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I don't have measurements, but if my memory is correct on the ceiling of my van I added (2) layers of 3/4" inch of foam between the ribs, (1) layer of 1/2" foam over the ribs and then a 1/2" layer of tongue and groove cedar (So the ribs were 1.5" deep). The walls varied greatly and it really depends on how you build tthem out. I basically just built them up with foam boards until they were level with the ribs and then added one 1/2" layer of foam board over that. In some places near the bottom it's close to 3" thick.
IMO it's important to have some type of insulation over the ribs as opposed to just furring strips. While some wood is a decent insulator, the humidity sends the R value of it crashing, and keeping the humidity levels low is a tough thing to do in a van. The conduction of heat (Lost or gained) through the metal parts of the van can be substantial and even if it's just a 1/4" layer of foam, it will make a difference.
2015 159" Gas Promaster (RV Build in Progress)