Steve, you need to insulate between the van walls and your ply lining. You can fill the cavity with a variety of insulates. Fibre glass, foam, .......... Ply lining alone will not or could not be sufficient. My van is insulated with fibre glass and then the ply is 6mm by 3 layers thick so obviously a total of 18mm. It is insulated everywhere, walls, doors and roof too. Although the floor isnt but has a vapour barrior on it as well as a 4mm altro floor. I havent had it freeze up yet and have had the van in temps of minus 20 before when I was in the states. If you dont want the hassle of taking the ply lining down again you can always have it foam filled as in cavity wall insulation. Then you would justhave to patch the holes up where you drilled for access. Best, Dave.