I work in a Builders Merchants , dealing with a customer who has ordered 16 shts off 8' x 4' x 80mm xtratherm along with 12.5mm plasterboard , he is putting these together for thermal liner and wants to fix these too an interior wall.
What sort of fixing or/and adhesive is needed for this ?

lafarge do a universal bonding compound which is designed to stick both normal plasterboard and thermalboards if thats any help. once the thermal boards are set they need 4/6 mechanical fixings incase of fire. are the boards polystyrene or foam backed. i've never stuck foam backed boards, always put them on studs. polystyrene are ok to dab though.

The xtratherm is foiled both sides with polyiso in the middle . Normally you buy the plasterboard already stuck onto the insulation tho it seems to work out cheaper buying the plasterboard & insulation separate then putting them together .
We have bags of the Bonding Compound that would do to stick them , need to source out these mechanical fixings you mentioned , going through 92.5mm in total, so would maybe need close to 130mm for the fixing ?

http://www.toolstation.com/shop/Hand+To ... /d10/sd140

they do a 110mm , i've got to be honest, i've never seen them before. best thing might be to stud the walls out and screw the boards on with some very very long screws. if the boards are foil then you're not meant to dab them.

I had mentioned using batons but he reckons it will bring the sheets out from the wall too much . I thought you could dab the plasterboard straight onto the wall then maybe use these fixings through the insulation then plasterboard on into wall ?

the fixings are only designed as a back up,like i said normally you would dab the boards and then put 4/6 fixings in afterwards. why not use 35mm polystyrene backed boards and dab them? i wouldnt personally dab foil backed boards but if he wants to its his choice

No probs , the customer has asked me how to fix them , this must be a first for him or something .
Thanks for all the advice , know a lot more now !