Yeah, I've found that with a moderate amount of insulation done, thing's stay moderately comfortable down to about the freezing point. After that, things go down hill pretty quick. When it's ~40 degrees outside I can do everything in my van comfortably without worrying about running my heater. From there down to 15 degrees or so I run the heater periodically, and from there down, I just run the heater to keep warm when doing stuff and then curl up in the bed. At -20 (the coldest I've camped in it in) It only took about 15 minutes for the temp in the van to drop from ~75 degrees to ~40 degrees so at that point your insulation is basically worthless unless you're running a nice Espar or something like Bbasso has.