Greenhouse design being used in Colorado at 30 below 0 temps with no added heat other than solar and stored energy. it will have 21, 55 gallon barrels filled with water only. These will be stacked against the inside back wall 3 high and 7 wide. During the day, the water as well as plants get the sunshine heating then at night the water has been warmed and will maintain temp over night. How simple is that? Mine is 14' X 16' similar to one on the web site.