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Am thinking of the following wall and roof assembly in BC as a cost effective way to achieve a high R-value and tight envelope.

Reggie Dubois | Posted in General Questions on

2×6 structural wall with roxul in the cavities and gaps in sheathing (drill holes as well) to promote drying to exterior. The inside of this wall is sheathed with 1 inch of polysio (air and moisture barrier). A 2×4 wall inside this provides chase services, a cavity for fiberglass batts, and protection of air barrier.
The 2×6 walls are straightforward construction, with most of the building wrap and rainscreen installed before lifting wall. Foam insulation is also easy to install with no penetrations. The only extra work for this R-40 tight wall assembly with a thermal break is a 2×4 wall framed on the inside.
The roof is a cathedral roof framed with 2x12s and sprayed with closed cell foam (rim joist cavities sprayed at same time). In my mind the money saved by using dimensional lumber instead of I joists (material and labour), as well as the superior insulating and air sealing benefits of the spray foam makes it a good place to splurge.
Any thoughts would be much appreciated.
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  1. GBA Editor
    Martin Holladay | | #1

    5.5 inches of Roxul = about R-21.5, and 1 inch of polyiso = about R-6, so you get a wall assembly rated at about R-27.5, minus the downgrade required for the studs.

    [Edit - I just noticed that you want to insulate the 2x4 wall also. That brings you up to R-40.]

    Drilling holes in your sheathing is silly. Most builders are aiming for airtight exterior sheathing; exterior air barriers are a good idea.

  2. Reggie Dubois | | #2

    My exterior air barrier would be building wrap, the holes and gaps in the sheating are for a vapour escape. The polysio on the inside face of 2x6 is a vapour barrier. Sheating the wall super tight would be a moisture trap. Tight sheating is somewhat of a moisture barrier, no?

  3. GBA Editor
    Martin Holladay | | #3

    Housewrap makes a lousy air barrier. It gets torn by ladders and fasteners, it flaps in the wind, and it is rarely airtight.

    Use your wall sheathing layer as your air barrier -- you won't regret it. Drilling holes in perfectly good plywood or OSB is a nutty idea, although I know that some builders have been told by the guys at the lumber yard that it makes sense.

  4. Reggie Dubois | | #4

    Sorry I wasn't clear. My air barrier is my polysio foam on the inside of the 2x6 wall (also my moisture barrier). The building wrap is more of a drainage plane behind the rainscreen that doubles as a "first defense air barrier". A few years ago home warrantee programs were specifying that exterior sheathing be vented with holes under tar paper (on Vancouver Island where moisture can be high). This makes sense to me, to allow the wall to breath properly to the outside. No? Do you have any issues with the rest of this assembly?

  5. GBA Editor
    Martin Holladay | | #5

    I know that the practice you suggest -- drilling holes in exterior sheathing -- is fairly common in British Columbia. It started as a fad among builders who were distressed at the massive failures that became known as the Leaky Condo Crisis.

    However, this method has no basis in building science. It was simply a panicked reaction by builders who were tearing out their hair because of the wet-wall disasters they were seeing.

    If you pay attention to proper flashing of wall penetrations, and if you include a rainscreen air gap between your siding and your sheathing, you don't need to drill holes in your sheathing.

  6. Expert Member

    The practice may have originated with builders, but it was backed by research into wall cavity drying by the CMHC, and although not specifically addressed in the code, was recommended by both building inspection officials and government publications such as the BC Building Envelope Guide for Houses.
    The inclusion of a rain screen, no matter how well detailed, does nothing to deal with the drying of moisture in the wall cavity and the permeable exterior sheathing is used to avoid the double vapour barrier that would result from sealing the exterior. This is recognized in the wall details for Pacific Northwest construction put out by BuildingScience too. You would do well to re-think your advice on this one.

  7. GBA Editor
    Martin Holladay | | #7

    1. I stand by my advice. It's nuts to drill holes in perfectly good OSB or plywood wall sheathing.

    2. You are wrong about the advice given by Building Science Corporation for wall assembly design in the Pacific Northwest. Here is a link to the relevant paper: High-R Walls for the Pacific Northwest. The paper by Jonathan Smegal and John Straube looked at 17 different wall assemblies, none of which included exterior sheathing with deliberate holes. The best three performing walls -- those with the fewest winter hours of potential condensation -- were (1) a 2x6 wall with 4 inches of exterior XPS, (2) a 2x4 wall with 2 inches of exterior XPS, and (3) a 2x6 wall insulated with 1/2-pound closed-cell spray polyurethane foam.

    3. Your statement "The inclusion of a rain screen, no matter how well detailed, does nothing to deal with the drying of moisture in the wall cavity" is absolutely false. Abundant evidence shows that a ventilated rainscreen leads to rapid drying of damp wall sheathing. I have no idea where you got the opposite impression.

  8. Billy | | #8

    I agree with Martin, and I think you should rethink your wall assembly. You should be able to get better performance by adjusting your materials and design.

    Why not put the polyiso on the exterior and perhaps make it thicker? If you are using fiberglass in the walls they will dry to the inside. With your thick wall assembly you could use cellulose or Spider fiberglass fill.

    Your cathedral ceiling sounds fine. As long as you have the foam guys there I would leave out the polyiso sheet in the walls, spray 3 inches closed cell into your exterior wall against the sheathing making an excellent air barrier, and add a rain screen to the exterior that will allow the sheathing to dry should it get wet. Then fill your wall with whatever insulation you want. You'll save a lot of labor and time and get a better wall assembly.


  9. Expert Member

    Let me deal with your response point by point:
    1. There practice isn't to randomly drill holes in your sheathing, but to increase the ability of the wall cavity to dry to the exterior in wall assemblies designed to dry that way. This is done by leaving a small gap between sheets when applied horizontally and drilling holes in the stud spaces that are fully enclosed, such as under windows.
    2. Your reference to Buidingscience refers ro several specific assemblies they recommend for high R walls. If you look at their general advice for the PNW you will see they warn against the inclusion of assemblies that trap moisture and have no drying path - exactly what you are recommending in the OPs suggested wall section - providing a fairly effective vapour barrier on both the interior and exterior of the wall. (
    3.I think you are still confusing the role of a rain screen assembly. It is used to stop the intrusion of wind blown rain and allow the siding and exterior face of the sheathing to dry. It does not play a significant role in drying moisture which is already inside the wall cavity..
    I guess my main comment would be that the combination of rain screen and a permeable exterior sheathing has proved remarkably effective as a wall assembly here in BC, and is the one recommended both by our code and most building envelope specialist here. It may not be you preferred method, but your dismissive comments about it being a panicked reaction, or something DIYers might be told at the lumber yard has no basis in fact.

  10. GBA Editor
    Martin Holladay | | #10

    Of course I agree with you (and Building Science Corp.) that it is undesirable to build "assemblies that trap moisture and have no drying path." That should go without saying. Even a cursory reading of the advice I have been giving here at the GBA website should reveal that I am in favor of wall assemblies that don't trap moisture and have a way to dry out.

    That said, I can't find any evidence in the paper you linked to that Joe Lstiburek or John Straube has ever recommended that builders drill holes in their wall sheathing. If you can find such a reference, please quote the sentence or indicate the drawing where this practice is recommended.

    One way to limit moisture accumulation in wall sheathing is to cover the sheathing with rigid foam that is thick enough to keep the sheathing above the dew point during the winter. Another is to be sure that the wall sheathing is protected by a vapor-permeable housewrap and that the wall assembly includes a ventilated rainscreen gap to encourage drying to the exterior.

    You are wrong when you state that a ventilated rainscreen "does not play a significant role in drying moisture which is inside the wall cavity drying." Check out John Straube's studies on this point; they are clear. When water is introduced into the stud cavities of a wall during testing, the walls with ventilated rainscreen cavities dry much faster than those without such cavities.

    I have no idea why you think I disagree with your advice: "The combination of rain screen and a permeable exterior sheathing has proved remarkably effective as a wall assembly here in BC, and is the one recommend both by our code and most building envelope specialist here." That sounds sensible to me.

    Just don't drill holes in your sheathing!

  11. GBA Editor
    Martin Holladay | | #11

    Here's a link to a relevant paper:
    “Moisture Recovery Rates for Walls in Temperate Climates”

    "Drying rates have been measured in 22 specimen walls on an experimental building following the controlled introduction of water into the insulated spaces and the water managed cavities. ... Summer and orientation-averaged drying rates for walls with absorbent claddings progressively increased from drained and ventilated to open rainscreen to drainage plane to direct-fixed wall types."

    In other words, the walls with ventilated rainscreens dried faster than the walls with "direct-fixed" siding.

    This paper also summarizes earlier work by Burnett and Straube: "An ASHRAE-sponsored project (ASHRAE 1091) developing the physics of heat, moisture and air transport in walls has provided theoretical support for ventilation assisted drying (Burnett and Straube 1995) and field drying rate measurements (Straube et al 2004) for a range of wall cladding types and cavity dimensions. ... Water was introduced in a reproducible way through dosing tubes to an absorbent material fixed to the insulated cavity side of the sheathing. .. They arrive at the following helpful conclusions:
    • Useful outward drying through the sheathing takes place and the presence of a water managed cavity assists with the drying process.
    • Ventilated wall cavity designs with large vent openings top and bottom dried faster than bottom vented designs. These faster drying designs coped with repeated wetting over several years and remained in perfect condition."

  12. Expert Member
    MALCOLM TAYLOR | | #12

    Here is a link to the CMHC research on the use of holes in sheathing to speed the drying of walls:

  13. GBA Editor
    Martin Holladay | | #13

    In the June 2003 issue of Energy Design Update, I wrote an article about the research report you linked to. The article included quotes from Brian Palmquist, Don Hazleden, and Paul Morris, all three of whom I interviewed when researching my report.

    Hazelden told me, “The only problems come in the execution. Some workers will very carefully drill holes and carefully install the membrane, while others might try to do it with an ax or a shotgun — they might end up compromising the membrane. You also have to know where your air barrier is. If you are relying on your sheathing to be your air barrier, obviously you are going to have some problems.”

    Here's how I ended my article: "In Vancouver, the oft-repeated mantra to builders is to remember the four D’s of moisture management: deflection, drainage, drying, and durability. “Deflection and drainage are still where you want to put 99 percent of your emphasis,” says Hazleden. Once it gets wet, even a well-built wall dries out very slowly.

    “These vapor diffusion ports may be a useful tool for some applications, but they’re only one solution to part of the problem,” says Morris. “I think what the envelope drying rates studies have told us is: Don’t rely on drying as a primary mode of moisture management, because drying is not going to solve your problems.” "

  14. Expert Member
    MALCOLM TAYLOR | | #14

    Let's back up a bit. The Op suggested a strategy that is widely and successfully used in BC, You dismissed it as being "silly", "nutty", "a panicked reaction" and "having no basis in building science". I pointed out that this was not the case, and rather than just admit your mistake you have dragged the discussion out on a number of tangents.
    I came here to see what The Green Building Advisor was about and although it appears to have some very interesting resources, I sense there may be certain orthodox set of views based on a close reading of Buildingcience that people are not comfortable stepping outside of. Thanks for the discussion, I don't see much point in sticking around.

  15. Billy | | #15


    There's some truth to what you say. Taunton used to be a lot better than it is now.


  16. GBA Editor
    Martin Holladay | | #16

    Malcolm and William,
    I certainly hope both of you stick around at GBA. GBA is enriched by the type of dialog found in this Q&A thread, and your side of the dialog is a necessary component.

    It's true that I believe in exterior air barriers, and for that reason I think it is important not to drill deliberate holes in sheathing. And I was certainly guilty of using colloquial language when I called the idea "nutty" or "silly." I'm sorry if my poor choice of words insulted either of you, or any other GBA readers.

    My conclusion that it is wise to maintain the integrity of exterior sheathing, and not to drill deliberate holes in it, is not a casually acquired whim. I'm willing to back it up with a reasoned argument, supported by research data. Anyone with contrary ideas is welcome to argue with me. Such debates are healthy (and frequent, frankly, here at GBA.)

    I'll try to be more polite in my characterizations of suggested concepts in the future, because it was never my intention to insult anyone or drive you away from the site.

  17. Expert Member
    MALCOLM TAYLOR | | #17

    We just somehow got off on the wrong foot. I am indebted to you of the many interesting articles you have written over the years. Got to go now and drill some holes in my sheathing.

  18. white_pine_vista | | #18

    Hi Reggie. I worked with a Vancouver Island architect to design a coastal high R value wall system (around R40) on a budget -not new but with some twists. It's a staggered 2x4 stud wall on a 9.25" plate filled with damp spray cellulose insulation, sheathed with glued and taped plywood, R 10 rigid insulation (ideally roxul drainboard or if that's too pricey then rigid foam) building wrap, rainscreen and hardie plank or cedar on outside. There will be airtight drywall on the inside with American Clay in some areas and latex paint in others (not the vapour barrier kind). With Roxul board on the outside and HRV (maybe with stand alone dehumidifier) the sheathing can dry either in or out or both.

    The house (2 storeys) and attached suite share an entry and washer/dryer and total is 2900 square feet. I have a quote from a panelized home builder for a house kit to beyond lock-up ($128,000 for wall panels, trusses, joists, lumber, plywood, siding, triple pane windows, doors, metal roof, drywall. A crew to put it together costs about 70,000 if I do the general contracting. I have lumber for trim, porch and flooring and will do the flooring myself. The insulation quote for installed cellulose is 15,000 (plus 6000 for Roxul accoustical insulation if we go that way on interior walls and between floors.) The attic is vented gable with R60 cellulose over poly vapour barrier (or blown in cellulose). I am waiting for a quote on Roxul Drainboard R10, or Polyisocyanurate or other foam. Under slab will be R20 foam. It looks like we can do the project for a little more than 300,000 after HST rebate and all the subtrades. I can send some more details if you email me at [email protected] I would be interested in your findings, approach and quotes as well - or maybe we can attach them here if its not against the guidelines



  19. czvTCsHuVD | | #19

    Hi Martin,
    During our reno in 2007 at our home in White Rock, BC, the building inspector required me to leave a 1/2" horizontal gap between the plywood wall sheathing he said was for expansion. I had purchased 5/8" T&G plywood in hopes of eliminating any air gaps. It does seem logical that the goal was to ensure prevent any water from getting inside the wall. We did the standard 2x6 walls with Roxul insulation and the 6 ml poly vapour barrier on the inside of the wall. In 2007 Delta Dry rainscreen by Cosella Dorken received some building awards, so I ordered some in, only to be told by the inpector it was not allowed or approved in our small town. So I had to use 15lb tar paper and wood strapping under my Hardi siding. Since plywood by design is very structurally stable, is a gap really required?I agree with your advise to keep the holes to a minimum since it seems any air borne moisture would be able to escape through the wood anywaywith the vapour barrier on the inside?
    Thanks - good debate....

  20. GBA Editor
    Martin Holladay | | #20

    The standard spacing at the ends and sides of wall sheathing panels (OSB or plywood) is 1/8 inch, as recommended by APA - The Engineered Wood Association. See this document: Cut Callbacks with Proper Spacing. I don't know why your local inspector suggested a 1/2-inch gap.

    Of course, if you are using your exterior wall sheathing as your air barrier, all seams should be taped.

  21. John Straube | | #21

    The summary report supporting Vapor Diffusion Port (thanks for the link) does not tell the whole story. The full report, and experimental setup should be looked at. The differences caused by the VDP are very small and not statistically significant. I am with Martin: drilling holes in your sheathing is nutty. It might actually help drying because it will tend to increase air leakage, but that is not a good approach.
    The research we did with ASHRAE support was for ventilated claddings. For decent ventilation rates (which can happen when exposed to the wind and sun not very easily in climate chambers), ventilation can be quite useful.
    The wall design being discussed is simply far too complex and unnecessarily so. If you want an expensive and very high performance wall with R40 (and this is on the high side in the Lower Mainland) I would recommend a 2x6 with Roxul batt, NO vapour barrier, OSB sheathing, a vapor permeable peel-and-stick (like BlueSkin VP100) as fully-adhered air and water barrier and 6" of Roxul insulation sheathing, furring strips, and then vented cladding (like fiber cement siding or if you must stucco). To save money, use a drained housewrap, taped and sealed (not a great air barrier, but not bad) and maybe only 4" of Roxul.

  22. Reggie Dubois | | #22

    If it's basic building science to not sandwich an insulated wall assembly with vapor barriers, and you have a vapor barrier on the inside, than why would you dance with the possibility of having one on the outside as well by making your plywood too tight?

  23. user-757117 | | #23

    If a wall isn't designed to dry to the outside, then it should be designed to dry to the inside.
    Drying in both directions is best, of course.

  24. user-869687 | | #24

    Reggie, I'm a big fan of the assembly John Straube described. There is no vapor barrier inside or out. Air tightness (including at the sheathing) helps to keep the wall dry, by reducing risk of condensation forming inside the wall. Remember that air leaks can carry a great deal of moisture, far more moisture than would diffuse through an airtight wall. Plywood is not a vapor barrier and it will become increasingly permeable if it becomes damp, so it's basically a "smart" vapor retarder.

  25. GBA Editor
    Martin Holladay | | #25

    John Straube,
    Thanks for supporting my position.

    Few people (if any) in North America have been involved in more wall-wetting and wall-drying reasearch projects than John Straube, and as a consultant and forensic investigator he has inspected countless failing buildings. If you are questioning John Straube's advice, it would be good to have data to back up your contrary position.

  26. Reggie Dubois | | #26

    thanks Martin,
    I realize that John is an authority on this topic. That's why I asked him a question. I'm not trying to challenge anyone here, just trying to further my understanding on what I do for a living. Don't get your back up

  27. homedesign | | #27

    Hi Reggie,
    This was a very good thread with a lot of good comments and questions.
    Absolutely nothing wrong with asking questions.

  28. wjrobinson | | #28

    John Straube is an invaluable asset to all of us trying to build superinsulated homes that do not become moisture nightmares.

    And Congrats John on your Lifetime Achievement Award

  29. GBA Editor
    Martin Holladay | | #29

    Fair enough; I apologize for not answering your question. I agree that there is nothing wrong with asking questions.

    Q. "If it's basic building science to not sandwich an insulated wall assembly with vapor barriers, and you have a vapor barrier on the inside, than why would you dance with the possibility of having one on the outside as well by making your plywood too tight?"

    A. John Straube recommended "I would recommend a 2x6 with Roxul batt, NO vapour barrier..." So you and John Straube are on the same page here -- no vapor barrier on the inside.

    U.S. building codes do not require a vapor barrier in any climate -- only a vapor retarder in colder climates. The vapor retarder requirement can be satisfied with kraft paper, MemBrain, or vapor-retarding paint.

    I have heard that some building inspectors in Canada insist on interior polyethylene. I don't know too much about Canadian building codes, but I believe that some Canadian inspectors have accepted vapor-retarding paint when the scientific basis for the selection is explained to them.

    In both Canada and the U.S., poorly written building codes that lead to undesirable outcomes should be clarified or changed. If existing codes confuse local inspectors into thinking that interior polyethylene is mandatory in climates where it often makes sense to omit the poly, the the code needs to be rewritten.

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