Replies

1. 
   yogumon | Jun 05, 2015 08:41am | #1

   Difficult to do blocking through the vapor barrier. And the vapor barrier should be further inside.

   Alternative: OSB sheathing on the INSIDE of the double stud wall, diffusion open fiberboard on the outside. Add a ~ 2" installation layer inside, allowing easy cable runs etc. This can also be insulated, but doesn't need sealing or particular attention to detail.

2. GBA Editor
   Martin Holladay | Jun 05, 2015 09:13am | #2

   Ryan,
   There are a couple of issues here.

   First of all, MemBrain is not intended to contain dense-packed cellulose. You need an air-permeable membrane like InsulWeb for that purpose. (Since MemBrain is an air barrier, it doesn't allow air to escape during the blowing process.)

   Second, the usual solution to your dilemma is to install a layer of OSB or plywood on the exterior side of the interior stud wall. This layer of OSB or plywood is a vapor retarder as well as an air barrier. Many builders use this approach, including Joe Lstiburek and Passivhaus builders.

   For more information on these issues, see:

   Lstiburek’s Ideal Double-Stud Wall Design

   The Klingenberg Wall

3. Expert Member
   MALCOLM TAYLOR | Jun 05, 2015 10:10am | #3

   Ryan,
   While your air barrier can be anywhere in the assembly, your building code wants the vapour barrier in a position to "protect the warm side of (the) assembly" (9.25.4.3.) It would be hard to make a case that installing the membrane that close to the exterior meets that requirement.

4. 
   RMaglad | Jun 05, 2015 11:31am | #4

   Martin et al,

   Thanks, I was under the impression that smart retarders could be used if holes were punctured top/bottom and re-sealed after density is achieved. Patch the hole with appropriate tape.
   video from 475: https://www.youtube.com/watch?v=hOhE-UP72xg

   If Ontario building code/building approval officials will require a vapour control layer, then the effort might as well be spent air sealing it, versus installing mid-point sheathing, and additional vapour open sheathing on the outside? I am thinking that having 3 sheating panes in a wall assembly is overkill (ext vapour open, plywood/osb at midpoint, and interior drywall). I hadn't considered blocking, and am not sure if it is absolutely required given that the interior wall is non load bearing/non structural.

   I was hoping to keep all electrical on the "inside" of my air barrier to help eliminate air leakage/the tedious task of sealing around all boxes/wire penetrations

5. GBA Editor
   Martin Holladay | Jun 05, 2015 11:43am | #5

   Ryan,
   The YouTube video shows Intello Plus. I have no idea whether MemBrain can be used to hold back dense-packed cellulose; I would contact the manufacturer (CertainTeed) to find out before proceeding with your plan.

   There are many ways to build a wall, and there is nothing wrong with choosing a wall assembly that seems to make sense to you (even if others disagree). In general, though, the closer your smart vapor retarder is to the interior, the better.

   MemBrain is a smart retarder, and so are plywood and OSB.

   For more on service cavities, see Service Cavities for Wiring and Plumbing.

6. Expert Member
   Dana Dorsett | Jun 05, 2015 03:25pm | #6

   MemBrain is unreinforced 2-mil thick nylon sheeting, which is pretty stretchy stuff. Dense-packing behind it would be nearly impossible without HUGE pillowing issues, and it would probably rip out at the staples unless reinforced.

7. 
   charlie_sullivan | Jun 05, 2015 03:56pm | #7

   Switching to Intello solves the strength issue for dense-pack. But it's still easier to blow in insulation behind a fabric that allows air to flow through, and allow allows seeing what is going on better than Intello does. I'd be inclined to use the "insulweb" or equivalent to hold the insulation during the fill, and then cover with one of the membranes. An additional advantage is that you don't need as many staples in the membrane at that point, so it stays intact.

   Note that dividing the space helps inhibit settling of the cellulose--very deep cavities are more susceptible to that problem. That's an added bonus to the mid-wall barrier scheme.

8. Expert Member
   Dana Dorsett | Jun 05, 2015 04:13pm | #8

   It's a bit of a pain, but using OSB on the exterior side of the interior studs as the vapor retarder works very well and can be dense-packed well in advance of the electricians, etc. allowing great freedom for scheduling when to insulate the interior studwall cavities. Dry OSB almost but doesn't quite reliably meet the vapor permeance specs for the Canadian code definition of "vapour barrier", but with a coat of half-perm "vapor barrier" latex it would.

   With fiberboard or gypsum structural sheathing under rainscreened siding on the exterior side of the assembly, even un-painted OSB in the middle of the stackup would be fine in an Ottawa climate, code definitions notwithstanding.

9. 
   user-4524083 | Jun 07, 2015 01:26am | #9

   Ryan - While Dana's suggestion is preferable to the air barrier on the inside face of the outside wall, as you originally proposed, I think your idea would work fine,(though I would prefer plywood or OSB to the Membrain).Trying to tape and caulk the OSB on the exterior side of the interior wall, as Dana is suggesting sounds very difficult. Even if you tape before erecting the wall, you still have the top and bottom to address.And what do you do on the inside corners? Your idea, but with OSB (or even your vapor permeable membrane)on the interior of the exterior wall, is easy to tape and seal. Sure it's not ideal, and it would be nicer to have more insulation outside the air barrier, but it's a dramatic improvement over a conventional double stud wall with the air barrier on the outside getting very cold and wet in the winter. And these double stud walls seem to be doing o.k., though they are admittedly worrisome. John Straube wrote a nice article on this topic. BSD-163: Controlling Cold Weather Condensation Using Insulation, in which he's looking at how much foam to use over the exterior sheathing to keep it warm enough to avoid excessive condensation. But if it's not foam doing that work, and instead it's something that allows diffusion, now the wall can dry to both the inside and the outside. Or as he writes, "If the selected sheathing layers ( including structural sheathing,water control,and insulation) are somewhat vapor permeable… less R-value can be used and diffusion condensation will still be controlled( because much of the vapor that diffuses or leaks with air into the stud bay will pass harmlessly through to the outside by diffusion)." This idea is a hybrid of the double stud wall and the foam- over- stud wall, just using another stud wall instead of the foam. Nicer environmentally, more vapor permeable, AND, an improvement over the single,deep, double stud wall. Good luck with your project !

10. 
    Dana1 | Jun 07, 2015 05:32pm | #10

    Putting plywood or OSB on the interior side of the exterior studs can be risky in cold climates, unless combined with MemBrain or similar just under the gypsum. The ratio of exterior-R to interior R ideally needs to be sufficient to keep the average winter temp at the air-barrier layer above the dew point of the interior air (about 4-5C in cold climates, about 6-7C in coastal B.C.), in which case there would be no need for interior side vapor retarders (other than latex paint.)

    Using fiberboard rather than plywood or OSB as the exterior sheathing does a lot for improving drying capacity toward the exterior, but won't completely mitigate a dew-point problem at a middle layer in every climate (though it would in some.)

    The argument that many double studwalls without vapor retarders in cold climates are doing OK doesn't mean that it isn't a problem for the long term, or in specific circumstances. The measured moisture content of the cellulose insulated test cavities in Kohta Ueno's test house in US climate zone 5 is high enough to be of concern, surely not "best practices", even though there was no apparent damage after 2 years of occupancy. With thousands of homes built similarly, over a handful of decades problems would very likely show up in some of them, even with only vapor diffusion as the only moisture transport factor.

11. Expert Member
    MALCOLM TAYLOR | Jun 07, 2015 07:37pm | #11

    Kevin,
    I think moving the sheathing to the outside of the interior wall and using it as an air barrier was Thorsten Chlupp's idea. He essentially frames the house conventionally, seals the joints, then frames another wall outside. The advantages are having the sheathing run continuously over the whole walls, not just from floor to floor as you would with the sheathing on the inside of the exterior studs, and having the floor system and rim joist stop at the interior wall, eliminating thermal bridging.
    Whether Ryan's idea will work or not, he still has the problem of the code requiring a vapour barrier of at least 60 ng(pa.s.m2) that somehow or other needs to be included as the code says:"sufficiently close to the warm side of the assembly to prevent condensation at design conditions."

12. 
    user-4524083 | Jun 08, 2015 11:15pm | #12

    Ryan, Malcolm, and Dana - The advantage of Ryan's approach in a cold climate is the ability to get the house closed in quickly, then finish from the inside. The uninterrupted sheathing/air barrier could be accomplished by "hanging" the ceiling joists ( or the second floor joists if two stories) from the sheathed wall. One foot of the sheathing could be put up behind where the floor would be hung from, and insulated later.This would require studs more than 8'. One compromise to the condensation problem would be to use 2x6 exterior walls, and get a little more R-value outside the sheathing, and enough to stop most condensation problems, by my rough calculations and using the rules from the Straube article quoted above. I was not going to reply to your thoughtful and intelligent retorts, but am planning a house myself and am not ready to give up on these ideas yet. Thanks for the dialogue.

13. Expert Member
    MALCOLM TAYLOR | Jun 08, 2015 11:49pm | #13

    Kevin,
    Lots of way to skin a cat. Swedish Framing, Remote, Mooney walls, etc, etc. I just try to occasionally remind people that apart from getting the building science right it's good to remember that once construction starts it's your inspector that will call the tune, and his reference is the code.. We have almost a dozen visits before final occupancy is granted.
    Edit: Here is a link to a good discussion of Chlupp's walls:
    https://www.greenbuildingadvisor.com/community/forum/general-questions/19942/sunrise-home

14. 
    RMaglad | Jun 09, 2015 10:47am | #14

    If the details are as follows (from inside-out);

    drywall,
    2x4 @16 stud wall with roxul insulation/dense pack cellulose
    plywood air/vapor barrier control layer, caulked, taped and sealed to be fully continuous (depending on building department review/approval)
    thermal break (dense pack)
    2x4 @ 24 exterior stud wall with dense pack
    exterior sheathing, fully vapor open fiberboard or equivalent
    Water resistant barrier
    1/2" furring strips
    Exterior Side/Brick Veneer where needed

    Does the interior wall become load bearing?

    My intention would be to construct the interior wall/air barrier system first, and return and construct the exterior wall. Intello would be used as a ceiling air/vapor control layer and would be draped over the interior wall prior to truss placement. If the air barrier is already constructed, and whole, the exterior wall will require some sort of sheathing. How is the dense pack installed? Are holes required through the fiber board, or do you wait for a sunny day, install insulweb, and quick close it in?

    What if my originally proposed simply used an intello vapor retarder, on the exterior side of interior stud wall. If the interior wall is built first, then the intello could be carefully sealed. The exterior wall could be erected and sheathed conventionally (this may be difficult, as conventionally it would be assembled on the subfloor and tilted up into place). Holes would be used per the 475 video to dense pack insulated the exterior cavity, while densepack or roxul batts used on the interior. I am stuck on the logistics here

15. 
    kcfink | Jun 09, 2015 11:29am | #15

    Ryan,
    I am currently preparing to build a double-stud wall in a manner very similar to your latter description (interior wall, intello+ on the exterior of interior wall, cavity space, exterior wall. In addition to some of the challenges you've mentioned, one other thing you should think about is that the Intello+ is only rated for a limited amount of UV exposure (2 weeks). As a result, you'll want to plan to get sheathing up on the exterior shortly after the Intello goes on.

    If you are considering using the Intello in this method, you may want to reach out to 475 to get some of their thoughts on sequencing this type of wall. I have found them to be helpful as we've thought through how to do this on our project.

16. 
    RMaglad | Jun 09, 2015 01:50pm | #16

    Kevin, I was just thinking about this.

    I'm sure the exterior 2x4 wall could be constructed as typical (1st). Then the interior wall constructed on the flat with the intello+ sealed and wrapped around the top and bottom plates. This would allow for the intello to be sealed to the subfloor after the wall is lifted up into place. This, also allows for the intello+ to be sealed to the ceiling intello+ installed in the future once the trusses are in place.

    This should also make local building approvals happy since there is a dedicated vapour control layer, on the warm side of the wall.

17. 
    STEPHEN SHEEHY | Jun 09, 2015 03:26pm | #17

    We installed the membrane as Ryan described. At the corners, we ran it long and then, when the walls were stood up, folded the two pieces together and stapled them inside a stud, then taped the joint. It's pretty obvious when you look at it, although hard to describe.

18. 
    user-4524083 | Jun 10, 2015 12:17am | #18

    Ryan, Stephen, Kevin F., and Malcolm - that sounds like a good plan and one that addresses Malcolm's pragmatic concerns as well. Ryan - You may want to ask Stephen more specific questions, since he's just recently built this wall. Plus, he's from Maine, and therefore "wicked" smart. Best of luck to you !