How are you trying to retain heat?
I've incorporated a modifications in an attempt to retain heat. First off I'm using a 20k BTU Heatstar LPG heater as my primary. I have covered the northernmost side and roof with used 6 mil uv resistant heat retaining poly. I got the poly from a friend who's recovering his hoop house. I covered the interior of those walls with 3/4" foil backed foam panels, and insulated the base and lower wall with covered straw. The factory supplied polycarbonate is a double wall 6mm and rather inefficient. Any future replacement will be with heavier triple wall to defray the increased material cost. Ric
I forgot to mention I have old cast iron radiators painted black under the sunward bench to collect solar. I plan to fill these with antifreeze and possibly attach a collector.