Ah, a question right up my street.

What is your target external wall U-Value target?

Why would you only install 50 mm of insulation between 100 mm deep studs? More insulation is always more, and a 50 mm void allows the possbility of air movement reducing performace.

Why not go for

Render on backed lathe
25 mm treated battens with dpc between render and batten
breather membrane
50 - 100 mm rigid insulation board (to achieve desired U-Value (0.15 or better))
100 mm studs with 100 mm foil faced insulation between
OSB sheathing board taped and sealed at joints
polythene vapour control and air tightness membrane
25 mm battens forming service void with medium to high density mineral wool insulation
plasterboard and skim

The cladding battens should be fixed through insulation to stud with either metal or plastic screws and if possible support at the top and/or bottom on either the rafters or some additional battening.

There are of course MANY ways to skin the cat. I persoanlly would go for some more breathable insulation materials between and outside the studs, but it would result in a thicker wall for a set U-Value.

It is also worthwhile considering how to mitigate any thermal bridges between the different elements. This may help to dictate wall, roof and floor buildup.

Timber

With no structural board on the outside of the studs, you'll need the OSB to provide racking strength.

Using insulated plasterboard makes it very difficult to achieve a good airtightness layer. It is difficult to seal the rear face of the plasterboard & the plasterboard is likely to be punctured in various places by electrics & other services.

Better to follow Timber's suggestion, fill the studs with the insulation, put any further insulation on the outside keeping the studs warm & put a continuous air barrier on the inside which can be sealed before services & plasterboard are fitted.

Taped OSB & seperate polyethylene vapour control layer is belt & braces from an airtightness point of view, but makes sense from a vapour control point of view if you have a non-breathable insulation board outside the studs.

The battens give the electrician/plumber somewhere to route cables/pipes without puncturing the air barrier. As an alternative you could use 2 layers of plasterboard with the services buried in the 1st layer.

David

Indeed the OSB goes on the inside. Helps with vapour control and air tightness.

If yoy are having minimal services, then you could use 15 mm OSB cut into strips to act as strapping to run the services between, only on the walls with services. Don't worry about the screw fixings puncturing the vcl. It will have very little impact on its performance.

For the insulation board on the outside, kingspan/celotex, eps, wood fibre, whatever you want really. EPS is more breathable than foil faced boards.

Viking,

What do you use to cut the grooves for the battens in the EPS?

Do you just use a router - I'd have thought that would make a heck of a mess cutting eps?

cheers

Mark

Hi all
Thanks very much for your input, have gone for an amalgam of the advise
Will see what comes out of the builders quotes
ideally will be able to afford Gutex and lime render but I doubt it!
All the best
Andy

Thanks for the tip JS, have you a photo of that soldering gun with the attachment?

Google for "hot knife eps" throws up quite a lot. Seems they are a US tool but they are being imported over here. Care I think is needed to get one with enough grunt for continuous use. A separate PSU is probably the way to go.

If you do find a good supplier give it a shout out.

Cheers