Boarding over celotex

Hi,

Not sure quite what your wall build up is here, but starting from the outside the recommendation is:

• the outer sheathing/cladding (in this case shiplap)
• a support system for the shiplap - usually 2 x 1 treated batten, this creates an air gap between the cladding (shiplap) and the underlying structure, allowing the cladding to easily dry out
• breather membrane
• timber studwork with insulation in between (no air gap needed here, but won't harm)
• vapour control membrane, tape all joints
• interior cladding (i.e. OSB, ply or plasterboard, or whatever your project demands, but beware building regs for combustible materials)

However if your workshop is not a residential space then you may not need to follow this exact spec, and there are other variations. I did something similar for our kitchen extension recently and added a fibreboard on the outside of the studwork for extra insulation, it was a tongue and groove structure and about 40 mm thick, but it makes a big difference to the insulation levels.

The other factor is, if you are creating an airtight wall with a vapour control membrane on the inside, you need to provide alternative means of ventilation to allow moisture to escape and air to circulate, thus losing heat in other ways, I don't now if the building regs have ever quite solved this paradox!

Hope this of some help.

 

From
inside out: insulated plaster board with integral VCL (or plasterboard, Foil faced insulation, AVCL ALL fixed back to frame), Foil faced boards between frame - full depth or less, building wrap membrane for timber frame construction eg Kingspan Nilvent stapled to outer face of frame. Fit treated Battens and then fix your outer skin. All a low level space for air to go in (install insect mesh too) and do same at top.

This will keep the heat in and deal with letting condensation out.

 

In fact I would fit a sheet of 18mm OSB class 3 to external frame prior to fitting the nilvent but you technically could omit it

 

I don't think the Vapour control membrane is necessary, this is not a kitchen or bathroom generating lots of moisture, just a workshop, you have already done more than most sheds get with the insulation and breather membrane, so if you want to just line with plywood to create nice clean interior that should be fine, lets the building breath, and in my experience most failures of timber structures are due to trapped moisture.

A 12 mm nice birch faced ply would make it look very smart, make sure you get something called WBP ply, go thicker if you can afford it and if you want to put lots of shelves and other fittings inside.

 

Is the roof also insulated? The point of vapour control is to stop interstitial condensation i.e. condensation within the structure. This is very important in timber from structures as condensation within the structure can lead to dry rot.

If you've insulated the roof and you are going to be heating the space then you really should have a fully sealed vapour control over the whole of the walls and ceiling.

 

If you aren't heating the workshop then why bother with insulation? Without heating it will just be the same temperature as the outside!

 

Agreed with Ben.

If heated you need a vapour control layer to walls and within warm roof package.