I just didn't have room for 4" of insulation all round. I did have room for 6-10 inches on the bottom and back, so that got XPS foam, no panel. The other sides got vacuum panels. Even with the vacuum panels you need at least 1" of foam on the outside, to minimize edge effects. To keep everyone honest, I embedded thermistors in the center of each panel between the vacuum panel and the 1" of foam.