Sticking plasterboard to insulation board

Insta-stik or mega-stik foam to the battens or wall (pva'd) Stainless steel washers with screws, but not too many as you'd reduce the vapor properties. Can use the foil tape over the screws and joints to each-other and perimeter. Below link is for the washers only as the screws will need to be longer depending on the thickness of the insulation board https://www.wickes.co.uk/STS-30mm-Screws-and-Stainless-Steel-Washers-%E2%80%93-Pack-of-50/p/101133 The same applies to the plasterboard but this time without the washers and you'll need longer drywall screws (+12.5mm longer) Hope this helps

There are a number of solutions but this is a possible one. Firstly ensure that any water ingress issues in the wall are addressed. You'll need 2x2 treated battens, 50mm PIR, aluminium tape, expanding foam, 100mm drywall screws, plasterboard (and a plasterer). Remember to ensure the vapour control layer, on the warm side of the insulation, is fully taped to the perimeter so that absolutely no warm air can escape behind the insulation, which causes interstitial condensation. You could use 2x2 battens, fixed off the wall on plastic packers to plumb them up, at regular 600 centres, with additional batts along the top and bottom to fully support boards. You could now install 50mm (or 40mm if you can find a source and it fits better) PIR or phenolic insulation between the battens. Tape all joints and foam any larger gaps - and tape over. Make sure it doesn't stick out anywhere. Worst case, you'll have to gouge out a bit off the back and repair it with aluminium tape - or chip off a rogue bit off mortar that's stopping the board from sitting flush. It's a good idea to seal the entire perimeter with expanding foam as you do this, as a belt & braces approach to preventing interstitial condensation. Now you can install 50mm PIR or phenolic over the battens. You don't need a lot of screws- just 'tag' it on. Use insulation washers to secure it. Again, tape joints and foam the perimeter. If you've done your work well and the PIR is in good condition, then the foil layer may be sufficient as a vapour barrier now. Install vapour-check plasterboard over the insulation for good measure, using screws that are long enough to go most of the way through the batten. You can get 100mm drywall screws. Ensure the screws only pinch the paper, and don't break it. You will need to push against the plasterboard as you screw, to ensure you're fully pulling the layers together. Skim the wall for best results.