pmurphy

I find it all depends on the type of material the greenhouse if made of.
My first greenhouse if a single-walled poly and keeps the inside only a couple of degrees warmer than the outside temperature.
My newest greenhouse is a double-walled poly and without a heater keeps the inside temperature about 6C warmer than the outside temperature....at 0C (32F) outside it is 6C (44F) inside. With a heater inside I can maintain the inside temperature at a balmy 15C (62F) even when the outside is below well zero.