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Double wall or other option?

9BiXyph5XJ | Posted in Green Building Techniques on

We’re going to build a house in zone 7. Northwest Washington state, up in the mountains. I’m trying to figure out what’s the best wall/insulation method on a modest budget (I know that doesn’t tell you much, I’ve got to do the whole 2 bed, 2 bath house for about 150,000). Is double wall with blown fiber glass insulation worth the extra cost? And can it be done on an already laid concrete block foundation? Or is there another equally warm option that I just don’t know about? Thanks for any input – I can only check email once a week, so bear with me.

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  1. Riversong | | #1

    I don't think there's any climate zone 7 in Washington state. It's zone 6 north of Spokane. But that's cold enough.

    I've been designing and building an improved version of the double-stud system, called the Riversong Truss, typically 12" thick and filled with dense-pack cellulose, which is superior to fiberglass in every way.

    I typically build with locally-sourced green rough-sawn lumber, no exterior sheathing and horizontal shiplap siding, with the Air-Tight Drywall system as the interior air barrier. This makes a highly breatheable, highly insulative (R45) wall with almost no thermal bridging, and requires no more lumber than a conventional 2x6, plywood sheathed house. And it's about as green as can be done with conventional materials.

    Details can be seen at

    If you have a competent straw-bale builder in your area, that's another wonderful, potentially low-cost and very green option.

  2. Rob Harrison | | #2

    Robert, there are areas in Washington with 7,000+ HDD. We're working on the design of what we hope will be Passivhaus cabin on a site in the Okanagan Highlands near Wauconda with ~7,800 HDD, and not a lot of sun. Our current thought on the assembly, from the outside in, is: fiber cement siding on 1/2" x 3" PT plywood furring strips (rainscreen) on 1/2" fiberboard over 15" Larsen trusses 24" oc on structural sheathing that is also the air barrier, on 2x6 studs 24" oc, with 5/8" gypboard and latex paint. Both Larsen trusses and studs will be blown with dense-pack cellulose. This yields a wall of R-70. Roof assembly is ~R-85, floor over crawlspace is ~R-75. Windows are U-0.13.

    With respect to the foundation, a concrete block foundation may have trouble meeting current seismic requirements.

    Depending on whether you are eat or west of the passes, straw bale could be ok. As I understand it, R-value has tested closer to R-28, which barely exceeds the 2009 WSEC. I would not recommend it west of the passes.

  3. Rob Harrison | | #3

    Oh, and I would strongly caution you against building with green lumber in western Washington. I don't have first-hand knowledge to know if you could get it and use it in the way Robert suggests in eastern Washington.

  4. GBA Editor
    Martin Holladay | | #4

    If you are an owner-builder, it's hard for people in other parts of the country to recommend an inexpensive way for you to build your wall. It depends on who is doing the work, whether you are paying for labor, how much time you have, and whether you are willing to scrounge for recycled materials.

    If your local building officials allow its use, rough-sawn lumber from a local mill can save money.

    Many owner-builders have gotten great deals on recycled polyisocyanurate insulation that has been salvaged from commercial roofing jobs. Ask around to see if anything like that is available locally.

    If you are paying for the labor, it's hard to beat double-stud walls filled with cellulose insulation for a low-cost high-R wall. But even that option may be too costly for your budget.

    Good luck.

  5. Riversong | | #5


    Climate zone 6 is from 7200 to 9000 HDD.

    And your "strong caution" against using local lumber is for what reason?

    And, as one who has been doing superinsulated construction for 30 years, I think you are over-building for your climate.

  6. user-659915 | | #6

    Karen, I have to ask: how come the foundation is in place before you have decided on what wall system is to go over it? It can't be over-emphasized that good forward planning of your whole-house construction strategy is essential if you a) want to build green and b) do it on a tight budget.

  7. davidmeiland | | #7

    Karen, I'm not sure where the dividing line is, but the Washington State Energy Code for 2009 divides the state into zones 1 and 2. Over here by the coast we are in zone 1, and colder areas are zone 2. There are prescriptive guidelines you can follow to get approval for your plans. Lacking that you will need to take the performance based route. The folks at WSU Extension's Energy program are ready and waiting to help you with energy code issues. Suggest you look carefully at the topics on this page:

  8. [email protected] | | #8

    Karen, where in fact are you? (That would have been a good first question, sorry!) Perhaps you are looking at a USDA map, like this?
    I'm in Seattle. Like others here I'm sure, I'd be happy to chat with you on the phone, if that would be helpful.

    Have you considered somehow paring down the program? Let's say, doing one "rush hour" bathroom instead of two full baths?

    Robert, it's possible that we are overbuilding. We're still tweaking the envelope. Though I've been designing "green" houses for 20 years, I feel like we still have a lot to learn.

    I caution against green lumber because the builders with whom I work here (who have been doing super-insulated houses for 30 years, back to before the BPA Super Good Sense days) have had trouble with it here in western Washington. That is, based on their feedback I don't think you can just substitute green lumber in a conventional building assembly, and not have some issues. (Twisting, shrinkage, mold.) I'm sure green lumber works in your complete system, where you are.

  9. Riversong | | #9

    While the species, availability and quality of local rough-sawn lumber varies from region to region, there are a lot of misperceptions around it's usefulness in residential construction, and codes often made it difficult to use.

    The first principle of sustainability (which should be the basis of what is called "green" building) is to use locally-sourced materials that are processed as little as possible. This not only limits transportation costs and embodied energy and global warming contribution, but also supports the local economy which sustains community and allows us to live responsibly within the annual output of our local environment.

    All KD lumber is visually graded, and the same visual grading techniques can be applied to local lumber. In fact, I'm often unimpressed with the grading and quality of KD lumber, but code only considers whether there's a grade stamp and not whether it's reliable.

    Much of today's KD lumber is fast growth and poor quality, with too many knots, splits, checks and other weaknesses. As soon as it's removed from the tightly bound cubes it's shipped as, it begins to warp and I've had to return or discard or cut up into blocking a great deal more KD lumber than rough-sawn.

    The advantage of using rough-sawn green, fresh off the sawmill, is that it cuts like butter, nails with no resistance (even with 20d galvanized common nails) and stays straight once it's nailed in place. It's drying on site or at the lumber yard which results in most warpage.

    And, once it's stood up in a house frame, it dries quickly (as long as there's some sun and wind and your relative humidity isn't rainforest variety). There's a common myth that KD lumber doesn't shrink. Assuming it arrives at the 19% moisture content that the KD standard requires at time of milling, and dries to an equilibrium moisture content of 10% in the house frame, a typical 2-storey house will shrink about 13/16" at the outer walls (more at the center bearing wall if there are built-up underslung beams).

    Unless you can frame and close in a house in just a few weeks, or do so in the winter when lumber isn't going to dry, a green lumber house will equilibrate quickly and will not shrink after completion much differently than a KD lumber house.

    Depending on the type and quality of the sawmill that produces local lumber, inaccuracy in sizing may not be a problem. I've been ordering lumber from a computer-controlled bandsaw mill that produces amazingly accurate sizes and leaves a finely textured face that is even suitable for exterior trim (as it takes stain or paint much better than a planed board).

    I've found that the only disadvantage to using green, rough-sawn lumber is that it's quite heavy. But the benefits far outweigh that problem. And I'm amazed that "green" builders don't insist on using locally-sourced low-embodied energy lumber. If we did, then codes would allow it.

  10. David Meiland | | #10

    We have excellent locally grown fir available here. The inspectors are familiar with this material and don't even blink, although maybe they're supposed to have you bring a lumber grader on the job to stamp everything. It is a little more expensive than commodity lumber and it's harder to get an order filled and delivered (the sawmills are small and manual here).

    We also have great locally grown cedar, and I sided and trimmed my shop with it.

    Some of the commodity fir here is now from "sustainable" sources, whatever that's worth. The cedar is from Canadian clearcuts until proven otherwise.

  11. Riversong | | #11

    the sawmills are small and manual here

  12. 9BiXyph5XJ | | #12

    Hello all – dial up seems functional tonight. Wow, thank you for all the responses. To answer a few questions.

    Robert: I got the “7” off the map that was on the site or linked to the site (can’t recall which)

    Rob and David: I’m near Glacier, Washington about 30 miles east of Bellingham (zone 2 in your definition) For the western Washington folks, the huge drop in “zones” occurs once you pass the town of Deming. We can be under a few inches to a few feet of snow here when it’s raining in Bellingham. Mind you, the skiing’s good up road a piece.

    James: The foundation is in because we inherited it. It’s a well made foundation, concrete block, basement, nice crawl space, so I’m going with it. The original house was to be traditionally built. So I’m trying to figure out how to adapt what I have to something that would be more insulated than a traditionally built wall. If it’s not possible, I’ll live with it.

    Martin: I figure that no one could really give advice re expenses, but it felt rather silly not to give the general amounts I’m working with. Otherwise there could be losts of wasted effort on the part of you all. Which, by the way, was incredible.

    Thank you for the input and for the links.

  13. Riversong | | #13


    It appears you're in climate zone 4C. Do you know what the heating degree days are for your area? You can ask any fuel supplier, as they generally keep track, or a local weather station. What's your elevation? What's the size of the foundation and what size house were you considering?

    It generally costs in the range of $150/SF for a well-made house if you're having someone else design and build it. If you can do some of the work yourself or with help from friends and neighbors, that's often the best way to bring down the cost. And keeping the geometry of the house as simple as possible also goes a long way to keeping costs reasonable.

    I've been designing and building affordable double-wall houses for 30 years. I'd be glad to advise.

  14. FELIX | | #14

    40+ years as a carpenter/designer/builder, all of it searching for the best way to build an energy efficient structure.
    Forget wood, except for roof trusses. Too many "band-aids" required to even approach an energy efficient house.
    Build with ICFs, anybody's ICF; you'll be way happier (I'm partial to NUDURA). The only caveat here is that you have a soils test done for bearing capacity, and a structural engineer verify your "good foundation" is indeed good; ought to do this regardless of what sort of structure you put on top of it.
    Have fun if possible.

  15. Riversong | | #15


    If you spent 40 years looking for an answer and came up with ICFs, then you were looking in the wrong places.

    ICFs use a lot of petrochemical foam and a lot of high-global warming concrete to produce a mediocre-performance wall that doesn't even offer any dynamic thermal mass advantage.

    And there is nothing green about them.

  16. Just-An-Idea | | #16

    Thanks everyone for all the info on framing with green wood, can’t wait to try it. I wonder, does anyone know the effect of shrinking wood on pull out force for nails, seems wood might really tighten up on nails.

    To answer the main question though

    Double wall, is it too much? The single most effective means of reducing energy use in building is INSULATION. Snow not withstanding, you aren’t in an extremely severe climate temperature wise, ie, weeks of sub – freezing temps 24 x 7, lots of single digits, sub 0, etc.. So, just off the cuff, a foot thick double wall is probably a reasonable technique for you. One can debate the carbon footprint creating the insulation creates. I believe this misses the point that the energy saved is far, far more than the energy used in creation of any insulation, so in the end, there is little difference. Use what’s best, use a lot of it. There is also debate as to the cost effective amount, well, in my experience, nothing is every manufactured or built perfectly, so if you’re over the “cost effective” amount by even 30%, probably all you’ve really done is leave in a decent safety factor to ensure you get the thermal performance you want, and at little cost.

    Foundation: Sure you can use it. Overhang (cantilever) the floor deck joists over the edges by the few inches the double wall will take. BONUS this allows you to install 2 – 4 inches of foam over the foundation wall, and not end up “sticking out” from the outside sheathing on the rest of the wall above (or shorting what you really need on the foundation). You need to insulate foundation and crawlspaces with at least 2 inches of foam, down below the frost line, or horizontally a similar distance out from the building. BONUS for installing horizontally is kicking water away from the building, which limits basement moisture, mold, and insect damage. You’ll want a wide termite proof flashing from the foundation wall out to the edge of the deck though, shouldn’t be a big cost at all for this.

    Small budget: As others said, keep bathrooms to a minimum, forget the Jacuzzi, big energy hogs anyway, keep kitchens small, keep tile work to a minimum. Keep windows to a minimum. All big ticket items. You might consider “shelling out” as much as you can and finishing it latter. This can include framing for windows, but walling over (oddly this is almost cheaper than all the time intensive attention taping, spackling, trimming openings take) and installing windows later, also stubbing out pipes for bathrooms, bigger kitchens, gas fireplaces, even that Jaccuzi on the deck, and the deck, just get some pavers and lay them on a woven weed barrier after you rake the area flat, it’ll do you a few years.

    Other issues:

    Air Infiltration: It is vital that you tighten up the house as much as possible. Air tight drywall is great, but requires insanely good construction. You’ll help this process by not installing recessed lights all over the place, doable, but harder. Biggest issue here is a taped house wrap, which is outside the sheathing, coupled with flexible flashings on all openings, taped to the house wrap (Tyvek is the big brand name, and it’s pretty important, so why not spend the extra pennies and get it). Don’t forget filling up all pipe and wire penetrations, ALL along with gaps from shimming out doors and windows. Insist your builder provides a blower door test after the housewrap is done, and at the end of the job. Have your professional establish guidelines to meet, and have the professional confirm the test results.

    Professional: Yes, I did say professional, yeah, a good builder is enough, if you get great recommendations from folks you trust. A professional is usually very liable, often personally, ie, their house and wife’s jewelry is at stack for problems, a builder, just declares bankruptcy, re-incorporates and keeps on going.

    Waterproofing: You’ll want to keep all that insulation dry to keep it maximally effective, prevent excessive settling, and mold. Inside that’s a vapor barrier, outside, it’s your siding, backed by the Tyvek, flexible membrane at openings, and I’d put builders felt under that stuff too. I’m a belt and suspenders guy, I’d like my buildings to last as long as maint. free as my grandfathers. Oh, put WR Grace Ice and Water shield at eaves and valleys, full width of roll, which usually means on small house you might as well underlay the whole roof with it. This stuff will pretty much stop water from ice dams, wind driven water up the edges of eaves, etc from getting in and messing up the insulation.

    Local production / Green building: Do the best you can, but buy the cheapest materials your builder is happy to build with. You get something really cheap and your building is unfamiliar with it, you’ll get charged a “pucker factor” so to speak, and probably get a bad installation. For fixtures and cabinets, haunt all the local supply houses, see what’s on sale, see if it’ll do. Especially good to get tile and stone, some cabinets. As to the LEED requirements for materials and equipment made within a few hundred miles of use, get real. The world where every locality is an independent whole that can exist separate from the world is gone. Many things are just far more efficient and probably less carbon intensive when mega mass produced and shipped, sorry, it is if you take the time to do carbon calculations taking into account EVERYTHING required for locally vs. centralized production. So unless you’re one of the top 5% richest Americans, shop with your wallet and just do the best you can. Let the top 5% make up for any sins by supporting every green initiative, no matter how expensive.

    If you’d like to discuss further, feel free to contact me here or at my web site roughdesigns dot com.

    PS: Reviewers: I liked my response so much when the internet failed during submission, I repeated it, so you might have two of these. Sorry

  17. GBA Editor
    Martin Holladay | | #17

    Don't follow Just-An-Idea's suggestion that you install an interior vapor barrier. All you need is a vapor retarder, not a vapor barrier. Depending on your wall construction details, interior polyethylene can cause problems.

  18. Riversong | | #18


    Ignore everything Just-An-Idea said. What little is intelligible is almost all wrong, violating principles of building science, standards for responsible building and common sense.

  19. Just-An-Idea | | #19

    What the heck, I was silly enough to try to help, might as well go on.

    The lady asked if doubled walls with insulation was too much, and could the foundation be reused

    Most of the comments are about using green wood, and thanks, I learned alot about it.

    A whole bunch more debated the exact energy zone, and while knowing exactly where the house is allows a definitive answer, seat of the pants should suffice until a professional of some sort is brought in.

    As to ideas being wrong, what, cantelevering a deck? nope, done all the time, shelling out and finishing later, didn't someone suggest having friends help, just the type of thing they could help with, stacking and backing up plumbing, pretty basic cost savings method, what else, air tightness, pretty sure that's a big requirement now a days, now i should have mentioned get it tight enough, and you need to supply fresh air, and will need an efficent heat exchange ventilator, what else, keeping wall and roof insulation dry, yeah, I've heard tale of people with problems with wet insulation, I appreciated the vapor retarder vs. barrier, sounds good, though I've never seen a poly barrier installed that wasn't swiss cheesed by the trades, no fault on the trades, pretty hard not to move fast and efficently and be compulsively neat too, ending up with ice and water shield on a whole roof, when you see that half was ending up covered, yeah, seen it, read it, what else did I say, yeah, buy stuff on sale, pretty sure lots of people believe that can save money. my little story about windows, it's true, do some research, Canada is ahead of the US when it comes to energy conservation, they have to be. The one thing I just noticed was someone quoting $150 per sf, which means the lady gets a thousand square feet of building, assuming there's no site work I'd guess. $150 per sf certainly isn't the low range of construction in this country, and this lady has only $150,000 available. now when someone tells me that, i take them at their word, and know that they don't really have enough unless they can accept compromises, thinking outside the box, and getting as low as possible on all aspects of the building to get that house built, unless it's a DIY job, or has a lot of handy friends with time on their hands, who knows, in this economy, might be.

    Finally, this is why I just don't post much, anywhere, people prefer to just say, so to speak, you're crazy, rather than engage in rational discussion.


    That's all from me, get on with that debate on green wood y'all.

  20. Riversong | | #20

    $150 per square foot? The last super-insulated passive solar natural healthy house I designed and built here in Vermont (Energy Star 5+, HERS 46) cost $105/SF including excavation, driveway, well and septic, all permits and engineering.

    See it here.

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