Yeah, I don't heat it in the winter, too expensive. I live in the Upper Peninsula of Michigan, stuck between Wisconsin and Lake Superior. It's too cold at night here to start using the greenhouse until about the end of April. I have some passive heat storage in the greenhouse that helps keep the greenhouse a bit warmer at night, but if the outside temps go much below freezing, it loses heat pretty quickly.
I'd like to have a sunroom attached to the south side of the house, it would be easier to keep that usable in the winter.