After spending a couple of hours reading this thread yesterday, here comes my 2 cents worth straight from Richard Cranium.........................as my little woman calls me............
While in the process of rebuilding my house last year, I was checking out several different methods of insulation and here is what I found out......
Full foam closed cell insulation in the walls = R value 30+
Full foam closed cell insulation in the ceiling = R value 40+
1" thick foam sprayed as a sealer + fiberglass bats over the top of it in the walls + Celatex on the outside = R value 26
Blown insulation in the ceiling 14" thick = R value around 40
I am not a rocket scientist but the numbers speak volumes as far as I am concerned and would think the same would hold true for a hot tub