Hi Darrell, if I were to "solidify" the back wall I think I'd cut TechShield panels (foil side facing-in) to fit between the studs, and then secure them with horizontal braces that are bolted to the studs & corners--not sure maybe perforated steel or something. Then to dress-up the outside I might screw siding into the TechShield (I think it's thick enough such that it would clear or be-even-with the studs). I know you said "rigid foam" but thought I'd toss this out anyway... Here is a link that might be useful: Our 10x12 HFGH

Yes, It definetly helps. I am currently using this method, without it every thing inside froze with ice all over it. Since I put up the plastic sheeting nothing freezes anymore.Just make sure that the plastic doesn't lay against the windows or the corrugated glazing or it will suck the heat out of the greenhouse. The bigger the air cushion the better the insulation. Even the windows don't freeze over like they did before.The nighttime temps are about 15 degrees warmer than without plastic. That plus a small electric heater and you've got yourself a heated greenhouse. To go one step farther channel the heat to access two small tabletop greenhouses inside and the temps can stay around 78 to 80 degrees all night long even on bitter cold winter nights. I use two cornhole boards turned upside down and let the heat come up through the holes to heat the small greenhouses Perfect for propagating or seed starting, and it works great. My greenhouse is 10'x 16' with 12' height. Plastic drop clothes on walls and ceiling, wooden door insulated sheeting. Space between plastic and walls is about 3 1/2 inches. I have yet to insulate the floor but that is next springs project. I have many small veggies that I have started from seed and all are doing well. Good Luck.

To help answer some of your questions, the attached link gives a start. There are hundreds of web sites that have good info on greenhouse energy conservation ideas. Here is a link that might be useful: greenhouse energy conservation

the floor is a work in progress...I take it a little further day by day. I've got 3 rows (of the 19 it's going to take to fill er up) I need to get my laser level in there tomorrow, because as I look at what I've done from the doorway, it doesn't look level. as I'm on my hands and knees in the sand, it does seem to be level. It might be an optical illusion, but if I can run a piece of string over the top from one end to the other, at least then I'll have a better point of reference. I took pics today, but my camera battery is recharging :) so I'll get back with some pictures soon.