Garden Office Wall Composition - Do I need Vapor Barrier?

[Cliffoshay]

I'm just trying to finalise the design and exact construction of the walls of a garden office. It's somewhat of an 'organic' process as I keep changing my mind or getting advice that changes the spec.

This is what I have got so far - (outside to inside obviously):
Western Red Cedar cladding TGV
12mm plywood
Breather membrane
6"x4" joists at 400mm centres
Insulation between joists
12.5mm Plasterboard

I'll also have DPM between the concrete base and floor timbers, folded up between the cladding and ply.

I also plan to add ventilation on each wall into the cavity from inside and out.

So, do I need to add a vapor barrier, if so where?

Thanks in advance!

 

[r896neo]

You need a vapour barrier between the plasterboard and the celotex.

Assuming you apply the outer membrane carefully you could omit the ply lining, it serves no purpose. I would sooner counter batten for the cladding. giving you a nice air circulation void instead. Cheaper too