I'm going through the same process. Actually thinking about buying a roll of Reflectix tomorrow.
Reflectix is best used along with something else. It doesn't have a good R value but reduces 93% of the radiative heat transfer. My plan is to do a 3 layer installation in my V berth. There is a fiberglass grid that stands out 1" so I am thinking of reflectix/closed cell foam/reflectix.
Here are 2 links. My plan is the method in the first link.
The Frugal Mariner: Insulating your boat
Capt'n Pauley's Virtual Boatyard -- Projects Galore!!!: Installing Insulation in Your Boat
Hope this helps.