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Community and Q&A

New home without sheathing acceptable?

rsmith02jp | Posted in General Questions on

I am working on plans for a new house with a builder in northeastern Japan (Sendai region) which is a mixed climate with lows of around -.4C (31F) to 28C (83F) with high summer humidity.

I just realized that their diagram for the wall include no sheathing (OSB, plywood, etc.), which came as a shock to me.

Is this is a reasonable building approach for a safe and long-lasting home?

They layers from the outside are:
ceramic siding
air gap
housewrap
blown cellulose fiber contained by non-woven polyester
drywall

There does appear to be extensive bracing for earthquakes (meeting Japan’s highest standard), but structurally is housewrap enough to hold the house together for the long-haul?

Adding OSB or plywood would introduce another potential problem- the risk of condensation in the wall in winter which not having it avoids. I’d probably need Intello or the like on the interior if I forced the housemaker to add such siding based on a condensation risk calculator I tried. This may be why they omitted it in the first place.

I’m not able to add pictures to the forum at the moment but here’s a diagram of the home:
https://www.dropbox.com/scl/fi/3ly97nywxds6f3qp8galq/PXL_20241020_125028555.jpg?rlkey=7b9gpq81us7i0tgy61e24b1bb&dl=0

and here’s an example of the wall bracing:
https://www.dropbox.com/scl/fi/5qsdztsukie9yzdsompqr/PXL_20240924_034545545.jpg?rlkey=8hmeab76qm8mowj59ll6owggo&dl=0

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Replies

  1. Expert Member
    Michael Maines | | #1

    Building without structural sheathing is allowed in the IRC that most of the US uses; you just need to provide another way to resist lateral loads: https://codes.iccsafe.org/content/IRC2021P1/chapter-6-wall-construction#IRC2021P1_Pt03_Ch06_SecR602.10.4. Homes built before the late 1800s virtually never had structural sheathing; the timber framing did all the work.

  2. Malcolm_Taylor | | #2

    rsmith02jp,

    I think it will perform well from both a structural and moisture perspective. What it lacks, like most new Japanese houses, is what we would consider adequate levels of insulation, and strategies to reduce thermal bridging. I'm not sure there is much you can do about that.

    1. Expert Member
      Michael Maines | | #5

      Malcolm, I have heard the same from others who have lived in Japan. It's amazing to me that as advanced as they are in many ways, such as manufacturing, that they don't address the wasted energy and the discomfort of living in poorly-insulated homes.

      1. Malcolm_Taylor | | #7

        Michael,

        From what I understand it's tied in with other issues. The average lifespan of a new Japanese house is only around 20 years.

        1. rsmith02jp | | #9

          None of this is really true or up to date.

          In absolute terms, the energy usage of a Japanese home is lower than most "green" homes in the US. They are smaller, there has been a focus on heating inhabited areas vs the entire structure, they use the latest ductless minisplits, and solar PV penetration is greater than the US. That said I agree there is significant potential to improve with air tightness, insulation and better windows. I want to help create a new norm for what a good home is with even this better than average house.

          I met with multiple builders and two that I met with make homes that would be considered high-performance in the US. There is a small but growing Passive House scene as well. The builder I went with is less advanced, but had access to a great property that the others didn't so I've been retrofitting improvements onto their plans (like replacing plastic with natural materials and tripling attic insulation). To get a mortgage tax deduction homes now have to meet standards and this one meets a zero energy homes standard and a long-lived homes standard. The latter includes wall condensation calculations, which is how I got onto this topic to begin with (when I redid it myself with plywood sheathing it failed; then I realized there wasn't any such sheathing!)

          The average lifespan of a Japanese house is closer to 30 years but that's more about deprecation than potential usable life. In the countryside near a major city where I was looking for houses to renovate there are many from the 1960s and 70s available for next to nothing. If I waited longer I might have found one worth renovating but got impatient.

          The house I am planning to build will have 7KW solar, all local cedar beams and floors. R-40 attic insulation and 4in blown cellulose walls, continuous subfloor insulation, low-E resin windows, EV charger, 5 min. walk to a train station, plaster walls, cork floor kitchen, metal roof, custom tile bath and bay views for about $250K. It's on stable land well above a tsunami inundation area. It should outlast me and hopefully retain value more than cheaply built disposable homes.

          1. Malcolm_Taylor | | #11

            rsmith,

            Sounds great. Good luck with your build. I think you misunderstood our comments. We were advocating for the same improvements to insulation levels that you have added to your house.

          2. rsmith02jp | | #12

            Thanks Malcolm. I didn't take it as criticism of me, I just think the understanding of Japan is outdated. The Japanese government is promoting long-lasting and more energy efficient homes and this house is built to those higher standards (which I raised even further, in part as an example to others of what is possible).

            Normal home loans here are 35-40 years and there are many multigenerational houses so the idea they are all torn down in 30 years is just not right.

          3. Malcolm_Taylor | | #14

            rssmith,

            That's heartening to hear!

  3. rsmith02jp | | #3

    I really appreciate these replies as this is far outside of what I had experience with in the US and it makes me more comfortable with the idea.

    I do wonder how blown cellulose will fare supported by plastic sheets on both sides (houswrap and whatever they blow the insulation through) vs the wood cavities I'm used to. What happens when we cut through these layers to run minisplit lines?

    Great points on the thermal bridging- the amount of framing here is huge. The wall cavity will have about 4 inches of cellulose which isn't awful. The attic area was only planned to have the same which I found unacceptable and will be doubled to 8 in around the narrow eaves areas (so as not to block ventilation) but tripled elsewhere in the attic, so should be in the R-40 range which isn't terrible.

    I'll have a somewhat oversized solar PV system so should be able to control heating and cooling costs with that.

    Roger

    1. Malcolm_Taylor | | #4

      Roger,

      High performance houses without exterior sheathing - both on walls and roofs - have been around for a long time. It's something it takes a bit to get comfortable with, but there are real benefits in terms of the way they perform. Moisture problems are often associated with the sheathing, so removing it removes a big risk.

      I'd have all the services roughed-in before the walls and roof get blown, but poking a few holes through cellulose for mini-split lines and then sealing them isn't very difficult.

    2. Expert Member
      Michael Maines | | #6

      As long as the plastic membrane is stretched tightly and fastened securely, it won't be much different than blowing against sheathing.

      1. rsmith02jp | | #10

        Thanks for that reassurance, I will meet with the builder this weekend and ask about roughing in openings. I think we'll prepare service sleeves for the minisplit lines in advance as I trust them more than the local electronics stores to do a careful job. I don't really want cellulose falling into the air gap and ending up blocking a bit of the bottom grate either.

        (FWIW minisplits are cheap here. I should be able to get a larger 200V cold climate Fujitsu one for about $1500 installed. I got a smaller Daikin recently with an outdoor ventilation function for about $1000 installed).

        1. Expert Member
          Michael Maines | | #13

          Sleeves are definitely the way to go. Using flexible gaskets helps with air-sealing but linesets still require a bit of extra attention.

          Those are amazing prices. Just a little of 10% of the cost here in the northeast US.

          1. rsmith02jp | | #15

            Thanks Michael, I'll go with your advice.

            I had lived and worked in CT (did residential efficiency for 7 years) and when I visited last year was shocked at what people were paying for Japanese heat pumps. Just so you know this is what they actually cost in a mature market (price is for a set with one head and one outdoor unit- installation is usually <$100 from a big box store) so hopefully they will decrease in the US as well.

  4. begreener | | #8

    I recently came across an EPS foam "SIP" type of wall system that doesn't have exterior sheathing either ...

    https://gsbp.ca/

    Interestingly, neither does Raycore (polyurethane) "SIP" type of wall system

    https://raycore.com/

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