@ZeroKelvin, I think it depends upon where you live. The heater will be less efficient with (I assume) twin wall if you live in Alaska or Minnesota. I live in central California. My twin wall greenhouse stays pleasant enough for citrus and a few other not quite winter hardy plants with a milkhouse heater (5100BTU) but my low temps are in the mid to upper 20's.
If I expect a really cold night, I have a small propane Buddy-Burner.
