Yes, It definetly helps. I am currently using this method, without it every thing inside froze with ice all over it. Since I put up the plastic sheeting nothing freezes anymore.Just make sure that the plastic doesn't lay against the windows or the corrugated glazing or it will suck the heat out of the greenhouse. The bigger the air cushion the better the insulation. Even the windows don't freeze over like they did before.The nighttime temps are about 15 degrees warmer than without plastic. That plus a small electric heater and you've got yourself a heated greenhouse. To go one step farther channel the heat to access two small tabletop greenhouses inside and the temps can stay around 78 to 80 degrees all night long even on bitter cold winter nights. I use two cornhole boards turned upside down and let the heat come up through the holes to heat the small greenhouses Perfect for propagating or seed starting, and it works great. My greenhouse is 10'x 16' with 12' height. Plastic drop clothes on walls and ceiling, wooden door insulated sheeting. Space between plastic and walls is about 3 1/2 inches. I have yet to insulate the floor but that is next springs project. I have many small veggies that I have started from seed and all are doing well. Good Luck.

boomantoo-
Can you show me photos of what you are talking about? I recently erected my first HFGH and it got down to 38* last night here in NC. The inside temps were only 10* higher. I use 1 - 1500w milk house heater. I'm terrified for next winter! I know I'll have to add another heater, and maybe insulate. Any advice will be greatly appreciated!
kasha77

This is of benefit to me too.
Pictures maybe?
KennyP