How cold does it get during winter where you are?
I have been brainstorming how to have a lean-to green house on by lower patio next to the house. We have an all brick home, and the back faces southwest. The bricks absorb a lot of heat during the day.
However, I think I would still need a little heat during cloudy or extra cold winter days, and that has me a little stumped.
We can have unheated garages stay above freezing if opened/closed judiciously and a section of it protected with plastic tarp to keep cold drafts off the tender plants. Our dryer vents into the garage. Not a great idea in the summer but in the summer I usually hang the clothes out to dry. Heat in the deep south is FREE in the summer, danged if I’m gonna pay money to make my garage even HOTTER in August. In January if I run the dryer once a day it’s usually enough, combined with the deep freezers out there, to keep the garage fairly toasty (always above 50 even if the outside temps are in the 20’s at night).
Maybe put the lean to on the outside of your house near where you’ve got a fireplace or somewhere you can vent a dryer? Maybe pave it with concrete block tiles that are darker in color to absorb more heat?
You get a good deal colder than we do though. And stay colder for longer. We’ll have a few days with the highs in the upper 30’s to lower 50’s range and lows in the high teens to upper 30’s. Followed by 2 or 3 days with highs in the upper 60’s. I kept a pepper plant in a low tunnel in my garden till january one year. Let it freeze then because the rabbits had found it and didn’t want to feed the problem animals!
There are some good sites out there that talk about green houses both heated and unheated. I’m sure there are lots of really really good ideas out there. An unheated greenhouse is on our ‘to do list’. Just not this year!