Chicken-heated Greenhouse Foundation

A year or so ago our friends Kelly and Keller in Minneapolis gave us a greenhouse from the backyard of a house they were renting out. It is Juliana brand greenhouse, about 12 x 12 with corrugated plastic glazing. In order to have a year-round greenhouse in Zone 4 winters you basically have two options: provide a source of heat (usually electric or gas), or design a solar greenhouse. We are creating a combination of the two options by attaching the greenhouse to a horse shelter built by the previous owners, insulating it well, and housing chickens in it to provide some extra heat during the winter. Here's a picture of the two structures independently situated:There are many design problems to solve with such a structure, but the first and most important in many ways is how to do the foundation. The frost line in this area is down to 4 feet, meaning that in order to have a structure that is insulated well the foundation needs to be insulated down to 4 feet. After considering several options, we decided to build the foundation walls out of below grade certified treated wood. 2" rigid foam insulation was used on the inside of the walls, and 1/4" treated plywood on the outside. We put the boys to work on this one so Jay and I could work on getting the hot tub in working order. Here are some pictures of the process:



First they dug the trench:They they checked to see if it was level:
They added gravel on the bottom where the frame would sit:Then they started building the walls (and apparently got a little sidetracked):
They added the rigid foam and the plywood and surveyed a job well done: