The floor will be a lot cooler, but your heat loss is through the roof. I have a heater in my greenhouse and keep it about 12ºC to 15ºC at night. During the day, if it is sunny, it will get above 30ºC, with an outside temperature of 5ºC. I have a couple of roof vents that I open to keep it cooler during the day. I don't run the heater during the day, unless it is cloudy and cold. I cover the greenhouse with blankets at night for temperatures colder that -10ºC. The blankets on the roof reduce the heatloss significantly.
I keep bigger plants on the floor and smaller seedlings higher. I also keep the temperature sensitive plants like coleus and such higher up too. It really helps if you have a fan mounted about 6 feet high. The house will stay much warmer at night with a high fan and a heater on the floor.
