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@Emyr
Thoughts on insulation:
Aerogel e.g Proctor Spacetherm:
Conductivity: 0.014 W/(m.K) about as good as it gets.
Relatively stable.
Available as a plain blanket (polyester infused with aerogel "sand"), good for wrapping around stuff and bunching into gaps, or bonded to a rigid sheet (plasterboard, ply or Magnesium Oxide). MagOx is nicely fire resistant but I don't know how it handles (bendier or stiffer than thin ply?). Would need to consider how to incorporate it without cold-bridging since the blanket part isn't strong. Spacetherm multi has a 6mm MgOx sheet and the "floor" version of Multi has the blanket encapsulated in metalised plastic, which should help keep the aerogel in the blanket.
PIR (e.g Celotex) Foil-faced foam sheets.
Conductivity: 0.022 W/(m.K) Almost as good as Aerogel, but 10 times cheaper.
The gas used to blow the foam can leach out over time, which is unpleasant and reduces its performance.
Can be cut using a craft knife, which is useful since it's too rigid to match the curve of some van ceilings.
Cut edges can crumble if they move relative to the adjacent structures. The dust is an irritant, so you want it outside your vapour barrier.
Performs better when warm, so you'd want it nearer the inner face of your insulated wall.
For awkward spaces, can use expanding foam to hold smaller pieces in place and fill gaps.
XPS:Extruded Polystyrene
(e.g Styrofoam)
Conductivity: 0.026-0.035 W/(m.K), less insulating than PIR but Better loadbearing and lower vapour permissivity
Traditionally blown using HFCs, but recent versions are air-blown (hence Styrofoam's blue-> grey transition) yet have almost identical performance.
Panel Systems in Sheffield do CNC hotwire-cut Styrofoam, which might be useful for corrugated panels.
EPS:Expanded polystyrene
Conductuctivity: 0.038 W/(m.K)
Same and bike helmets and packaging.
Cheap but lower strength than XPS.
Fibrous insulation:
Glass (0.034 W/m.K), mineral, sheep, recycled plastics...
Porous, so less insulating for each unit thickness
Porous, so humid air can pass through it and condensate on the first surface on the cold side, potentially trapping moisture against your sheetmetal.
Tends to sag over time, leaving poor insulation at the top where the hottest air is.