To build a geothermal greenhouse, I need to insulate a thermal mass — basically the ground directly the greenhouse. The insulation will be for the underground "walls" only, no floor, and 5 feet deep.
As an alternative to having to break up my 6-inch-concrete-paved backyard and then digging a hole, putting up the insulation walls, and refilling the hole, I thought of cutting out only the "slots" for the walls (basically just cutting out a 2-inch "moat" that goes 5 feet deep, around the greenhouse, and then filling it with insulating foam.
How would I achieve this, what tools/techniques would I have to use? What type of contractor would do this and how would I find them?
Best Answer
Boy, that sounds like the hard way to make a thermal mass, especially in light of the already present concrete. I’m also concerned with how you are not insulating the bottom of the “dirt thermal mass”.
Dirt is a weak thermal mass, and concrete isn’t much better - but more importantly, both are rather poor thermal conductors. Water, however, is an excellent thermal condutor and the best thermal mass known.
You’ll want these plants set up on shelves anyway for ease of gardening, so here’s what I propose.
Lay thick layers of insulation on top of the existing concrete; this will be the subfloor of your greenhouse. Come up the sides with also very good insulation. Everywhere except the aisles, put tanks of water. Say, 55-gallon drums stood vertical and packed dense. Set the planters on top of those.
Packed all in heavy insulation, that water will hold heat like crazy. In fact you’re betting on that because you don’t want the water to freeze (though you can certainly use non-toxic propylene glycol antifreeze if needed).
All this doesn’t require digging, hard construction problems, or modification of the concrete slab.