A lot depends on your build and the materials you use but again you are starting out with a metal box with glass windows and doors. If a high top most likely fiberglass also. You are trying to isolate the interior from the exterior while still having enough space to sleep, change clothes and maintain physical hygiene in comfortably. Any time you connect anything to the metal exterior it will conduct temperature so it becomes a concern, using aluminum strips with metal screws to hold panels for example. Opening a door, window or vent quickly equalizes the inside temperature to the outside temperature. One reason a curtain or wall is sometimes used between the driver’s front area and the back sleeping area is to reduce the effects of getting in an out of the vehicle and the large amount of glass in that area. Main reason we follow the seasons is we mainly live outside as there isn’t really enough space in a van to “live” inside for most of us and insulation or maintaining temperature in the van takes up a lot of space we need to do the other things well enough. Just my opinion stated here. I built a foam box with insulated air spaces with a entry chamber and just enough venting for two people to sleep in 7’ x 7’ x 6’ and it was still a pain to heat and cool. Main problem was keeping enough air flow to avoid condensation build up and stay alive! Lol!!! In a van insulation on the roof in the summer and on the floor in winter is important but head room for me was more important. 1/4” Luan or plywood panels with thin carpet that was easily removed and replaced worked best along with a very minimalist build worked best for me.
