Interesting. I had seen this in Mike’s design as well as others online. But like you say, hard to adjust when the majority of framing you see online places OSB on the exterior. I was probably thinking of doubling up and using osb on outside and inside of the frame. However, concern with that would be trapping moisture within the frame.
Ok - back to the drawing board with an internal wall height of 2.4m and just the breathable membrane, battens and wood cladding on outside.
I’m not yet sure about size of floor joists. We have incredibly rocky ground and the least amount of excavation the better for my wrists! I’m thinking concrete piers and the least possible, so it helps me to push spans and go for studier timber sizes eg. 6x2. I’m getting a local architectural tech to draw up my sketches so hoping he’ll assist. If not, we can get some structural calcs done. Anyway... my 100mm osb overlap was simply there to cover the join between bottom plate, floor sheets and joists. It doesn’t need to extend all the way to the bottom of joists, just enough to cover the above. I suppose with this new design (no external osb) it’s just a case of extending the breather membrane and battens below the bottom plate (sole).
Thanks for the correction on terminology... there does seem to be quite a bit of variation out there! I guess some of it is UK/US variations? Top and bottom makes sense to me anyway!!!
How big is the building?
Could you not just use span tables to get the right size joists rather than get structural calcs done? That's what I did for my floor and also my roof.
I've been thinking about this OSB on inside thing and I can't think of any reason not to do it other than a few minor things.
1. electrical second fix might be more hassle because there is OSB and plasterboard to cut through to fit the sockets. Not exactly a deal breaker.
2. No service void behind plasterboard to put the electrical wiring, assuming the PIR is tight up against the OSB on inside.
3. The PIR isn't 'sandwiched' unlike with traditional method, unless you count the breather membrane, but not sure if that is going to be as robust.
4. You might not be able to build the wall flat, including sheathing, breather membrane, battens, which is more efficient than doing it when it's upright. I'm again thinking of sequencing of the electrical first fix and how that works with this method.