GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Double walls: where to locate the vapor retarder?

GBA Editor | Posted in Green Building Techniques on

My new house will have a double-wall design, similar to what I did in ’80, but better…. I think. In my present house I used two, 2×4 walls, insulated, around a 5 1/2″ gap, stacked full or fiberglass batts; pretty normal, I guess. What I did NOT like about it, after years of thinking, were all the penetrations in the VR (vapor retarder) by electrical boxes, etc, and all the flipping metal strips over wires that were too close to the sheet rock. On my next house, I am planning on putting the VR on the outside of the inner 2×4 wall to avoid all the prior nuisances. Yes, it will be a bit tricky at times to hang visqueen on the inner wall through the outer wall, but I feel the tightness I’ll gain will be worth it. (The inner wall will not be load bearing, so I may build it as a partition wall, standing it up from the inside, w/ VR already on it… mostly.) I have NO INTEREST in sealing around and in electrical boxes; been there, done that. I will also likely leave the inner 2×4 wall devoid of fiberglass, for a couple of reasons. One, I can insulate first, then run wire and the odd plumbing pipe while being relatively warm, and I won’t have to fiddle around fitting ‘glass around wires, which is never perfect anyway. Secondly, leaving the inner wall devoid of glass will keep the VR well w/ in the “1/10 of the (temp) way through the wall”. Thirdly, I’ll have no particular amount of time to emit water vapor into the insulation whilst I am working on wiring, etc. Lastly, I won’t need little metal strips over wires/pipes that are too close to the sheet rock. Yes, I’ll have to insulate like a maniac and get sheathing/house wrap up before the insulation gets rained on, too; that will be fun for all. QUESTION for you double-wallers: Is this how it is normally done? Anyone see any problems w/ this idea? Better ideas? Thanks for any thoughts. john. PS: (Type of insulation is another topic.)

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. MICHAEL CHANDLER | | #1

    Where are you building? most folks aren't installing a vapor retarder on the inside these days. There is a discussion of how we did this here in NC here https://www.greenbuildingadvisor.com/homes/nc-home-grabs-five-green-ratings but if you are in a northern climate you will have a different vapor profile and what works in NC will not work for you.

    In any event you'll likely be blowing dense pack Cellulose or Spider micro-filament fiberglass rather than dealing with fiberglass batts so you won't have to wory about fitting around the electrical boxes and you'll be able to run your wires between the walls and not have to worry about nail plates either.

    Things have changed a bit in the last thirty years. We don't build em like they used to!

  2. jklingel | | #2

    Michael: I am in Frb, AK. 14K degree days, so a VR layer is mandatory. I am considering blowing in insulation, but have not re-researched the pros/cons thereof as yet. If it is practical to avoid dealing w/ fiberglass batts, count me in! Thanks for the comments.

  3. Riversong | | #3

    John,

    There are too many liabilities to double stud wall construction (I built several in the 80's). The Larsen Truss, developed by John Larsen of Alberta, is a far superior system. You build a conventional platform frame and get it under roof, sheath the walls, wrap them with high-quality vapor barrier material (like Swedish Tenoarm) just like housewrap with one taped seam in the middle, then hang site-fabricated parallel-chord trusses to create an external insulation cavity.

    You may have to also sheath the outer skeleton if you want to blow insulation (don't use f-glass batts, they're an inferior product), or leave enough room at the eaves to fill the walls from the attic. The trusses can be supported on an extended plywood subsill that covers the foundation insulation and tied into the rafter tails or rake overhang.

    If the trusses are deep enough You can leave the load-bearing walls uninsulated if that's your preference. Or blow densepack into them through insulweb netting after wiring and mechanicals are in place.

  4. GBA Editor
    Martin Holladay | | #4

    Translation for people living in the lower 48: I believe that "Frb Ak" means Fairbanks, Alaska.

    J., Your proposed method of installing a polyethylene vapor barrier 3 1/2 inches back from the drywall is an old method used by Canadian builders in the late 70s and early 80s. It works well. That way you have a 3 1/2 inch wide cavity for running your wiring, and you don't have to worry about insulating around your wires. You can make your wall as thick as you want if you need more insulation.

    My own advice:
    1. Use dense-packed cellulose, not fiberglass batts. Choose an insulation contractor with experience blowing cellulose in wide walls; it can settle.

    2. Don't forget to come up with good insulation details for your rim joists.

  5. jklingel | | #5

    Robert/Martin: OK. I have to get my brain wrapped around this Larsen Truss system; it sounds good, but it sounds like 3 walls, too (wall w/ VB, then two "walls" in the L Truss). I see advantages in it, esp covering the foundation insulation. Yes, Fairbanks, Alaska; sorry for being too brief. The appeal of using fiberglass batts is minimal; I hate the stuff. I'll have to call insulators to get costs on cellulose or fiberglass blow-in. I blew in my own lid (cellulose) in '80; will have to learn what is different about walls, other than being very careful (which is a concern when hiring work; some pros are good, some are not). House is one-story, so no rim joists. Glad to hear the 3.5" space to the VB is OK; I just read a "1/3 of the way through" rule of thumb; sounds kind of generous. Much to think about here. Larsen Truss, huh? Hmmm. Thanks again. john

  6. user-723121 | | #6

    John,

    I have built both ways with a warm side vapor barrier, right behind the drywall and with the vapor barrier on the outer surface of the inside wall. The homes were both 15" double wall and the ach50 was the same for both methods. For me, the easiest method is the warm side vapor barrier and you have the drywall to mechanically fasten the joints in the poly and the seal on the plates. With some care you can get 1 ach50 or less with a minimum of cursing.

  7. Riversong | | #7

    "1/3 of the way through" rule of thumb

    The thumb rule is 1/3 of the way through the total R-value of the wall system, not the thickness of the wall. In other words, an R-39 wall can have the vapor barrier outside the inner R-13.

    The original Larsen Truss is hardly "three walls". Just like a roof truss uses light members so that it takes less wood than independent ceiling joists and roof rafters, or a parallel-chord floor truss uses less lumber than heavy framing, a Larsen Truss can be made from 2x4s ripped in half with 3/8" plywood gussets.

    The way I build, with a modified Larsen Truss (using the load-bearing studs as the inner chord) and eliminating the sheathing, requires less lumber than a 2x6 house sheathed with plywood or OSB and creates one of the most thermally-efficient breatheable wall systems every devised.

  8. jklingel | | #8

    Doug: Good to hear. Thanks. I am glad I asked about this whole thing here. I asked about double walls on another forum, and some SIP freaks started ranting about wasting all those precious trees.... I pointed out that SIPs use wood, foam, and energy to make, too, and that a double wall actually SAVED trees by burning less wood for heat. Some rather closed-minded souls over there.
    Robert: Roger that on the 1/3 rule, and thanks for explaining the build on the LTs. Now I understand. Sounds like you have refined a wall to the minimum materials, maximum R. I'll have to ask about skipping OSB up here, but I think w/ all the quakes we have (mostly mild, thankfully) it would be ill-advised.

  9. Riversong | | #9

    I'll have to ask about skipping OSB up here

    I was referring to skipping the sheathing (and using code-approved metal t-bracing instead).

    But I would advise always skipping the OSB. If sheathing is necessary or desired, use CDX or diagonal boards.

    OSB is one of the worst building materials ever created. It's highly vulnerable to moisture damage, from swelling to mold to rot. And it has too little vapor permeance to allow a building envelope to breathe properly for long-term durability.

    It's become the norm because it's cheap. And that's exactly what it is.

  10. jklingel | | #10

    Robert: Points taken. I have never really liked OSB for screwing into, and have noticed how much it swells when it gets wet. Not being a builder by trade, I've tended to observe what is usually done and then copy it, or research it first if it seems "big ticket". I have not really thought of researching OSB, so I am glad you mentioned it. Definitely is food for thought.

  11. user-723121 | | #11

    I am also in the non OSB camp, I always use plywood when 4 x 8 sheets are appropriate. Robert and I agree on at least 2 things, no OSB and always vented roofs.

  12. Riversong | | #12

    Doug,

    Wanna try for three? Which way is the right way to hang toilet paper?

  13. user-723121 | | #13

    Robert,

    That is way too controversial, as a builder, I prefer to play it safe.

  14. jklingel | | #14

    Vertically is the only way to hang it. It is much harder for the cat to unroll it. j

  15. tim fieweger | | #15

    i will soon be breaking ground on a larsen truss project of my own, and was wondering if i could have some feedback from you guys? my project is my home, and it's part renovation, part new construction. i'll be doing it myself with the help of a local highschooler in the neibhorhood. it's going to be in two phases, 1. i'm going to build it using 2x4 construction sheathed with 1/2" cdx, roof overhang 2', then add on 12" larsen truss with 1/2" cdx, roofing, windows, doors, trim, siding, then blow cellulose in from the inside. phase 2 over the next year or two plumbing, heating(hrv), electrical. insulate 2x4 wall, then wallboard, finishwork, pop the cork! would i put the the vapor barrier on the first sheathing and not on the second or both?

  16. jklingel | | #16

    Tim: I am not a builder, but I think it is safe to say that vb on both sides will kill you. Any moisture that sneaks out of the house, and there will be some no matter what you do, will get trapped, condense, and water-log the insulation, cause rot, etc. I tried that when I built a wannigan on a trailer 35 yrs ago; the next spring I had to remove the insulation, dry it, and cut out the outer vb. Depending on where you live, you may not even need a vb. I am sure the heavy-weights will weigh in on this soon. Good luck w/ the project; the hs kid will likely get some great lessons. john

  17. Riversong | | #17

    Tim,

    Where are you building? Unless you're in an extreme cold climate (>9000 HDD), you don't need a vapor barrier and many cellulose manufacturers advise against it (at least one won't honor their warranty if a VB is used). Codes generally require only a 1 perm vapor retarder on the warm side.

    There's no need for double sheathing, either. Depending on what you plan to use for siding, you can install that directly over a WRB on the Trusses. I've built several Larsen Truss houses, including a couple with no sheathing at all, using code-approved metal T-braces for shear resistance.

    Are you planning to insulate only the truss cavities and not the stud bays? I've built three buildings with a modified Larsen Truss wall system that has a single continuous insulation cavity. You can see it at http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm

    I'm designing and consulting on several Larsen Truss projects around the country right now. If you have questions, you can contact me at HouseWright at Ponds-Edge dot net.

  18. tim fieweger | | #18

    robert, i've seen all your postings and projects that have been online, totally awesome stuff. my project is in glastonbury, ct. just outside of hartford, as for the double sheathing, i know it will cost a bit more, the siding will be red cedar shingles so i'll need the sheathing there, the sheathing on the inside 2x4s is for an envelope for the cellulose and also because i won't be finishing it inside right away. this way i'll have the house inulated and tightened up to the weather and can then over time install plumbing, elec., all mechanicals, then add more insulation in the 2x4 wall. it is possible that the project could stretch out up to 2 to 3 years, that's why the double sheathing. i guess i didn't mean vapor barrier, i really meant more of a tyvek type product, and would i just put that on the exterior sheathing or would i want felt paper under the shingles and maybe an air barrier on the inside sheathing.
    john, thanks for the reply, i guess i meant to say air barrier

  19. jklingel | | #19

    Tim: A curiosity thing here. If you are going w/ 2x4 and L Trusses in the first phase, then couldn't you use the diagonal straps for bracing, like Robert R uses, then something like 1/8" paneling or peg board to hold the cellulose up? I'm considering that kind of system and reinforced vapor barrier or Tenoarm. Robert R mentioned a desire to minimize the "film" in walls from flexing too much, and that ought to do it, no?

  20. Riversong | | #20

    Tim,

    With your system, you may be better off applying a taped housewrap breatheable air barrier over the inner sheathing and #15 felt for siding underlayment, which will serve as the secondary weather barrier and drainage plane but not trap condensation behind it as plastic housewraps can. Do not use anything less vapor permeable on the interior than latex vapor retarder primer.

  21. tim fieweger | | #21

    john, i was thinking about building 2x4 walls 16"o.c. and L truss 24" o. c.. and the 1/2" sheathing on the 2x4 walls would let me screw the trusses in where the studs don' line up.

  22. tim fieweger | | #22

    robert, does the latex vapor retarder paint allow moisture in wall to dry to the inside and/or prevent moisture from getting into the walls?

  23. jklingel | | #23

    "john, i was thinking about building 2x4 walls 16"o.c. and L truss 24" o. c...." •• OK; got it. Might 2x4 fire stops, turned vertically if desired, suffice for anchors? It would make insulating a PITA, but save on CDX. Just a thought. Either way, you're gonna have a nice place. Good luck. j

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |