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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. Expert Member
    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.

  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. Expert Member
      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.

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