These vans aren’t built of much and what they are built of vibrates a lot, with that vibration comes noise and a lot of it. The idea is to stick something on the side if these sprawling panels to stop them vibrating so much. You can buy special sound deadening stuff for cars but Bitumen is (apparently) exactly the same stuff and a damn sight cheaper.
A covering of around 20-30% should suffice, it isn’t going to get rid of the reverb completely but it does a lot and makes you van sound a lot more solid. Anything the Bitumen doesn’t get the insulation should get.
The Bitumen I bought is self-adhesive, however, as I’m doing this conversion in January/February and the glue on the back of this stuff is more of a summer fan sticking it was challenging. Having left it overnight I came back to the van to find a lot of the ceiling strips I had put on lying on the floor the van. I went straight out and bought a little fan heater which I stuck in the van for a couple of hours to warm everything up. This seemed to do the trick for the most part and I’m hoping once the insulation is stuck over the top I’ll have no further problems with it.