Synopsis: I'm toying with building a highly unconventional "insulated cold-frame" with off-cuts I've collected. I'm soliciting feedback on whether this is a good or terrible idea, and problems I might not have considered.
I live in northern New England and have long thought about making a few 'cold-frame' boxes to extend the growing season by ~6 weeks on each side. Recently my workplace has undergone a very large renovation project, and I've been collecting the 'leftovers', including some very large insulated panels and a bunch of polycarbonate greenhouse sheets Now I *think* I have the materials to make a truly epic cold-frame, but since no one builds cold-frames this way, I can't find much info.
To be clear one would never build a cold-frame this way if they had to pay for these materials - the insulated panels are prohibitively expensive (each full-sized panel costs >$1,000... even the offcuts represent a few hundred each in material)- but these are just off-cuts destined for the landfill. It sickens me how wasteful the construction industry is.
Materials:
Walls - (sunk at least 24" into the soil... maybe deeper): 6" thick insulated panel off-cuts from our walk-in freezer. Both sides are stainless and food-grade. I've got 4' x 4' offcuts, as well as some 4' x 6' offcuts. Should give the walls an R-23 with no thermal bridging (which I know is insane for an insulated cold frame. Most seem to use an inch or two of XPS foam at most).
Roof: Triple-wall polycarbonate greenhouse panels (10mm), sloped ~15 degrees and hinged on one side. Again, these are offcuts and way, way overkill, but it's way I've got.
Design:
Largely the classic cold-frame shape with a sloped roof and southern facing, but with insulated walls. The footprint would be either ~4' x 6' or (if I decide to do more cutting) 3' x 4'). I like the 3' wide for accessibility (with the 'roof' it's very hard to the back wall of a 4' deep bed) but it involves more work (cutting) and I might benefit from the volume-to-perimeter of the larger frame. Dunno. Cutting these panels is a bear, as they have metal on both sides). I have a spot in my yard that should get 6+ hrs of direct sunlight on March 1st and October 1st. My hope is I can keep the soil temp warm enough to have frost-tolerant greens in March-May and again in Sept - mid November. The personal-value of having fresh-picked greens in the extreme shoulder season is the entire reason for this project. FWIW our "frost-free date is May 16th and we typically get our first snows in early October. Also, I've accepted that this frame will likely not be used during June-Aug as it'll probably get too hot (even with the roof off). That's ok as we'll have lots of other beds.
With such large and well insulated panels I could bury the panels 24" (or should they be 36"?) into the soil to form the box, and then have a foot (or more) of good soil above ground. Having the panels so deeply buried and so well insulated should (i think) allow the soil to not freeze during even hard frost events and night-time temperatures into the upper teens (~-10ºC).
Questions:
I'm worried about frost depth and whether this will "upheave" the cold-frame each year. On one hand our frost depth is ~52" here (can't go that deep!) but OTOH with the cold frame in place I think the frost won't ever reach more than 12" inside the frame (probably less) in the coldest months. I'm not sure how it works around the exterior. I guess I can just try and see. I might be able to mitigate this by mulching heavily around the edges...
drainage - I know I want ground-contact. Right now I think I might add a layer of coarse sand directly on the earth, and then fill with my soil to a depth of 8-12". I guess drainage isn't a big problem since I will completely control the watering schedule.
venting - simplist solution is just to lift the lid on hot days and prop it open with a stick. But I know me, and know I'll either forget or be out of town. So I might try one of the auto-vents common on greenhouses that rely on the expansion of wax to open/close. Looks like I could get one for about $35, so worth a try?
air-exchange. With these panels and a too-tight fitting roof I run the risk of having too little air exchange. Not sure if this will be important on a practical level or not.