I agree with the person who said not to go 3 feet down below the soil. Everything I've read about geothermal and passive solar greenhouses in zone 5b say not to dig your greenhouse into the ground. It's best to keep the whole greenhouse above the soil. That is because temps aren't warmer in the soil until you get 6 ft to 15 ft below the surface. I've also been researching if I can use passive solar and geothermal to keep the greenhouse warmer. So far that's gonna be my best bet here in Massachusetts. You also want to build the greenhouse somewhere where it's getting the most use of the sun obviously. Your gonna need to think about fans, humidity and air exchange. Most important is also insulation, I've read you need to use foam board to insulate the walls. Really do alot of investigation b4 you build. Someone else i was reading about said he built his greenhouse off his main home and wished he hadn't done that. Tropical plants need alot of humidity and he found that the inside of his home , even with a door between the main living area and greenhouse was being effected by the humidity . Good luck and let us know what you decide to do.
