Opinions vary wildly, the two main ones that come up are Celotex (https://ukvanlife.co/celotex/) which is sheet foam that is used in houses as well and then Insulation Roll (https://ukvanlife.co/insulation-roll/). I’ve used both and they both work great. It really comes down to your budget, your environmental ethos and how comfortable you are using the products. You can't really go wrong with either of those two options and no matter what you read, all insulation ends up making the van feel pretty much the same, insulated.