The first thing I did when building my VanLife van was rip out all of the factory-installed cargo gear to make room for insulation. Insulating a van requires understanding how (and where) you want to live the VanLife.
I wanted to be able to park my van to the base of a ski mountain, like Tahoe, so that I could get first runs in the morning. In order to not freeze my butt off, I knew I’d need some good insulation.
Heat retention in insulation is measured by its R-value; the higher the R-value, the more heat is retained.
The U.S. Department of Energy provides R-value recommendations for new and existing homes. The maximum recommended value (for walls) in my home state of California is 15. It’s not until you get into the depths of Alaska that up to 25 is recommended.
Of course, this is just a rough estimate, giving me something to shoot for when insulating a van.