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Can someone explain the science behind a mid-wall vapor barrier on a double stud wall?

alumniu | Posted in Green Building Techniques on

In order to save money, I’ve switched my wall assembly last minute from using exterior rock wool board to a double stud wall construction. I would like to speak with my county inspector about using this approach to make sure I don’t have problems during construction. I’m struggling to understand how I’m going to explain how a vapor barrier on the back side of the inside wall is a good idea.

I’m in climate zone 5, and here is my intended wall assembly:

From inside-to-out:
Insulweb netting (affixed to stud to hold cellulose)
2×4 stud wall (with cellulose) (also serves as utility chase)
Certainteed MemBrain (affixed to backside of stud. Taped and sealed)
5″ gap filled with cellulose
2×4 stud wall (with cellulose)
Insulweb netting (affixed to outside of stud to hold cellulose- blown-in from outside?)
Housewrap (brand yet to be determined)
1×3 rain screen gap
cement board siding

So, I have several questions.

1. How is a mid-wall vapor barrier a good idea?

2. Do I need to install MemBrain on the inside of the inside stud wall as well? Every house I’ve dug into has a 6mil standard vapor barrier directly behind the drywall. I’m wondering if my county inspector will want a vapor barrier here. If they do, I imagine ill want a smart vapor barrier here as well rather than standard plastic sheeting.

3. How do I install horizontal fire blocking to separate the two stud cavities? At what point in the framing process should I do this?

4. Can I fill the full 12″ depth full of blown-in cellulose as planned?

Sorry, so many questions.

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  1. STEPHEN SHEEHY | | #1

    I don't know what your inspector will want, so can't comment on that. Don't know why you'd need fire blocking.

    But our double stud wall may be easier. From inside:

    2x4 stud wall, filled with batts. Functioned as utility chase.
    Membrane stapled and taped to outside of inner stud wall.
    5" space, filled with cellulose.
    2x4 stud wall. This wall is structural. Filled with cellulose. The cellulose was blown from inside, through holes cut in the membrane and then taped.
    WRB, seams taped
    Cedar breather
    White cedar shingles.

    This was a single level house. We ran the membrane long at the top and lapped it over the ceiling membrane stapled to the bottom of the trusses.

    I like the midwall location for the air barrier because it eliminated most wiring and plumbing penetrations

  2. GBA Editor
    Martin Holladay | | #2

    There are many, many ways to build a double-stud wall. You are proposing one way to do it. What surprises me is that you are asking, "Why am I doing it this way?"

    I suppose my answer would be, "I don't know -- you tell me. If you don't think this way makes sense, why are you proposing it?"

    I'm assuming that you copied this detail from somewhere, but are unsure of the logic behind the wall. Is that what's going on?

    Q. "How is a mid-wall vapor barrier a good idea?"

    A. Proponents of mid-wall vapor retarders usually point out that this location (a) allows electricians to work without worrying about penetrating the vapor retarder, and (b) protects the vapor retarder from abuse.

    Q. "Do I need to install MemBrain on the inside of the inside stud wall as well?"

    A. Not necessarily.

    Q. "I'm wondering if my county inspector will want a vapor barrier here."

    A. If that's what you are wondering, it's time to talk with your inspector.

    Q. "Can I fill the full 12 inch depth full of blown-in cellulose as planned?"

    A. Yes, but your plan to blow insulation from both the inside and the outside is a little unusual.

  3. Expert Member
    Dana Dorsett | | #3

    In a zone 5 location as long as at least 33% of the total R is on the exterior side of the vapor barrier there will be no wintertime moisture accumulation at the interior side of the vapor barrier (nor any summertime moisture accumulation on the exterior side.)

    The fact that it's MemBrain and not 6 mil polyethylene means that it can't accumulate moisture under any conditions other than bulk water incursions.

    Locating a vapor retarder on the exterior side of the interior studs means there are many fewer electrical & plumbing penetrations to seal to detail it as an air barrier.

    In a zone 5 location the fact that the siding is rainscreened means that there is not a code requirement for a vapor retarder other than standard interior latex paint on wallboard:

    Your stackup meets the "Vented cladding over wood structural panels." exception.

    BTW: Use 1x4s instead of 1x3s for the rainscreen furring. That makes it far less prone to splitting.

    Dense packed cellulose qualifies as fire blocking all by itself in most locations as long as the vertical depth is over 14-16 inches or so. See section 718.2.1 Fireblocking materials in the IRC:

    See also: (<<See section 4.4 on fireblocking)

  4. GBA Editor
    Martin Holladay | | #4

    Good answer -- especially on the fire blocking. Thanks.

  5. alumniu | | #5

    Awesome. You guys rock! Great answers as always.

    Great suggestion on filling the outer 8.5" from the inside by cutting slits in the membrane. For some reason I was thinking after this was carefully taped and secured, that I wouldn't want to punch holes in it...but I suppose its no big deal if its just taped right back up. No need for netting the outside this way.

    You are correct. I saw the details of double stud construction on GBA, and BSC. I was unsure about the logic behind the wall. Specifically why its ok to put a vapor retarder 3.5" back from the inside surface. I guess I didn't understand at which depth in the wall this becomes a hinderance rather than a benefit. I think Dana answer that with the "33% of R-value on exterior side" answer. I've been so focused on the exterior rock wool details the past couple of months, that I've got some catching up to do now after switching to double stud approach.

    Great info here. Thanks so much for the detailed explanation....and extra kudos for the links!

    Thanks all!

  6. Expert Member
    Dana Dorsett | | #6

    The recommendation is a MINIMUM 33% of the R value on the exterior side of the condensing surface (= the vapor retarder), and that's only valid for climate zone 5. In this case more is better (which you have.)

    In reality you'd likely still be able to skate-by at 25-27% if the IRC chapter 7 prescriptives mean anything, but at 33% you have some margin. At only 25% you would risk periods of wintertime liquid condensation on a polyethylene vapor barrier, not so much with a MemBrain or OSB/plywood vapor retarder. In colder climates than zone 5 that fraction goes up. In climate zone 7 it would 38-40% absolute minimum, 45% minimum to provide some margin.

    In your stackup well over half the total R would be exterior to the vapor retarder, so it doesn't much matter. You would have HUGE margin in any climate zone.

  7. alumniu | | #7

    Thanks for the further detail Dana! This is info I couldn't find when investigating double stud wall articles. Probably has more to do with vapor barriers in general which I didn't delve into.

  8. Expert Member
    Dana Dorsett | | #8

    In general you don't need vapor barriers at ALL on a zone 5 climate- rainscreened siding alone is sufficient, but the MemBrain won't hurt, and will even help a bit.

    If you had located the MemBrain at exactly 33% of the R in from the exterior side it's usefulness would be limited, since the relative humidity of the entrained air between the wallboard and MemBrain at that cooler layer would be high enough to make it more vapor open. At your placement within the stackup that won't happen- it'll be a pretty good vapor retarder all winter long, and won't block drying in either direction the way 6 mil poly would.

  9. StollerB | | #9

    Is this a single storey home Adam? If not, I am wondering what people would recommend one does with their floor system in these suggested double stud walls. Running it all the way out to the exterior wall means that you have all the thermal bridging of each of those joists, and additionally the rim board is placed in the coldest possible location (displacing insulation as well). It also means that your smart membrane can't run through all those floor joists and continue up the stud wall on the floor above. Would you just closed cell spray foam the entire perimeter of your floor system and accept the thermal bridging of the joists, or what? And what about the exterior plywood also being a bit of a bottleneck in terms of vapour diffusion to the exterior?

    Dana- Regarding the placement of the vapour barrier, perhaps I haven't kept up with recent changes in the thinking or there are nuances regarding zones, but my understanding has always been that in heating dominated climates there should always be 2/3 of the r-value on the exterior side of the vapour retarder, not the other way around?

    I am basically in the same boat as you Adam. I liked the idea of doing 6" of exterior mineral wool on the outside of a standard 2x6 wall UNTIL I started adding up the costs associated. Got real crazy, real fast. So, I am back to puzzling my way through how to do a cost effective double-stud wall that is quick and easy to build, permits simple air-sealing details, and most importantly, avoids the risks of cold/wet exterior sheathing that plagues double stud walls. I think I am getting close, but the above concerns I have are some sticking points for me. I will probably do a sketch of the assembly to post and ask for critiques soon, but am basically in a similar boat to yourself!

  10. GBA Editor
    Martin Holladay | | #10

    Your questions about rim joists are good ones. One common solution is to make the inner wall the bearing wall, and to hang the outer wall on the exterior side of the foundation and rim joist. That is the approach suggested by Joe Lstiburek, as discussed in Allison Bailes's article, Lstiburek’s Ideal Double-Stud Wall Design.

    The illustration can be found at Double-stud wall.


  11. RMaglad | | #11

    To add to Burkes point, i was recently discussing my planned wall assembly with my friend who is also a building official here in Ottawa, USCZ7.

    I had planned on the following wall construction, from the inside.
    Interior Gypsum Wall Board (26 perms)
    2x4 @ 24" stud wall, dense pack cellulose R12
    3.5" gap, dense pack cellulose R12
    1/2" plywood sheathing (1 perm, primary air and vapour control layer)
    2x6 @ 24" structural wall (undecided insulation, assume dense pack) R19
    Exterior Grade gypsum sheathing (26perms)
    1x4 furring strips (rainscreen)
    fiber cement siding/stone veneer

    In this assembly, the inside edge of the vapour retarder has 44% exterior insulation (not including the sheathing or losses for thermal bridging).

    My assembly meets the previously mentioned 33% exterior insulation, is vapour open in both directions. However, my friend mentioned that the Canadian rule is 66% insulation on the exterior side of the vapour retarded/barrier layer. This means that my wall is no good up here, yet it seemed like a fairly simple wall to build. All air sealing done from the inside against the plywood, easy to apply liberal amounts of sealant and tape. No service penetrations. Very durable.

    He did mention that one has the option to perform a dew point calculation on the vapour barrier, and if it passes, would be accepted. Then he went on to throw another wrench into my plans, and that the 2017 ontario building code will likely require continuous insulation through a wall assembly. I am not sure on the specifics of this, or if insulation may change for thought.

  12. Dana1 | | #12

    Burke: Under the IRC for wall assemblies it's sufficient to use an interior latex paint as the interior side vapor retarder as long as the exterior R values in Table R702.7.1 are met:

    While these are minimums, it is substantially LESS than 2/3 of the total R on the exterior.

    In roof assemblies it's a somewhat higher fraction, spelled out in Table R806.5:

    These prescriptives presume the code minimum performance as spelled out in Table N1102.1.1:

    if the total center cavity R increases, the exterior R has to grow proportionally, in order to keep the average temperaure at condensation surface (the sheathing) the same. In this zone 5 case, R13 + R5 continuous insulation meets code min. That's a center-cavity total of R18, with R5 being exterior to the sheathing which is R5/R18= 28 % of the total. With a presumptive R20 cavity fill for a 2x6 framed assembly the prescriptive exterior R goes up to R7.5, for R27.5 total, making the exterior fraction R7.5/R27.5= 27% of the total.

    Under the Canadian code there are similar but not identical requirements, as discussed in this thread:

    In Canada the interpretation of the NBC by inspectors regarding eliminating the Class-II vapor retarder is usually more severe than the IRC, but there is no building science case for requiring fully 2/3 of the total R to be on the exterior in walls. There IS a rationale for 2/3 (or more!) of the total R on the exterior for unvented roofs in some very cold locations- roofs experience nighttime radiation cooling, and have a lower average temperature at the roof deck than walls. In Table R806.5 of the IRC, R35 out of a presumptive R49 (71%) is required in US climate zone 8.

  13. Expert Member
    MALCOLM TAYLOR | | #13

    The complication that many others don't face when designing double walls is our seismic requirements. If you make the inner wall load-bearing, and keep the floor system back from the exterior, I don't think you can get away with substituting a smart membrane on the outer face of the inner wall for the sheathing necessary to provide shear.

    GBA posters from other areas have suggested running the subfloor through to tie the walls, or using let-in bracing, but short of installing steel moment frames I don't think a non-sheathed lead-bearing wall would pass our code.

    Once you sheath the inner wall you are faced with the unpalatable choice as to what to use as a non-structural sheathing on the outer wall. This second exterior layer represents a lot of expense.

  14. StollerB | | #14

    Martin, Ryan, Dana, Malcolm- thanks all for your clarifying points and thoughts.

    Martin- I have indeed poured over this article many times. I get the science of it, but I think Joe's wall is quite impractical/labour intensive to build. The real economy and beauty of the double stud wall is being to build them as a box frame of sorts. A past article of yours ( actually shows a picture of the very double stud walls of the first Net Zero house I helped Peter Amerongen build in Edmonton, Alberta quite a few years ago. You can see in the photo that Peter devised a very elegant and fast system for building double stud walls. On that project, the framer figured that building the double stud walls only added about 15% additional labour time on framing the entire house. And it doubled the R-values. The real beauty of it was being able to lay out identical plates, frame the walls on the ground right on top of each other, and use the sheathing "spreader plates" at top and bottom to create a box that gets tilted up as one piece. Incredibly fast and simple. Peter even told me at the time that this double 2x4 wall @ 24" o/c used no less lumber than a single 2x6 wall @ 16" o/c. I didn't believe him (being a know-it-all apprentice at the time!), so I went home and calculated the actual board feet of lumber in each wall section, and the double stud wall actually used 1.5% LESS lumber!!). Joe's wall requires, essentially, framing the whole house, air sealing if from the outside of that inner layer, and then starting all over again and essentially balloon framing another whole house outside of it. That's just crazy, in my opinion. If you're going to do that, then you may as well do the bulletproof Passive House wall, with a 2x8 exterior wall, dense-packed with cellulose, fibreboard on the exterior, plywood/OSB/structural sheathing on the interior face (as an air and vapour barrier), then building a service wall inside of that. Both are time consuming and expensive, but the Passive House style probably less so than Joe's.

    Dana & Malcom- where I am, on central Vancouver Island, on the West Coast of Canada, the building inspectors still seem stuck in interpreting the code as 2/3 of insulation on the exterior side of the vapour barrier. If you stray from that, you basically need to get an envelope engineer to sign on, or have the architect debate with them and prove it. They do want the continuous insulation layer (which is great) to ensure an effective R-20 in your walls, but the vapour barrier issue still seems to be a tough one to negotiate in any kind of prescriptive project.

    I have posted on my proposed double-stud wall design asking for critiques:

    Please weigh in!

  15. Expert Member
    Dana Dorsett | | #15

    Regarding the intrangence of inspectors on all topics involving vapour barriers, Lstiburek feels your pain:

    I can't find where the NBC says anything about locating it no less than 1/3 of the R from the interior. It does show up on page 44 of the CMHC guide though:

    "Location of the Vapour Barrier

    To prevent condensation from forming
    within building envelope assemblies, materials
    that act as vapour barriers (vapour retarders),
    including certain types of insulation, must be
    located within the assembly so that moisture
    moving from the inside to the outside does not
    condense and accumulate within the assembly.
    This means that vapour barriers should be
    located on the warm side of insulating materials
    (Figure 14). An exception to this rule is permitted
    for walls where no more than one third of the
    total thermal resistance or RSI-value (R-value) is
    located on the interior side of the vapour barrier. "

    This seems crazy in a climate as temperate as Vancouver Island.

    It also shows up in an NRCan discussion of attics, which is fine for most of the country but potentially problematic in attics in locations as as cool as Whitehorse Yukon or Churchill Manitoba:

    The one-third two-thirds rule of thumb feels a bit like an old wives tale masquerading as code. I don't believe it's in the NBC. The relevant section of the 1995 version of the NBC reads:

    "A- Location of Low Permeance Materials
    Generally the location in a building assembly of a material with low air permeance is
    not critical; it can restrict outward movement of indoor air whether it is located near
    the outer surface of the assembly, near the inner surface, or at some intermediate
    location, and such restriction of air movement is generally beneficial, whether or not
    the particular material is designated as part of the air barrier system. However, if such
    a material also has the characteristics of a vapour barrier (i.e., low permeability to
    water vapour) and low thermal resistance, its location must be chosen more carefully in
    order to avoid moisture accumulation."

    The, "...chosen more carefully..." is pretty far from a prescriptive 1/3-2/3.

  16. Expert Member
    MALCOLM TAYLOR | | #16

    I think you are right. The code section on vapour barriers says it must be located close enough to the interior so it doesn't cause a problem. I guess that puts the onus on the designer to show moving it in from the interior works. I've never seen the 2/3 anywhere, but is sure floats around in everyone's head.

  17. StollerB | | #17

    Great post Dana- thanks! You and Malcolm are lightyears ahead of me in terms of your code knowledge, so I really appreciate you expressing that expertise in clear terms. I think you are both right in that perhaps it is just such an ingrained "rule of thumb" that it has become more like religious dogma among builders than actual code language. Though often mistaken for it, myself included. Thank you for the clarification, though you're right in that it puts more responsibility on the designer to understand the physics at play with far greater finesse.

  18. StollerB | | #18

    BTW Malcolm- regarding your post #13. I spoke with a structural engineer we routinely use for residences and asked him if our seismic region's shear bracing requirements could be met by cross-bracing alone with Simpson framing straps or lumber, and he said that he knows of no thorough published data for cross-bracing in seismic regions, unless it is continuous 1x dimensional lumber cross-bracing (old-school sheathing style). Because of that, he said he doesn't think it would be possible to do in our region on the coast.

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