Breather Membrane and/or Vapour Barrier

Hi All,
I'm about to start construction of a large wooden shed, 8m x 4m, and need some advice on where and what types of membrane to use please ?
The shed is effectively a studwork arrangement from CLS timber and it will be clad with shiplap, and in time I will fit Celotex type insulation into the stud spaces and then line the inner face with ply.
I intend to fit a breathable membrane between the inner face of the shiplap and the outer face of the CLS studs - is this a good idea or just a waste of money ?
Do I need to leave a space between the shiplap/membrane and the insulation board ?
Or should I leave a space between the insulation board and the inner ply lining ?
Finally should I fit a vapour barrier i.e. impermeable membrane, between the studs and the ply lining ?
I'm looking to build the shed to a good long term standard so it's reasonably warm to work in so all advice welcome.
Many Thanks
Steve
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On Wed, 13 Aug 2003 18:00:48 +0100, Andy Dingley

Celotex has a foil barrier on both sides, so AIUI from the manufacturer, this acts as a vapour barrier in both directions.
.andy
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wrote:

But unless you take care of cold bridging - and ensure Celotex covers the studs as well - the moisture dew point will occur on the studs - which may end up causing rot. Put in a vapour barrier over the celotex - just buy temporary polythene protection sheet .... it's about 15 for a roll of it.
Rick
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On Thu, 14 Aug 2003 19:43:44 +0000 (UTC), "Rick Hughes"

Good point. I taped over my studs with the metallised tape that they recommend. I also treated all the timbers for belt and braces.
.andy
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wrote:

That is the usual way - the other way is to have a 20mm thick celotex board over studs - and fix boards through it .. supposedly better for cold bridging, but rarely done on site.
Rick
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