This link might be interesting Capt'n Pauley's Virtual Boat Yard -- Projects Galore!!!: Installing Insulation in Your Boat
I am insulating with both Reflectix and closed cell foam. In the V-berth my boat has vertical beams glassed in that are 1" thick. I am using a layer of Reflectix, a layer of 1/2" closed cell foam and another layer of Reflectix. The V-berth is probably the most important area as when sleeping you have a lot more body heat close to the hull than in other parts of the boat which often have lockers between where you sit and the hull.
I don't think Reflectix is that effective by itself but it would help on the inside of a cabin top that is balsa cored.
As described on the Frugal Mariner site there are 3 types of heat loss and one type of insulation can't eliminate all 3.
I have a neighbor with a Spencer 35 that is insulated with 1/2" closed cell foam - it was done over 20 years ago - and it is one of the warmest boats I have been on in cold weather. The boat was built in the 60's and other than the 1/2" foam the only insulation is a thick hull.