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HELP!, roof/ceiling venting plan, is this an isolation disaster?

I am having a back garden practice/recording cabin built. Structure will be (from outside to inside)

Concrete base with DPM
timber frame (total area 4x5m)
OSB with 18 mm shiplap cladding
4x2in wooden studs
100mm RW3
2 layers of 15mm acoustic plasterboard.

Roof
OSB covered with EPDM