Aluminium tape for Insulation
If you use permeable insulation material such as sheep’s wool, rockwool or fibreglass, then on the warm side of the insulation and its accompanying timber frame you should install a vapour barrier, commonly a plastic sheet of sufficient density. Expanded polystyrene (EPS) sheets are semi-permeable, so for safety should also have one. See Wikipedia for a brief discussion.
Extruded polystyrene (XPS) has good vapour resistance, while most other insulation boards (polyurethane et al) have aluminium facings which make them impermeable. However, joints between insulation boards as well as joints between floor etc and the boards must be sealed.
You can use Celotex or Kingspan insulation boards which are foil faced, and then cover them with separate plasterboard. The insulation therefore forms the vapour barrier, but it is essential to seal all joins and gaps; Use Aluminium tape (sold for this purpose) to seal joints between boards as it sticks well to the clean new Aluminium foil on the boards.
However, its adhesion to old walls, floors etc. is suspect, so here I leave a deliberate 15mm gap which I fill with expanding polyurethane foam; the expanding foam is semi-permeable, but this is better than impermeable tape which might in time come off and leave gaps. A separate foam gun (R250 to R420 from Builders Warehouse) is invaluable for this; the foam canisters for it are slightly different from the regular ones.
If you use the insulated plasterboard this should come with an Aluminium foil vapour barrier between the plasterboard and the insulation, but check a corner to make sure. The cheapest insulated plasterboard which uses expanded polystyrene does NOT have a vapour barrier, so don’t use it!
The skim coat of plaster will give some degree of vapour resistance at joints, but make sure that the joint between the bottom of the insulated plasterboard and the floor is sealed, as the plasterers bible seems to say that they must leave a gap there. I recommend asking the plasterer to leave a 20mm gap which one of you can then fill with expanding foam before the skirting board goes back on.