It’s no fun being cold and I wanted a van I could use all year round without being too worried about the weather. I decided upon 25mm celotex (ebay link) type insulation board for the ceiling and walls as it’s a reasonable price and has very good thermal properties.
I stuck the celotex (ebay link) to the walls and ceiling using expanding foam. It’s insulating in itself and dries quickly to stick things solidly in place. I used this applicator gun which made life much easier.
For the floor I used 30mm xps board (extruded polystyrene). The reason for this is as it has high compressive strength so it isn’t damaged by walking on it. Board like celotex is very easily damaged so you wouldn’t want to consider that for flooring unless laying thick plywood on top of it. After laying the xps I covered it with 3mm hardboard as it was the thinnest board I could find. I would have preferred plywood but couldn’t find large sheets in that thickness. If I was laying it again I would make the extra effort to find 3mm plywood as it is much more resistant to water
Moisture will condense onto the coldest surface it can find and as I didn’t want any condensation forming places I couldn’t see I used bubble wrap insulation (ebay link) on top of my celotex to form a vapour barrier.