We often see forums repeating bad advice, here are a few common mistakes you should watch out for....
​
Beware of any product that promises to cure all issues in one application. Structural noise, soundproofing and insulation require specific layers.
Adapted building materials are cheap but cheap for a reason. Their primary purpose is within a house not a steel vehicle with little to no natural ventilation
Flashing tape is not the same as sound deadening. While it adds some mass to the panel it is bitumen based and will perform very badly as a vibro-damping material and will degrade over time with vehicle temperature variations
Foil bubble wrap is not adequate insulation. While it is a very cheap solution it offers minimal thermal insulation and has very poor to zero soundproofing abilities. With basic thermal conduction principles it will very quickly become as cold as the outer skin causing condensation not only behind the foil but also on the inside of the van
Glass fibre and wool based "loft" insulation while thermally very good can absorb moisture. When used in a van prone to condensation, it is not the ideal environment for this type of product.
Vapour Barrier. A total big no no. Barriers have been adapted for use with also adapted domestic insulation materials such as PIR boards for use in a van which is not their primary purpose. If you line it properly using vehicle specific materials, you only need to run the thermo liner over the panels to stop any warm air from reaching the panel and that's it. Nothing more needed, it needs to vent and breathe with plenty of air flow around the linings. Celotex, foil bubble foil, you are asking for issues! You need your liner and insulation to breathe, not seal in any warm air. A vapour barrier will not stop heat reaching cold panels which will condensate when cool... its simple science of heat conduction. A thin foil liner will conduct heat through the material giving way to warm air in the sealed cavity due to poor insulation in air cavities.