In reply to Nicola:
I'm coming to the end of a ducato conversion.ivused 25mm kingspan/celotex for the roof, 50mm for the walls, then ybs airtech bubble foil over the top. The airtech in theory creates a vapor barrier and air gap.
After that ply lined and carpeted with easyliner. So far its been really good. The celotex is pretty cheap too, I got mine from a local builders yard and it was much less than screwfix, wicked etc...
We also got a 6ft bed widthways (cut the foam for the bed last night.
I have some pictures if that would be helpful, send me a message.
Alex