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Community and Q&A

Energy inefficient Japanese walls – am I missing something?

EricMatsuzawa | Posted in Energy Efficiency and Durability on

I am from Connecticut but am about to build a house in a climate zone 4A section of Japan. I have talked with several Japanese builders and am concerned about the energy efficiency of their building envelope design. Before abandoning local practices, I would like to read GBA readers’ opinions to help decide if my concerns are well-founded.

Here is an overview of what I have learned of walls in Japan. The standard Japanese 2×4 wall from outside to inside consists of panels of artificial siding, a 1/2″ vented rain screen, house wrap, 1/3″ plywood sheathing, a 17″ OC 2×4 stud bay insulated with 2″ fiberglass batts, polyethylene vapor barrier sheet, and 1/2″ sheet rock. Windows are aluminum frame and single pane. Also the amount of wood in the walls make them the polar opposite of advanced framing. 2 photos are attached.

I originally planned to build according to local custom as I assumed the builders would know what was best for the area. However, I later learned that the life expectancy of a newly built Japanese home is 35 years and know first-hand how cold in the winter and hot in the summer they are (most homes are heated with one or several unvented kerosene heaters and there is an air conditioner in each room). I would rather build one house and build it right so my family can live in comfort without exorbitant energy costs. It seems to me that local building practices have too little insulation and I am worried about the interior vapor barrier with air conditioning on during the hot and humid summers. I have raised these concerns with local builders but they have tried to assure me that their methods are correct. Are my concerns well-founded? Would it be advisable to abandon local practices for a more efficient building envelope?


  1. heinblod | | #1

    This company build timber frame homes up to energyplus standard:

    There are many more in Japan, but this one has the longest record.
    You can go to the pre-fab factory and watch your own house being build, taking friends and a camera with you. High quality, but propably not as cheap as the card board house you have described above.

  2. user-1135248 | | #2

    *35 years*?? You might as well buy a trailer.

    I thought Japanese culture was far more into durability
    than that...


  3. GBA Editor
    Martin Holladay | | #3

    The house you describe is similar to homes that were being built in the U.S. in the late 1950s and early 1960s. You shouldn't put up with such a low-quality envelope.

    It's hard to buck the trend here in the U.S., where we have more than our share of builders whose methods are stuck in the past. I don't know if you speak Japanese, but it can be even harder to buck the trend if you are an American in a foreign country where respect for one's elders is honored. Good luck.

    I would advise you to follow Hein Bloed's advice and look further into the possibility that somewhere in your area, there are progressive builders who are paying attention to energy efficiency. I hope you discover some.

  4. Mike Eliason | | #4

    i think the 35 year issue is more about turnover (e.g. houses making way for taller/denser housing).

    historically, japanese walls had no insulation - so i guess 2" fiberglass batts is somewhat of an improvement. there is a small passivhaus community in japan, so there are progressive builders and there is interest - and from what i understand it's grown fairly well since fukushima daiichi.

    that being said, when one of the top architects puts together a cold-weather experimental house like this (with no sub-floor insulation and a few inches in the wall/roof) - one gets the feeling a fundamental paradigm shift on wall performance ain't happening.

  5. heinblod | | #5

    Thanks Mike Eliason, interesting Japanese structure !

    We shouldn't forget the tectonic problems in some places on our planet, lightweight construction can safe lives.
    And the still kickin' atomic village responsibel for building legislations, high fuel bills feed the suckers.

  6. Expert Member
    Dana Dorsett | | #6

    The whole-wall R of that stackup at a 25% framing fraction (typical for US 2x4 16" o.c. framing, but nothing like the 8-9 stud stackup in the photo) would run just shy of R6 after thermal bridging, and presumes that there is a tight fitting facer on the batt that faces the remaining 1.5" of air. That compares to roughly R9 whole-wall if the cavity is completely filled with mid-density fiberglass (R13 batts, not R11s, not high density R15s.)

    With the rainscreened siding there is no point to the poly sheeting in that climate (other than an air-barrier, if detailed thusly), which would only serve to become a condensing surface during the summer should you air-condition the place, since it's vapor-impermeable. But even moderate air-tightness is a big improvement over pre-1970 construction in Japan, where homes were pretty drafty too.

    Traditionally Japanese homes aren't fully heated, and are often point-source heated, making the net-present-value of the energy savings pretty low, but from a comfort point of view It's substantial. I've stayed in Japanese homes in winter of the barely-insulated construction described, as well as homes that were of masonry construction with ~R8-R10 continuous insulation (all located in Kochi prefecture), and the upgrade in comfort was significant, despite the similarly low room temperatures and point-source heating strategy (vented kerosene space heater in the living/family room.) And that's a local climate comparable to the WARM edge of US zone 4.

  7. KHWillets | | #7

    There was a thread on FHB a few years ago about building a modern house in Japan:

    Lots of interesting info. The guy had some building experience, so he manhandled the contractors into doing it his way.

  8. GBA Editor
    Martin Holladay | | #8

    K Willets,
    Here's a quote from the owner of the house who posted the information on the Fine Homebuilding website: "I think that Japanese carpenters, no matter how good they are with wood, are resolutely stupid when it comes to insulation. This is understandable, from the standpoint of their traditional role as workers in wood alone, and their pride in this. However, for some reason, they have been stuck with the job of insulating, and I think it would behoove them to increase their knowledge."

  9. jinmtvt | | #9

    "However, for some reason, they have been stuck with the job of insulating, and I think it would behoove them to increase their knowledge."

    i believe it is the same around here with most tradesman :)

  10. EricMatsuzawa | | #10

    Thank you for mentioning Sekisui. It is one of the biggest building companies in Japan and I live in one of their newer apartments at the moment (it's like living in a pup tent as far as thermal comfort). They might make houses to the energy plus standard but my guess is they are leaky structures in a mild climate with loads of PV panels. Please point me to any info to the contrary and I would be happy to give them another look. You can read a description of their walls near the middle of the following linked page:

    Two thirds down the following linked page in Japanese, they extoll their building envelope's low thermal transmittance of 2.1W/m²k which beats my climate zone's government standard by 0.3W/m²k:

    I wasn't sure what 2.1W/m²k meant. I searched a bit and found out that it seems to transfer heat similarly to "double glazed windows with advanced coatings: 2.2 W/m²·K". After living in this apartment, I believe it. See the Wikipedia article on thermal transmittance:

  11. EricMatsuzawa | | #11

    I was wondering how much R-value would be left in the wall with so much thermal bridging studs instead of insulation and you answered that for me. I toured the 2 largest 2x4 framing companies in the area and they both used solid stacks of 2x4s as headers all the way from the window to the top plate without any space for insulation. I talked to them about advanced framing but they were sure that a house using it would immediately topple over in an earthquake. We are in the one of the most earthquake-safe areas of Japan so I'm not so sure.

  12. KHWillets | | #12

    W/m²k is the metric U-value, the inverse of R-value. You have about R-2.7 in English units.

  13. EricMatsuzawa | | #13

    You hit the nail on the head about the difficulty of bucking trends in Japan. There is actually a saying here that "the nail that stands up gets hammered down" which means that conformity is enforced. I can speak Japanese well although I am far from fluent. In my last job, I was the head of research and development for a local mushroom producer. As one of the youngest and newest employees, and as a foreigner, it was much easier to triple their efficiency in some areas than it was to suggest those improvements without ruffling any feathers of my superiors. Being tactful enough requires skills beyond anything I could have imagined back in America.

    Last year, I did manage to discuss my house project with Miwa Mori who is the president of an architectural firm which made the first certified passivhaus in Japan. She was the first person I had met in Japan who shares my concerns about energy efficiency in Japanese houses. I originally approached her with a more ambitious house plan and she explained that it would be out of our budget. We have since decided on a small, 600 square foot Tumbleweed house so it may be worth talking to them again.

    Here is a link to an interview with Miwa Mori for German TV program, Global 3000:

  14. user-869687 | | #14

    I have noticed this time and again with Japanese buildings published on -- no concern about insulation (or privacy, or waterproofing, or safe stairs, but that's another story). At least among the modern / photogenic projects, it's common to have single glazing, solid concrete walls (exposed inside and out), and some rather iffy waterproofing details for skylights or roof deck spaces. But on the other hand these buildings are generally ultra compact and heated with mini-splits.

  15. user-304075 | | #15

    I was in Japan in December and visited a site with over a dozen model homes by Daiwa. I was very surprised when I saw a wall section. There were no thermal breaks in the walls or windows, windows were all metal frames, insulation was pretty loose fill and would settle and I saw no real evidence of good air sealing. The rain-screen and roofing details were good, as one would expect in rainy Japan. They would not let me take a picture. To me the structures didn't seem to be energy efficient but the homes were heated by mini-splits.

    The cost of the homes per sq foot were not expensive by American standards and the interior finishing was very nice.

    I gather it is true that the homes are designed for a 35 year life but I suspect in reality that they would last as long as an American spec. built home. It is probably some legal definition for mortgage paperwork. I was told that the reason is that the land prices are often multiple times the cost of the building and homes are often knocked down after 20 years or so and new built on the site. I have seen evidence of this on my visits to Japan in the cities but these model homes were 2000 sq feet and over and would never be built in the city.

  16. user-1087436 | | #16

    For what it's worth, I Googled this and found that Precision Panels, according to their website, have been shipping SIPs to Japan since 1996. The link is here:

  17. user-1087436 | | #17

    Also, try this link:

    I Googled the phrase [no quotes] "building with SIPs in Japan". Evidently there are people there who have built and continue to build with SIPs. They're expensive here, and I'm sure they're expensive there, but you'd get an energy-efficient earthquake-proof house, for sure.

  18. heinblod | | #18

    Hi Eric !

    Here a page from Sekisui explaining the wind proofness of their constructions:

    If going with this (or any) company then get everything into a written contract. Including guarantees on performances and even fines for not delivering what was promised/agreed on.

    Demand the air blower test being done.

    Sekisui have sales people to contact you. These will refer you to a meeting with their engineers.
    Write down what you want, incl. the energy usage you want (kWh/a) for this house with it's enrgy supply in this position (heating degree days) as paid for.
    Fix this into a contract and see if they are willing to sign, to build it.

    This is a normal purchasing method, they are familiar with 'difficult clients'.

    Buying a show house is for the show, getting the real thing is down to contracts and controls.

    That the building world is now opting for less insulation thickness compared to 20 years ago is normal, economics are important. The price for electric energy is going down, home-made PV now becoming cheaper and fossiles being phased out pricewise and by legislation.

    But this shoud not mean leaky, unhealthy and noisy homes.

    If you are not familiar with building methods and -phrases (the written contract!) get an independant advisor for the legal part and an independant supervisor for the site.

    Good luck!

  19. EricMatsuzawa | | #19

    Thank you for the two links. I have considered Precision Panel. Then, I read many articles on GBA and decided that although SIPs are more energy efficient than most houses in Japan, they might not be the most cost-effective choice for getting R-40 in the walls and R-60 in the roof. I am starting to think that a double stud wall might be the best way to go because it could use materials that are locally available.

    I really like reading the blog from the DIY house in Kyoto. It's nice to see how the whole community is helping out. I have been here for 7 or 8 years now and have found that people here are extremely generous and kind. Plus, they really know how to timber frame.

  20. Expert Member
    Dana Dorsett | | #20

    Eric: The R-value of a 2x4 edge-to edge is about R3.5 give or take a bit depending on exact species & density. The larger the fraction (often called "framing fraction" in the US) of the wall's total area the studs, plates, fireblocks are, the lower the average "whole wall" R value is, at any center-cavity R greater than R3.5. In the US typical framing fractions on 2x4 construction runs about 25% of the total wall area, and with an R13-R15 center cavity the whole-wall performance is in the R10 range when wood siding is used, a bit less with most other siding types.

    But it's the framing fraction and insulation-R that determines just how much of a performance hit you take.

    Code required seismic bracing & anchoring does not need to dramatically raise framing fractions in most US locations it is enshrined in code. The key components are tighter specifcation of anchor bolts designed to keep the bottom plates from separating from the foundation, and brackets keeping the studs from separating from the top & bottom plates, usually only at critical junctures like corners or window/door framing. A massive 8-stud post (as in your attached picture) without the metal bracketing tying it to a well-anchored studplate comes apart just as easily as other framing in a earthquake.

    The rest of the seismic resilience specs usually include cut-in or other bracing against racking forces, or tighter fastener spacing for some/all of the sheathing, (with sheathing much thicker & stiffer than 8mm plywood (which wouldn't meet code in any part of the US that has building codes, even where there aren't earthquake or hurricane wind issues to deal with.)

  21. EricMatsuzawa | | #21

    Thank you for the further information about calculating R-value loss due to framing members. With what you told me, I can now make my own rough estimates about R-value losses.

    I would really love to use advanced framing (or at least standard US framing) on my build and from what you wrote, it seems that it should be possible with the proper engineering but every one who I have talked to in Japan about it essentially says that I would be building my own tomb. I can do it, but they wouldn't advise it.

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