I debated at length as to how much insulation I should use. My kitchen fridge/freezer is insulated with a little over an inch of injected polyisocyanurate which provides something around R8.
An inch of extruded polystyrene yields around R5. Add that to half inch plywood with both sides coated in epoxy paint and you get something in the neighborhood of R6.
I pretty much only brew ales so this cabinet will be around 65F. It gets up to 100F and down to 25F here so I'm looking at a max of 40F differential. So I made a hipshot decision that the 1" of blue foam would be fine.
If I ever feel like lagering, I'll probably just put another layer of insulation on the top half, separate the two halves, temp control with a fan yada yada...