Breather Membrane and/or Vapour Barrier

[Newsreader]

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

[Andy Hall]

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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[Rick Hughes]

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

[Andy Hall]

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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[Rick Hughes]

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