woodsmith
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Vapour barriers sit on the inside of the insulation and prevent warm moist air from inside condensing on the cold inside face of the outside covering. What you are talking about is called a moisture barrier and works in a similar way to roofing felt.
If I build a timber frame wall starting from the inside,
plasterboard
vapour barrier, (polythene sheet) or can be incorporated in the plasterboard if you use the foil backed type
stud wall with insulation
11mm OSB board (This one is optional but gives a much more substantial wall)
moisture barrier I find it is best to fit a breathable membrane such as Tyvek or Rubberoid, expensive though. If you fit the additional sheet of OSB the membrane needs to be firmly fixed with staples or the wind gets underneath it and it makes a hell of a rattle.
Batten using tiling battens, as this will provide an air space
external finish
Even though the plastic cladding is waterproof, I would still fit a moisture barrier
Hope this helps
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