Vapour Control Layer

[chirpychippy]

I've built I timber framed garden room in my garden.

I'm not sure about a detail relating to a vapour barrier or breathable membrane.

The area of concern is the floor, it's a timber suspended floor, which has celotex between the joists. The underneath is vented for airflow but otherwise sealed.

I am about to put 18mm MR Chipboard flooring down but am wondering do I need a breathable membrane or vapour control layer between the two?

Equally whilst mind is whirring, same question, the walls are externally to internally, 20mm Cladding 2x25mm batton, breather membrane, 11mm osb on 4x2 stud, I'm putting knauf accoustic insulation in the walls and celotex in the roof then plasterboard, do I need a vapour control layer behind the plasterboard please?

 

[catlad]

I don't see any of that being necessary as long you have ventilation behind the cladding.
If you are planning of pumping a lot of heat in there for extended periods then it might be an idea to use foil back plasterboard.

 

[chirpychippy]

Yes the cladding has full airflow behind it, but behind that is rhinovent and osb.

The roof is flat and after the celetex goes in there will be a 50 to 125 mm air gap with vented soffit.

It will be heated, it will be an office.

I don't understand "dew point" where the hot air inside meets the cold air outside and dew form's, that's what I'm trying to protect the building against.

 

[freddiemercurystwin]

Warm air will naturally want to penetrate through the wall through all of the various layers. The dew point is the theoretical point in the wall, somewhere in the depth of the insulation where the temperature reaches a level low enough to enable the water in the air to condensate allowing it to enter the insulation which is obviously no good for the insulation/wall. So yes you should be fitting a vapour barrier behind your plasterboard to stop any air from entering the insulation thus preventing any condensation occurring within the insulation. The rest of your wall construction is fine.