Get some flash band on the doors and footwells, best £10 you'll have spent. Once that's out of the way then get some thermal insulation. The are a few key issues (do some searches on the self build camper sites, some good info) :
- condensation - this will occure at the cold boundary which is the inside face of the metal. To stop that you have to glue the insualtion to the metal really well so it behaves like one thing. This is why some is sprayed on. But if its not perfect it will trap water and turn inot a soggy mess and corrode.
- you can have an airgap between the insulatition and the metal so it gets wet but dries out. This needs some ventilation - gaps, vents, grills.
- you can make it removeable - that's how I've done it. This lets me make mods and run wires behind it.
- if its all glued in its very hard if you need to do a repair.
- from expreince i would say none of the bits I've used glue on have stayed stuck (unless of course I wanted to get it off...), but with the removable panels I take them out and re-glue the layers.
- if you do for the airgap then make sure everything is waterproof. This may be as simple as a plastic sheet behind the insulation to keep it dry.
My panels for the roof and sides are made of 4mm corrugated plastic floor protector with a layer of foil/bubble insulation glued on then carpet. They are 100 waterproof, stiff enough to hold their shape and work well. The tub is the foil glued to the tub then carpet. The headlining is 3 panels - a centre flat one and two curved sides. They are held up by ally strips screwed through the roof under the cappings.
https://www.toolstation.com/ybs-general-purpose-thermawrap-insulation/p16779
B&Q do the 2mm, Selco the 4mm
https://www.wickes.co.uk/Proplex-Bl...VxbHtCh2KcwChEAQYASABEgLzF_D_BwE&gclsrc=aw.ds
For the sides I made up the panels then cut the windows out so I can push the window bit back in at night.
