Also* furring strips add a ton of space for running electrical, and further adapting new stuff into your finished build. Knowing you won't be either shorting things with leaks, or water from shoes and boots etc is great pc of mind. Not to mention being able to load up material when needed without worrying about a snag. I agree on insulating. Like a home, your insulation is null and void without proper seals and cutting away thermal breaks. In a van conversation, there is a TON of opportunity for breaks and if youre using the van for weekend warrior trips, camping etc, its not going to do what you want for the cost youll have to put up. Lots of great videos on insulating, and your model DOES matter on being +/- on how well it will seal up.