How far below grade should I insulate 12 inch poured concrete walls for a greenhouse?
My 48 x 24 greenhouse has a 9' tall and 12" thick poured concrete north wall, which will be back filled 8' in the ground, leaving 1' exposed. I'm using the mass of concrete as a heat sink to help regulate my greenhouse temperature. I'm using 2" foam insulation sheets on the outside of the wall anchored to the seal plate. Should I Insulating 3' in the ground or run them vertically insulating 7' in the ground? I figure I'm exchanging heat between concrete and ground contact in the summer to be used in the winter. If I create a heat bubble of earth underground from 50 deg to 60 deg, I can reclaim that in the winter time. Setting sheets horizontally I would be exchange heat with a larger earth mass although closer to the surface. Vertically I would have less concrete to earth contact but it will be concentrated deeper into the earth. Wall is on a 3' x 1' slab, w/ french drain. Opposite of a living basement, I believe the tall concrete
Insulate to the frost line.
For more information:
Read "How far below grade should I insulate my house's exterior? What material should I use?" a Q&A answered by Mike Binder.