Greenhouse subfloor (greenhouse on top of concrete) -- best material?
You will lose VERY little heat through the floor . Heat loss will be through glass/plastic and a little through the walls ; assuming you insulate the walls. I have some experience ; an 8 X 12 semi-greenhouse with tropical fish aquariums , about 70 F minimum water temperature. The only heat is a few hundred watts of aquarium heaters. And several tender plant pots sitting on the floor. My coldest weather is about 20F but many nights below freezing.  Concentrate all your heat loss plans on the window area.