I am designing a greenhouse to be built next spring in Oregon. The freeze depth is 24 inches and winter lows are in the 5 to 15F degree range (Zone 5), although it occasions gets colder. Our soil is very bad so I am planning to fill the foundation with compost to the native soil height. I have already hired a contractor to excavate the foundation and we ran into too many rocks to go any further down (not even close to freeze depth). So, I will need to back fill and insulate to protect the foundation from freeze. I have hired an architect to design the foundation and he recommends a 12 inch thick slab to support the back fill and internal water heat sump. OK, perhaps, but that puts a slab foundation under 18 - 24 inches of compost.

I want to ask what some of you think about this plan. Install a slab and bury it in compost or build a perimeter foundation. I plan on installing hydronic heating with the intent of heating the soil - where should the insulation and hydronic tubing be installed? Most of what I have read suggests installing hydronic tubing in the slab but most slabs are at floor height not buried. So if I install the slab as the architect suggests and install insulation on top of that. How should I install the tubing - in a layer of sand, or in the soil?