This week has been 20° and lower at night (supposed to get down to 0° this weekend).
During the day, it can be 20° outside and 80° plus inside. Being sunken helps retain the heat we add via a propane heater at night.
We also use black barrels filled with water to absorp heath throughout the day and bleed it off at night.
Thanks, that's helpful. Our temps are similar. I've been looking into greenhouses and high tunnels and I'm trying to get info from people who have experience with them in climates, especially winter temps, similar to ours before I dive in!
Earth sheltered is the way to go. It made a huge difference in the amount of propane we used last year vs. the year before. So much that we built a 2nd bigger one this year (30 ft x 48 ft).