Nobody likes sitting on a cold floor and a cold floor will make your van more difficult to heat thus consuming more power. I mention this because I have seen factory built conversions with zero floor insulation, other than a wood sheet, wood panels or lino. I dont agree with this, I believe the floor needs to be properly insulated like the rest of the van.
Below: I have fitted a 25mm wood sections to the floor. I have also bonded them as well as screwed them down with self taping screws. I also under-sealed each screw under the van to prevent them rusting. The reason I bonded the wood shown below is so that if any sections did come a little loose then they wouldn’t “flap” when walked on. It helps keep the floor quiet.
Below: I have added 25mm foam panels inside each of the wooden sections. Each panel is a tight fit and any small gaps were sealed with spray foam.
Below: The next stage is to seal all the edges with aluminium tape and make sure you have a tight seal everywhere.
Below: Once sealed I have added 15mm of ply on top to form a good base.
Celotex 25mm Foam Boards (or Kingspan)
Board size – 1200 x 2400mm
Silver Foil Tape
https://www.screwfix.com/p/diall-aluminium-foil-tape-silver-45m-x-75mm/4415v