I have a 50 sq m shed I want to insulate as cheaply as possible, it has a concrete floor, thin t & g cladding walls, and a profiled plastic coated steel pitched roof. The plan is to lay an 18 mm osb floor onto treated timber 47 x 75mm & press fit 50 mm EPS in between. The same for the walls with 11mm board and the ceiling with 9 mm board and 75 mm EPS as the rafters are 100mm.

Assuming the above is ok, my question is can I do without breather membrane as there will be a gap between the cladding and the insulation, and also do without a vapour control layer as I intend to seal the OSB joints.

My apologies for my query basically being one of corner cutting, I've only a four year lease and its only going to have occasional use - part workshop, part showroom.
Thanks

Assuming the wall insulation is getting pushed against the t+g, any water penetration will just sit there rotting the wood?? Might last 4 years but landlord might not be too impressed if its started rotting!

Many thanks all, I did wonder about using 9 mm board on the ceiling as the max rafter centre is 670 mm, so its either battened, 11mm OSB, or plywood which clearly is lighter and easier, any steer on minimum thickness, 6 mm ok ?

I wasn't just opting for OSB on price I quite like the textured finish,happy to have ply on the ceiling, maybe I ought to go with 18 mm for the walls, centres are 645 mm. There will be a 25 mm air gap between the insulation and the t & g cladding, I've had no problems with damp or penetration apart from massive amounts of snow blown up and in last winter, I've since filled the roofing profiles with matching foam strip.

Will ask the Landlord's agent to sign off the plan, their attitude is that the shed is such a state that anything I do should be an improvement, especially if it's at my expense, but obviously I'd rather do a decent job and appearance is important.