50mm Celotex over or in between studwork?

[Some bloak]

I've pretty much decided which way to go with the insulation order of things on my new workshop build.. I intend to put a heavy duty DPM sheet against the (stone) wall. Then build the 3 x 2 stud wall/battening on top of/ against that, and where any fixings penetrate the DPM I'll make good with some silicone at those points. There would be a small void within the depth of the studding behind the insulation but I can't see that being a problem.

My main question; is there any reason why I can't cover the whole stud frame with the Celotex sheets and foil tape and foam all the joint lines/ any gaps ?. This would save all the laborious cutting and wedging of the insulation between the stud work. Presumably it would also cut out any thermal bridging through the studs than if I was fixing between them. And with everything taped and sealed I don't think I would need any further vapour barrier on top of all this?.

I would then clad the whole lot inside the workshop with 18mm OSB, fixing the OSB boards through the 50mm Celotex in to the stud work behind.
Does anyone envisage any problems in this method?. If the Celotex or rigid insulation is good enough to float floors on
then boards fixed against it should be fine me thinks?. Thanks for all your valuable help and advice out there.

 

[HOJ]

On our timber frame buildings we fix the internal layer of insulation, only 20mm thick though, over the face of the studs and foil tape all the joints, as you say, easier, and breaks the thermal bridge, also gives better air tightness and acts as a vapour barrier, as has been said, check your sheet dimensions for the stud centers, the insulation will probably be 1200mm x 2400mm, the 18mm OSB will be 1220mm x 2440mm (for reference, 9mm OSB is 1197mm x 2397mm, designed for timber frame walls).

In the past, on several grade 2 listed houses/barn conversions, we have fixed and packed level, treated battens to
the walls with a strip of DPC behind each batten to give us a means of securing the inner linings.

 

[RobinBHM]

Celetex over, rather in between is a much better solution. In between has thermal bridging but also it is diffult to avoid some gaps so in practice the u value will be lower.

Taping joints instead of a vapour barrier works well.

The thing to remember is that you may crush the celetex somewhat at the studs, so either do 3 x 2 flat, have studs at 400mm centres, or clad your studwork with 11 osb to fully support the celetex (or strips to cover the studs).

Dont forget to mark your stud centres carefully before cladding with insulation!