In reply to marsbar:
My build went something like this:
Layer 1 - 2mm silent coat on interior of van outer panel (stops the van exterior panels vibrating whilst traveling so its much quieter)
Layer 2 - recycled plastic bottle insulation (B&Q) or save on type stuff where I needed something a little thinner.
Layer 3 - 10mm of closed cell foam (camping mats)
Layer 4 - vapor barrier (foil coated bubble wrap stuff) sealed together using foil tape
Layer 5 - ply wood with a 2mm adhesive closed cell foam backing. This ensures there's no rattles, seals screw holes and adds extra insulation. I also use a triple layer of foil tape around the edges of the ply to seal it further and it makes the carpet appear a little more seamless where the ply meets metal.
Layer 6 - 4 way stretch carpet
Maybe all a bit overkill but I wanted to be warm in winter without a heater.
Post edited at 13:15