I wasn't planning on heating the above deck area under the shrink wrap with the furnace; only using that area as the "furnace room" with insulated flexible ducts running to the hatches. This way I wouldn't have to find somewhere to install a heater inside the boat.
Also, the insulation rolled out on the deck does a great job holding in the heat. Again, when spring hits, I can just roll it up and toss it away with the shrink wrap.
Catalina 38, Hull #112
Grosse Ile, Michigan