Last night my greenhouse got down to 23.8F while the outdoor temperature was 10.1F, and I usually don't heat it at all. I do however grow strictly temperate plants, mostly bamboos in there so I don't turn on my paraffin lamps unless it can get below 14F in there. Last year, I never had to turn on any heat. I don't have that many bananas so I store them right in the attic which never gets below 40F.
One thing I like about my unheated greenhouse is the a majority of the time, the temperatures get stuck in the 32-55F range which is fairly idea for vernalization which usually gets them taking off much quicker in the spring.
Here are some pictures of it.
I solarize the floor too so it can get warmer during the winter.
Here's the exterior.
