by my understanding if you are looking more for sound proofing rather than heat retention then you may want to look at a more dedicated, denser, heavier soundproofing material rather than Cellotex etc.
However, from experience my feelings are that if you are going to be spending any time in the van, rather than just keeping tools in it then insulating is a very good idea.
There is a counter argument which posits that one should just let the van breathe but it seems your are looking to insulate in some form.
Gluing foil-bubble insulating material directly to the exposed metal surfaces then sealing the edges with foil tape will minimise the "dew points" in your van (where warm air meets a cold surface and subsequently condenses into water droplets) Its important to stick it flat to the metal surface and seal the edges, as you don't want water collecting in pockets. At this stage I then left a cavity and foil-taped Cellotex into the front most areas of the voids. It seems that at this stage you may be more inclined to apply a dense sound deadening material, compromising a little on thermal effectiveness but achieving the results you need.
B.
