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I cannot find the answer to my question.  I just cannot believe it has not been asked before.  So can someone please direct me to the thread where it is answered?

My question is this.  What is a good average target R-value for a van in which the owner will be living in both extremes of hot and cold weather?  6, 10, 15, 20?  How about we consider each side of the van?  What is the target for the ceiling? walls? flooring? driver's cabin area?  

I am about to purchase all my insulation and I just want to make sure that I am targeting the right values.  I plan on living though out the Intermountain West primarily in Utah.  I would like to take more than a few trips back home to Alaska in the future and I don't want the winter cold to stop me from doing that.  

POLYISO board insulation has the very best R-value that I have found. At 2" thick, it would give you almost an R-12 value. If you want max protection... 3" thick should make anyone happy.

I installed two inches in a cargo trailer and was well pleased with it in every way.

Home Depot has it (in my store) in 1/2" sheets and 1" sheets. The 1/2" is more flexible but would require 4 layers to get to 2 inches.

It definately is not cheap to buy... it will take lots of 1/2" sheets to insulate a van well. It is slow to install... lots of cutting to get it in properly.

Note: You can buy cheaper, less quality insulation but you will probably regret it in the long run.
If my living space box could sacrifice the space I would go for average 4".

If using aircon a lot, thicker on the floor than the ceiling.

For boondocking in arctic cold, the opposite.

If both, then 4" all six sides.

Note theoretical R-value is just one factor.

A well-sealed envelope is critical, the only air infiltration allowed should be the controlled ventilation system. AKA vapour barriers.

Shading the outside against direct sun makes a huge difference.

Small windows, or none, or a system for well- insulating them when needed.

And a very strong ventilation system.
Assuming you are going to be in a hot climate in the sun, having most of the insulation in the roof is a great benefit because stopping the heat gain from the sun from going through the ceiling is of great benefit. Being in the cold, having more insulation in the ceiling is also good because that will keep the heat from inside from going through the ceiling since that is where most of the heat will want to go.

I can't remember the ratio thingy, but more insulation in the roof, then the walls, then the floor gets the least. I have 4 inches of polyiso in the roof, 3 inches of polyiso in the walls, and 2 inches of XPS in the floor. XPS starts to degrade and deform at 165F degrees if I remember right, so that was a factor for me. The floor has XPS because it doesn't get the sun exposure and has more moisture resistance. The 400 rated sheets also have a higher compressive strength.

Overlap the seams, reduced thermal breaks as much as possible, keep the windows small and to a minimum, and reduce drafts. You don't want it hermetically sealed, but not leaky either.

I can be any cold place in the U.S. and be fine with a small heater. It would cost me $60/month during a bad, cold month to heat my trailer In the summer just north of Meeker, CO, all I have is a fan and I'm comfortable while everyone else I've seen is hot or is running AC.
Reduce thermal bridging as much as possible.

Thermal breaks are the good kind.