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Would this building envelope work? Zone 4A in Japan

EricMatsuzawa | Posted in General Questions on

Thanks to GBA members’ answers I received to an earlier question ( ), I have decided to abandon local building practices. I am aiming to build a well-insulated & airtight 600 square foot house for my family. I would like to come close to the insulation R-values that Alex Wilson recommended in this blog post: Those numbers are R-4 windows (0.25 U-factor), R-30 walls, and an R-60 roof. I would love to read your opinions on my building envelope design. The house I plan to build can be seen here:

Double-stud 2×4 walls set 3” apart allow for 10” of dense-packed cellulose with the exterior wall bearing the load. This should give us an R-37. I am hoping the wall is thin enough to avoid too many window framing complications. The interior will be drywall using the Airtight Drywall Approach without a vapor barrier. The exterior would be sheathed with plywood or OSB and covered with housewrap. This is followed by a rain screen and siding. The vapor permeable wall would be able to dry to both the interior and exterior.

The roof for the house plan is not overly complex but there is a gabled dormer on the front and a shed roof over a room in the back which would prevent a straight run from the soffits to the ridge for a properly vented roof as mentioned here: I was thinking of doing an unvented roof using 2×10 rafters with bays insulated by dense-packed cellulose. Below the rafters, attaching airtight drywall to keep out most of the moisture-laden air. Directly above the sheathing, adding 2 layers of 2” XPS with staggered and taped seams, followed by metal roofing. This would give an R-54 which is not quite up to the R-60 that I wanted but I am hoping that it is enough. The 4” of XPS should prevent moisture accumulation in the roof sheathing. See

Foundation & Floor:
As far as I know, most residential foundations in Japan have crawl spaces. GBA mentions that crawlspace foundations have the same insulation and moisture control issues of basements without the usable space so I would like to avoid them. The building site is sloped so a slab foundation doesn’t seem like a good option. This leaves a pier foundation. One of my future neighbors’ house is on piers and they suffered from very cold floors. Later, they built a perimeter wall to keep the wind from passing beneath their feet. This may be more an issue with many Japanese builders’ lack of understanding regarding insulation and air sealing than the pier foundation itself and I am willing to give it a try. There is a passive house in VT that is on piers so I know that a comfortable floor on a pier foundation can be had. See: My plan is to get R-36 by insulating 8” joists with dense-packed cellulose and placing 2” of XPS sandwiched between OSB or plywood sheathing beneath the joists. Would it be wiser to instead use 2×10 joists for R-44? I will also caulk, gasket, and tape as if I owned stock in the air sealant industry.

I know there is a lot here for one post. Thank you for taking the time to read it and I appreciate any friendly advice.

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  1. EricMatsuzawa | | #1

    Update: This weekend, I learned about my uncle's recently built house back home in a climate zone 5. His house has 2x6 walls insulated with soy-based spray foam. Apparently his walls r-value is lower than my proposed wall plan and he is in a colder climate zone. Am I going overboard by trying to reach the r-values in Alex Wilson's post.

  2. user-659915 | | #2

    My two cents: size matters: this is a tiny house that will sip energy at anything over 20/40. Double wall is overkill in a 600 sf home in zone 4A, focus on stellar air-sealing instead.

  3. user-659915 | | #3

    Oh, and don't discount crawl spaces. Some of our yankee colleagues are disdainful but they're really not at all hard to seal and insulate and they're ideal for sloped sites, providing useful mechanical/storage space and protecting your plumbing from cold snaps. In an emergency they'll even serve as a storm cellar. Local insulation standards may not be up to par but it's not smart to ignore local vernacular form factors. They often have solid experience and good sense behind them.

  4. KHWillets | | #4

    It's a function of energy cost vs. cost of insulation. One cost factor might be loss of floor space due to thicker walls, if space is tight.

  5. EricMatsuzawa | | #5

    James, What you said about a smaller house needing less insulation is interesting. I wasn't sure about what you meant by "20/40." Do you mean that R-20 in the walls and R-40 in the ceiling/ roof is enough for a 600 sq. ft. house in zone 4? Also, can you point me to good resources on air sealing?

    I will look into crawl space foundations. I would like to avoid issues often brought up on GBA with hot humid air hitting the underside of cool air conditioned floors if possible. It seems like codes here require ventilated crawl spaces.

  6. EricMatsuzawa | | #6

    K Willets,
    I was planning on expanding the size of the footprint to keep the same amount of interior floor space. I didn't consider that the increased costs for sheathing, siding, roofing, etc with no increase in usable square footage might not be worth it to reach an R-30. I thought of another option which would be a dense-packed 2x6 wall with 2 inches of polyiso on the exterior of plywood sheathing. That would give us about an R-30 without having to build a double wall.

  7. user-659915 | | #7

    "Do you mean that R-20 in the walls and R-40 in the ceiling/ roof is enough for a 600 sq. ft. house in zone 4?"

    I mean you're reducing the overall environmental impact of your construction immensely just by building such a small house. This benefit alone will be far in excess of a few extra R's in your wall. Good for you. Folks who build McMansions should be the ones going the extra mile (or ten) on R-values.

    Of course they never do. But you shouldn't feel the need to compensate for their foolishness.

    If you do feel you need to go higher than R20 for the walls, 2x6 with external foam will almost certainly be a simpler solution than double wall which introduces a host of complications and should not be undertaken by the inexperienced. If you have a vented attic with a decent roof pitch pile on the ceiling insulation as thick as you like. It's cheap and easy. GBA has plenty of resources on air-sealing, also on creating a high-performance enclosed crawl space which will not have any of the problems which concern you. If I were as organized as Martin I would point you to them: I'm sure you can find them by looking around. For a long time North Carolina code did not permit insulated unventilated crawl spaces either but we were able to work with the inspectors and document the process to show we were doing it responsibly and to good purpose. It's not cutting edge any more here as it was twenty years ago: I believe you will be doing everyone a favor if you can be part of bringing the practice to Japan.

  8. EricMatsuzawa | | #8

    James, thank you for clearing up what you meant. I was able to find a GBA article which goes along well with what you said about smaller houses needing less insulation to be energy efficient. For any other readers interested, it can be found here:

    My perspective on house size changed a lot since moving to Japan. After living comfortably with a family of four in 430 sq. ft., 600 sq. ft. will seem spacious enough. I don't think I could enjoy living in a typically-sized American house. I would feel uncomfortable not being able to use each square foot of space as often as I do now. Spaces are more alive when in frequent use.

    I was able to find some useful info on air sealing in the Strategies & Details section.

    I will do my best to look into insulated unventilated crawlspaces. I want to make a success of my house project not only to have a green home for my family, but also so that others in Japan may feel inspired to build more thoughtfully insulated and sealed houses, too.

    Are you in NC? My grandparents live there most of the year in Southport. I helped them a little working on a house there when I was a teenager. The climate is very similar to our area of Japan so what works there ought to do well enough for here.

  9. user-1105327 | | #9

    eric, if your 600sq' plan is based on the footprint of your foundation (e.g., 20'x30') then you will be losing over 10% of your usable living space due to your double wall. if you were to employ a larsen truss detail instead, you could reclaim about 60sq'. that goes a long way in such a small house...

  10. KHWillets | | #10

    It sounds like you're able to expand outwards without any penalty. My concern was that you might be in a row house (as I am) or otherwise be constrained in your outside dimensions.

  11. user-659915 | | #11

    Hey Eric
    Yes, I'm in central North Carolina, and I've been told there are strong climate similarities with many parts of Japan. There are certain building traditions in common too: drafty, uninsulated thermally lightweight homes elevated on piers or 'candle stones', a foundation strategy which later transitioned to enclosed but vented crawl spaces. This was perfectly fine until airsealed, insulated conditioned interiors became standard, when the deficiencies of a moist, humid mold factory beneath the floorboards became glaringly apparent.

  12. EricMatsuzawa | | #12

    I wasn't aware of the similarities in building traditions. Japanese houses haven't become well insulated or air sealed yet so the town office and builders here aren't likely to immediately grasp the problem of vented crawlspaces. I'm going to be doing a lot of research on GBA and a lot of translating if I decide to go with an unvented crawlspace.

  13. user-917907 | | #13

    My perspective on house size changed a lot since moving to Japan. After living comfortably with a family of four in 430 sq. ft., 600 sq. ft. will seem spacious enough. I don't think I could enjoy living in a typically-sized American house. I would feel uncomfortable not being able to use each square foot of space as often as I do now. Spaces are more alive when in frequent use.

    I am considering building a very small, well-insulated, single-floor retirement house. Can you, or any other reader, suggest any resources or guides both for building an efficient small home, as well as comfortably learning to live in one -- in addition to the link mentioned earlier?

  14. EricMatsuzawa | | #14


    I am also always looking for good resources and guides. You might have better luck getting other responses if you start a new Q&A thread with your question. I can add my 2 cents here but I am sure others are more knowledgeable than myself.

    Green Building Advisor and are my favorite sources on energy efficient building. There is more information on those 2 websites than my whole collection of books on house building and energy efficiency But for a good overview on energy efficiency, I like "Green from the Ground Up" by David Johnston and Scott Gibson. You can check out the reviews on Amazon here:

    For info on comfortably living in a small house I sometimes look at the blog for the Tumbleweed Tiny House Company.
    It is essentially advertising for them but some of the articles spark ideas.

    As newlyweds, my wife and I shared a 190 square foot apartment. Living in that small apartment wasn't as difficult as I thought it might be. I didn't find any good resources to help. I just winged it but I would love to hear other people's suggestions. I learned that usually, buying something new was not going to add to my well-being but it would be sure to cramp my space. Living in a small place is a good way to keep oneself from buying needless things.

    Reading "Inside the Not So Big House" by Sarah Susanka and Marc Vassallo got me thinking about built-ins and attention to detail. That is one part of the equation that is lacking in living in an apartment. I am not allowed to alter the apartment any further than placing thumb tacks in the wall so built-ins are out. I will follow some of the suggestions I found in that book with my new house.

    Good luck in your project. I would love to learn how it goes.

  15. user-917907 | | #15

    Thank you for the suggestions. There doesn't appear to be a concise book on how to build and simply live in small places. Judging from reviewer's comments Susanka's books may have good ideas, but are geared towards a high-end market -- which excludes me. I may get at least one of her many books just for the inspiration. [Some of her books are available used very inexpensively through]

    I think that folks who live on sailboats learn how to maximize the use of small spaces. Since I've never lived on a sailboat (but not adverse to trying -- perhaps in some exotic tropical locale :) I've been looking for descriptions of how sailors effectively use their spaces. Of course, mariners have different needs than landlubbers (anchors vs lawnmower, etc).

    It's interesting how compact the bathroom and the kitchen were in your 190 sq ft apartment. I gather that the Japanese building industry has developed a lot of solutions to tight spaces that are not available to the US market.

    I hope you keep us posted on your building project.

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