I also live aboard in Boston Harbor (at Constitution Marina). Are we neighbors? Anyways, I have used the Reflectix on my boat and it seems to make a difference. My understanding of how those reflective type insulations work is that their R-value is dependent on the temperature difference across them. When there is a high temperature difference, more heat is being lost due to radiation, and it's the radiation they block, not conduction like other insulating materials. I'm not 100% sure that explanation is correct, so take with a grain of salt. I used my reflectix to insulate the lockers that the ducting for my Espar runs through. It has made the air coming out of the V-berth vent noticeably warmer. That's the only insulation I've added to my boat. My boat seems to have a lot of air spaces near the hull that I guess act as pretty good insulation on their own, since I'm plenty warm and only burn about 1 gallon of diesel a day.
1979 Gulfstar 37 Laissez Faire