. GBA Editor
    Martin Holladay | | #11

    Your questions are best addressed to William Rose, not me. I am not the author of the presentation.

    But here is the kernel: exterior materials (wood siding and sheathing) get wetter when insulation is added into formerly empty stud cavities. The reason they get wetter is because they are colder. The source of the moisture that these materials take on is the exterior air, not the interior air.

    The addition of an interior vapor barrier won't solve the problem, which is due to the temperature of the materials, not vapor diffusion from the interior.

    The origin of vapor barrier requirements in building codes was due in part to pressure from insulation manufacturers, who were eager to find an explanation for paint failures that didn't blame the insulation industry.

  12. tirfond | | #12

    dana, how would you improve upon this wall assembly? we are almost ready for drywall now...

  13. Expert Member
    Dana Dorsett | | #13

    With 8" of fiber in the Larsen Truss behind rainscreen & cedar shingles the OSB has excellent drying capacity to the exterior year-round, and with 3.5" of similar fiber in the 2x4 wall you have more than 2/3 of the total R on the exterior of the OSB. The OSB is the layer at most risk, but with >2/3 of the R on the exterior it won't be cold enough on average to load up with moisture very quickly under any circumstances.

    By using standard latex paint on the drywall it makes the gypsum almost as vapor retardent as the OSB, and should OSB gain some moisture during the cold weather it's vapor permeance goes up, which allows it to pass that moisture more quickly to the exterior, making it something like a "smart" vapor retarder. Even if it gains a bit in winter, it will be dry again by the time the OSB warms up to temperatures that would promote mold growth in the spring season.

    The key to maintaining this excellent moisture handling robustness is to make the wallboard as air-tight as possible, since a square inch of air leakage can move more water than a whole wall's worth of vapor diffusion through 2-3 perm latex paint. But even if minor leaks do occur (as they inevitably will at some point over the next 50 years), with 2-3 perm latex it can also dry pretty quickly to the interior as well as to the exterior.

    If you painted the interior side of the OSB with vapor retardent latex it might protect the OSB slightly by slowing the adsorption of wintertime moisture, but it really isn't necessary- it would be a neutral move. Putting vapor retardent latex on the wallboard would limit moisture diffusion through the wall, but would not protect the OSB from air-leakage from the interior as well.

    But as Martin aptly points out- this is a very robust wall stackup as-is, it doesn't really NEED a strong vapor retarder at any layer. But if the inspector insists, applying it directly to the OSB is more protective than putting it on the wallboard.

  14. user-1140531 | | #14


    Thanks for that explanation. I am going to study this in addition to my study of sorption and how it relates to condensation. It sounds like you are saying that diffusion never was a problem, but was blamed for a problem caused by insulation which increased moisture content of wood on outside of the insulation. Yet I have lots of reference material that describes the ability of diffusion to pass vapor through materials like drywall and into regions of condensiong dew points. I clearly understand that model.

    But it seems like GBA is replacing that model with an entirely different model. What I would like to do is make a big pictorial diagram of exactly what that model is doing in terms of vapor movement and transition from gas to liquid, and back to gas. I cannot question it until I get a clear picture of what is being presented, and it is hard to capture that whole big picture from fragmentary discussion here.

    I am particularly interested in getting to the bottom of some of those issues we recently discussed about adsorption.

  15. GBA Editor
    Martin Holladay | | #15

    You wrote, "I have lots of reference material that describes the ability of diffusion to pass vapor through materials like drywall and into regions of condensing dew points....But it seems like GBA is replacing that model with an entirely different model."

    While it is certainly true that water vapor can pass by diffusion through drywall, that fact does not usually lead to problems in building assemblies. Fortunately, GBA is not the actor responsible for "replacing that model with an entirely different model." The ones responsible for that "replacement," as you put it, are building scientists. The "replacement" happened between 25 and 30 years ago.

  16. user-1140531 | | #16


    I would like to find a clear and comprehensive explanation of the new model that replaced the old model. I have heard it clarified that diffusion still occurs, but it has been found to not be a problem. I once asked you how it is known not to be a problem, and you said it was learned by investigating buildings and looking for evidence of diffusion being a problem, and not finding any evidence.

    I will dig into this whole topic and look for further clarification. As it is, it seems like the explanation is being expressed in fragmentary relative terms, and it is very hard to add it all up to get the big picture. If you can provide any references to that replacement theory that was developed 25-30 years ago, I would like to read them.

  17. user-757117 | | #17

    Why not start a seperate thread of your own so as to avoid any further fragmentation of the explaination you're looking for?

  18. GBA Editor
    Martin Holladay | | #18

    Q. " If you can provide any references to that replacement theory that was developed 25-30 years ago, I would like to read them."

    A. If you are looking for just one book, read Water in Buildings by William B. Rose.

    If you have an appetite for more information, the next book to read is Building Science for Building Enclosures by John Straube and Eric Burnett.

    If you prefer your information in small chunks, you can read dozens of articles on these topics on the Building Science Corp. web site.

    Finally, there is always GBA. Here are some GBA articles to get you started:

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

    When Sunshine Drives Moisture Into Walls

  19. user-1105327 | | #19

    dana and martin, if the osb is the air/vapour barrier and if it performs well as an air barrier (say, <1.5 ach), then is it necessary to employ the ADA?

  20. GBA Editor
    Martin Holladay | | #20

    No, you don't need to follow the Airtight Drywall Approach if you have an airtight layer of OSB on the interior side of your insulation.

    But you still have to detail the OSB so that it is a true air barrier. That means that you need to pay attention to air sealing at the perimeter of the OSB and at all penetrations. You also need details that connect the OSB air barrier to the air barrier at the foundation and the air barrier at the ceiling or roof.

  21. jklingel | | #21

    Dana: "...The interior poly requirement for insulating foundation studwalls is an actively BAD idea, yet enshrined in code in Canada. ..." Are you sure about the enshrinement? I have no clue, but a guy on another forum, who CLAIMED to be Canadian, said the code really reads something like "... you need a poly vb unless you can show that the air movement.... blah blah." He further claimed that a lot of inspectors don't read the blah blah and pass misinformation on and among themselves. FWIW...

  22. Expert Member
    Dana Dorsett | | #22

    I haven't paid for a peek at the 2010 version of the national building code (which I believe is the most recent) but unless the relevant verbiage has been moderated, in practical terms it won't have changed much.

    The exception clauses found in earlier revs of the national code reads, vapor barriers are required unless...

    “ can be shown that uncontrolled vapour diffusion will not adversely affect any of,
    (a) health or safety of building users, (b) the intended use of the building, or (c) the
    operation of the building services.”

    This has been widely read to mean that it's required unless you can prove that it's NOT needed. Proving a negative is a tough proposition, which leads to the reductio ad absurdum exterior insulation keeping the sheathing above the interior dew point even at the outside design temp, (if it's ALWAYS above the dew point it will NEVER take on moisture) and the use of poly on basement walls, simply because the quality of proof in the clause is not defined, and inspectors demanded it.

    Running your own WUFI simulation on the stackup probably won't cut it as sufficient demonstration that meets the legal definition of "shown" in the exception clauses, but maybe if it came from an enginering company it would (?). SFAIK there's no prescriptive stackups by climate zone built into Canadian codes that are in any way comparable to Table R702.7.1of the IRC2012 widely used in US building codes.

    But maybe some sanity has crept into the new version of the national code (or at least some local codes). Until then, the onus is on the builder to have "shown" that the stackup works in the local climate, and the inspectors are not usually building scientists or engineers, and DO INDEED pass on the widely accepted disinformation about what does and does not work.

    It's not as if these issues aren't being studied in Canada (see: ), but I've yet to see an indication that it's made it into code yet. But I'm not building in Canada, and I may be operating on obsolete assumptions.

  23. user-1140531 | | #23

    I have not read the Canadian code. When they call for a vapor barrier, are they specifically calling for polyethylene sheet on the room side of the insulation cavity?

    It seems to me that if you had closed cell foam board on the outside of the frame, and if there was enough R-value in the foam to keep the warm side above the dew point temperature, then the interior side of the foam would be the vapor barrier. Assuming that there would also be insulation on the warm side of the foam, the form would just be a case of setting the vapor barrier somewhere midway in the total insulation layer, which is acceptable practice.

  24. user-757117 | | #24

    Provincial codes supercede the national code so if there is an argument to be made, it should be based on any direction (if any) given in the B.C. code.

    My own experience has been that some inspectors are more receptive to alternative paths to code compliance in this regard than others.
    If you are unlucky, your inspector will be covering his/her own ass and may not be interested in hearing about anything he/she isn't familiar with.
    If this is the case, Martin's earlier advice might prove best - try to go over the inspector's head.
    But in a "nice" way, so that you don't make more headaches for yourself down the road.

  25. jklingel | | #25

    My brother was having "issues" w/ an inspector, so when the inspector came back to re-check (while bro was at work) he helped himself to the conveniently placed bottle of scotch and signed off where bro desired. Simple. One never knows...

  26. Expert Member
    Dana Dorsett | | #26

    The "non-compliant inspector" problem knows no national, provincial, state, or municipal boundaries- they're everywhere!

    But the unfortunate "... can be shown..." turn of phrase in the national code on the vapor barrier issue puts a billy-club in their hand a club that's seen some real(ab)use. Try pricing out what it takes to build with enough exterior R to keep the sheathing above the interior dew point at the 99% outside design temp in in Edmonton sometime- it's RIDICULOUS! But building to keep it above the interior dew point at the average winter outdoor temp or even the mean January temperature isn't very onerous at all, and it's enough.

  27. Expert Member
    MALCOLM TAYLOR | | #27

    Dana, the BC code has distinguished between Air Barriers and Vapour Barriers since at least the 1992 edition. The code allows various materials to be used as Air Barriers and Vapour Barriers including rigid insulation. I can't find any reference to the need for poly to be used in any wall by reading section 9.25. which deals with Thermal Insulation and the Control of Condensation. Have you seen it somewhere else in the code?

  28. Expert Member
    MALCOLM TAYLOR | | #28

    Just to put the discussion of Canadian codes in perspective, the National Building code is a model created by the National research Council. It has no legislative authority, but is used by the Provinces and sometimes municipalities as a template from which to create their own codes, which vary quite widely and are regionally specific. So give that it isn't really a code, that nowhere uses it without adaptation and that inspectors can only interpret or enforce their local governing codes, there isn't much point in worrying about it's content or effect.

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