We did R6/inch, but used 2" of that, getting a total of R/12, plus I built this inside an existing icebox
, giving us more than R/12. That was the fridge. I used 3x R6/inch in the freezer. We had an enormous icebox. It had room enough for two cold plates, all that insulation, and still holds all the food
we needed even to charter
for a week. Now that we're not longer chartering, we have a lot of empty space.
Hope that helps.
PS: Make sure you get that book. There are a lot of places to make errors, and if you do it's a PITA to fix them once everything is already built. It'll pay for itself many times over, and it supports a guy who will answer any questions you have while you are building your fridge. For instance, you will want to wrap your insuation in plastic to ensure that no moisture gets in. He goes over all of this in great detail.