I did an extensive test of several methods of insulating
the shell. Prodex had captured many members' fancy a couple of years ago, so I decided to test fiberglass bats, Prodex, air gaps, and solid polyethelene foam. The thread is very long, but it has all the test methods, data, and conclusions.
Using Reflectex/Prodex provided only a small improvement over fiberglass bat. This was both disappointing (I wanted to find the magic bullet) and happy (glass bat is easy to install). The solid foam was excellent, but impossible to install in the curved shell.
Let me give you one caution--if you use Prodex and tape it perfectly, you run the risk of creating traps for condensed water. I think this risk is way worse than any benefit you get over fiberglass.