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Early days yet but I'm starting to turn my attention to future projects (yes, I know). I'm sketching out some ideas for building something small and cost-effective. This would be for construction under the 'portable building'; rule and thus building regs exempt. Critical factor is cost effectiveness. Application is for glamping accommodation, studio, garden room, etc. I'm playing around with performance and cost of different materials at the moment, and I'm a big fan of corrugated steel as a cladding material (cheap, easy, fast). I also really like the idea of doing away with the whole VCL/void/battens/PB thing and simply using a ply or OSB interior sheathing layer, sealed with appropriate paint for vapour resistance, and then with all wiring etc surface mounted. Now with no VCL I think there is a risk of vapour in the structure, even if I do my best to seal the interior. So I'm looking at the vapour permeability. The frame would be most likely 45x95 stick build, fully filled with glasswool. I think I then need some sort of exterior board to hold the wool in, prior to the breather, battens, and steel cladding going on. I know that I could just use OSB on the outside, but I'm very curious about the woodfibre boards that @ProDave used. A quick glance online shows that the thinner versions of these aren't silly prices. I don't need them for racking strength, as that's what the interior osb layer will do. Question is where is it best to spend the money: beef up the frame to 145mm and just use osb outside, vs keep the 95mm frame and use say 20mm woodfibre boards as EWI. Or, should I use a different material as EWI, e.g. PIR, PUR- my gut feeling is that this isn't vapour permeable enough. Another option would be to use Kingspan's insulated roof sheets as my exterior cladding, but how do you manage vapour permeability with this system?