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Double Wall Construction

steelejones | Posted in Green Building Techniques on

Guys I’m curious your thoughts on my building envelope on my home I’m about to build.

Building in cold climate MAINE

ICF foundation with 4″ of insulation under the slab.
Double Wall construction with 12″ wall, blown in dense pack cellulose.

I’m using Zip sheathing for the outside which allows me not to have to put up a vapior barrier on the outside as this will serve the same purpose. And covering with Hardi Plank siding.

Here is where I’m unsure, do I need a vapor barrier on the outside of the interior wall or just sheetrock it ?

Look forward to you reply, we are beginning construction next month.

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  1. Michael Chandler | | #1

    I'd suggest no vapor barrier behind the drywall. Just be sure the double wall assembly is well sealed against bulk air movement and the moisture that comes with it, seal the top plate really well and caulk the bottom plates to the slab.

    We're building pretty much like this here in NC and had to increase the exterior wall to 2x6 to get enough bearing on the edge of the slab / ICF transition. We had two inches of foam at the edge of the ICF which would have left only an inch and a half of bearing under a 2x4 wall and would have made the anchor bolts a little marginal, we went to a 2x6 exterior wall with 2 1/2" purlins bridging to a 2x4 interior wall with a 2x12 upper top plate. The inspector and engineer saw the inner wall as non-load bearing. All panelized off site and tilted up and roofed in under two weeks.

    I also think that you would do better to use regular OSB and House wrap so that the wrap would give at transitions as the house settles rather than throwing stress on the tape. I like the Zip for roofs but not to install windows and doors in.

    I'm also no fan of cellulose in walls and prefer the JM Spider Fiberglass Blown-in-batts but that opinion is due to the fact that we are in a hurricane area and that cellulose just takes forever to dry out when it gets wet due to a roof leak or whatever. It's not the wind, but the flying trees that lets the water into the houses down here.

  2. GBA Editor
    Martin Holladay | | #2

    I'd like to add a clarification that doesn't really apply to your house, but may be important for other readers. You wrote, "I'm using Zip sheathing for the outside which allows me not to have to put up a vapor barrier on the outside." But the use of Zip sheathing allows builders to skip the plastic housewrap, which is very different from a vapor barrier. It sounds like you may not realize the difference.

    Plastic housewraps are vapor-permeable. They are not vapor barriers.

    Polyethylene sheeting (for example, 6-mil poly) is a vapor barrier or vapor retarder.

  3. steelejones | | #3

    Why are you not a fan of installing doors and windows in ZIP ?

  4. steelejones | | #4

    Martin, yes...

    Im very much a rookie who is trying to find the best way to have my builder build my shell...

    Im VERY much lost at this point. As I have some people telling me which layers and how to build the layers of the home and other people disagreeing and telling me to do the layers a different way.

    Right now I have

    Hardi Plank Siding
    Zip sheating taped on walls and roof
    Dual 2x4 walls with dense pack cellulose inside.
    Sheet rocked

  5. Michael Chandler | | #5


    Why am I not a fan of installing doors and windows in Zip?

    I like having the "slip sheet" function that a plastic house wrap provides. We know that buildings settle and shift as they age. The goal is to allow for this movement without letting it induce failure in the weather resistance of the system as a whole. As the walls settle, the housewrap can shift behind the siding and take up some of the stress that otherwise would be placed on the flashing tape.

    Look at how windows are installed in house wrap, the flanges go over the wrap on the sides and are taped, the wrap goes over the flange at the top and is taped and the wrap goes under a drianage pan at the bottom and is left un-taped to allow the window jambs to fail over time at the bottom corners (as most of them will) and still give the introduced water a way to exit the wall assembly.

    I'll grant that you can still use a pan with Zip but the rest of the system is purely relying on the adhesive on the tape to keep water out for the life of the building as it settles and ages.

    And physics trumps chemistry every time.

  6. Riversong | | #6


    Sorry to say this, but if you don't understand the difference between a weather-resistant barrier and a vapor barrier, then you're not in any position to either second guess your builder or determine which of the contradictory advice is worth following.

    I've built a number of double-wall houses, using various techniques, and insulating with dry dense-pack cellulose. Such a wall system needs to be well protected from humid indoor air and from exterior weather and also be breatheable enough to dry as it inevitably takes on seasonal moisture.

    The Zip system is a fool's errand, as it is not only non-breatheable but excludes the redundancy of layered weather protection that housewrap or - better yet - 15# felt offers.

    The interior skin should be air-tight but approximately 1 perm of vapor resistance (such as latex vapor retarder primer), and the exterior skin should be about 5 times as permeable to vapor but impermeable to rain and wind.

    I would recommend that you find a builder you can trust who is experienced with double-wall superinsulated houses and let them determine the best methods for making it weathertight, breatheable and durable.

  7. Bruce Wilson | | #7

    Clyde, The problem here is that during the humid summer months water vapor can get inside your wall through your drywall, then when it gets cold outside the moisture could condense on the back of your sheathing causing moisture damage to both the sheathing and the framing. Either a vapor barrier adhered to the framing under the drywall or a thourough coating of primer sealer on the drywall will retard the movement of water vapor into your wall cavity. Time will tell if the Zip sheathing and tape system will create a lasting water tight barrier. I have read that house wraps have been found to react with the cement in cement siding causing them to deteriorate.
    After 32 years in the business I find I learn the most from experience and I am not sold on house wraps though they have their place. If only we could tear apart walls a few years after we buit them to see how they are holding up we could better understand what is happening inside the walls we build.

  8. Daniel Morrison | | #8

    Sometimes it's hard to find a local builder who has experience in non-traditional techniques (and let's face it double-stud walls are pretty non-traditional--they shouldn't be, but they are). In that case, you just need to find the best builder you can, one that's smart and is willing to learn. Then you need to go learn together. You've come to a good place.

    All of these guys are giving you good advice. I think the main question is about a plastic vapor barrier inside the wall. And I think most building scientists would steer you away from it. Most of the moisture that gets into walls doesn't get in there through diffusion, but instead by air leaks. A plastic sheet inside your wall will seriously cut the wall's ability to dry. With an airtight shell and a vapor-retarding primer or paint, your wall can dry in. If you let it dry out as well, you'll be in even better shape.

    I agree about steering away from Zip system on the exterior walls for a couple of reasons: the wall can't dry to the outside, and you can't weatherlap flashing; you can only tape it. (Although, I have to say that when my four year old daughter pulled the soap dish off the tub wall a couple of months ago (she's a brute) I saw it as a good opportunity to a) Test the Zip system tape, and b) Put off fixing the soap dish. I taped a piece of plastic to the tile and it's holding up very well despite getting two or three hot showers per day.)

    And I also like felt paper, but the 9-ft. rolls of housewrap, and their tear resistance is pretty tough to argue with. As for the cement siding reacting with the housewrap, I haven't heard of it, but I've been pretty busy lately, and I haven't heard of a lot. If you space the siding off the housewrap (with drainage mat or furring strips), you'll make it easier for the wall to dry to the outside, increase the life of the paint job, and eliminate the concern about cement and housewrap.

    Keep us posted!

  9. Jesse Thompson | | #9

    To focus on the Zip wall issue primarily, I'd like to continue this conversation:

    The reason we've been recommending Zip recently is that it is one of the few air barrier systems where the construction quality is verifiable during construction, while you still have a chance at catching installation errors. Tape it, install windows and doors, blower door / smoke test it, and you've got a very clear moment during production to find both air and water leaks in the shell of the building.

    Since leaks are the major moisture issue in buildings (much more dominant factor than vapor drive), we need all the help we can get finding them early in the construction sequence, and the Zip system seems to be the best system we've yet found for this. I have been feeling lately that there has been too much concentration on theoretical drying-to-the-outside issues, and not enough focus on methodology for finding and stopping air and water leaks.

    Air-tight drywall, for all its theoretical advantages, is extremely hard to detail and construct in a way that is verifiably complete, and it happens very late in the construction process.

    Tape only at the head flashings of windows makes me nervous as well, but people I trust have seen the Huber / Zip test sheds in person that have been left un-sided out in the weather for years now and describe the tape and sheet goods as holding up extremely well.

    It seems like we have a better chance of controlling vapor drive / humidity problems with good ventilation systems once we are sure we won't have leaks dumping gallons of water into our walls.

    I don't claim to have any answers, but would like to put this out there for conversation among the smart people here.

  10. Expert Member
    Michael Maines | | #10

    According to a recent FHB article, ZIP wall sheathing is vapor-permeable, with a perm rating of 2-3 for the osb and 12-16 for the phenolic-impregnated paper facing. We've used in on a few jobs now. It doesn't cost much more than CDX or Advantech (we don't use regular OSB) and it has the benefit of drying in the house as soon as the sheathing is up and taped.

    However, I'm in the camp that doesn't quite trust those adhesives to last a hundred years. I like Michael Chandlers' description, "physics trumps chemistry." We still use housewrap (lately Tyvek's Crinklewrap) and roof underlayment (Rooftopguard or 30# felt.)

  11. homedesign | | #11

    Jesse Thompson,
    I really did like your comments...
    Very good food for thought.
    thank you

  12. dankolbert | | #12

    We are just building a very similar house in southern Maine. Get in touch if you want to see what we did. [email protected]

  13. Anonymous | | #13

    What about using tyvek over zipboard to solve the window flashing issue?

  14. will goodwin | | #14

    vis a vis double wall framing with insulation on the outside of the foundation; if you make the interior stud wall the load bearing wall, you can make the exterior wall 2x4 and still have it overhang the foundation insulation (It won't need to be suppoerted completely by the foundation). There is a good discussion of double wall systems in THE CANADIAN BUILDERS ASSOCIATION HOME BUILDERS MANUAL.

  15. Riversong | | #15

    I don't understand why some of these old threads are coming to the surface without new posts, but I put some images of the varieties of double-wall systems I've designed onto my new Flickr site:

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