Garden Office Insulation & Damp Proofing

[musashi79uk]

I'm converting a 14x8 summerhouse into a garden office shortly and wanted advice on insulation and damp proofing as I've read some contradictory information.
The summerhouse will have a 19mm ship lap external layer with 40mm x 70mm treated timber frame behind it.
I was planning on putting 30mm Celotex TB4030 PIR Insulation Board in the timber frame, with Capital Valley Plastics 1000ga Black Damp-Proof Membrane over that and stapled to the inside of the summerhouse frame. Over the damp-proof membrane would be 12mm plasterboard and then skimmed with plaster.
For the floor I was going to lay YBS General Purpose ThermaWrap Insulation (Foiled) with 18mm OSB over it.
For the roof, I was going to use Knauf Eko Roll 200mm Loft Insulation Roll with the Capital Valley Plastics 1000ga Black Damp-Proof Membrane stapled to the inside of the battens with white plastic cladding over it.
The building will be heated by a 2kw panel heater.
Will this be OK or am I missing something? I've read that I need OSB over the wall damp proof membrane, some say not. Some say to use a breathable membrane, some say not. I was originally going to use Visqueen between the bottom floor layer and the OSB layer but thought the YBS Thermawrap would be better for the combined insulation and damp proofing qualities.

 

[Mike13]

when you have intermittent celotex as a wall construction you need to consider vapour control barriers on both sides of the wall to prevent frost-line condensation at the structural timbers that go from inside to outside of the wall structure.

 

[musashi79uk]

Instead of the Celotex, I may use 90mm Knauf FrameTherm® Roll 40 if that is any better for what I'm doing?