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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?

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  • FYI, foundations aren't usually built like you describe. You'd use a mini-excavator to dig trenches, not an entire basin. – isherwood Jun 23 '20 at 16:42
  • I trenching tool might work, but I don't think they will go down 5 feet. – SteveSh Jun 23 '20 at 16:43
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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”.

Here’s a quick science question. You want the largest thermal mass possible in a given space. Your choices are a) depleted uranium (freakishly dense) or b) plain old tap water. Which would perform better? Answer is b) tap water. Thermal “mass” has nothing to do with actual 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.

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  • Perfect answer and confirms my "alternative" solution for warming the greenhouse using water tanks. Was just looking at them yesterday, 45-gallon vertical black ones that will absorb, store and radiate heat. Good point on water being better for heat capacity. You're right, that's why the lakes take longer to change their temp compared to land. Yes good tip re: insulating the subfloor. I decided to do that when I was formulating my alternative-heating solution. – Jinren Jun 24 '20 at 21:24
  • The floor/ground will be too cold in winter and will quickly conduct heat away so it must be insulated. These water tanks I have in mind, will actually be holding rainwater from the rooftop, and will be used to irrigate the plants. They will all be connected to each other on the bottom so that they're always all at the same level. One of them will have an overflow output hose. I may have to fill them manually in the wintertime if they go low... but that might still be OK... – Jinren Jun 24 '20 at 21:25
  • 45 X 4 gallons should be quite a lot, and evaporation loss in wintertime should be minimal since the greenhouse will be closed (the actuating vent won't be open). Thanks again for your great answer! – Jinren Jun 24 '20 at 21:25
  • I forgot to mention, the thermal column wasn't intended only to serve as a thermal mass but also as a "connector" or conductor to the rest of the earth and to access its latent heat in wintertime. I read somewhere that this can be done if we go below the frost line, hence "geothermal". Also originally my plan was to dig up that space and fill it with rammed earth, with the thought that rammed earth would have good latent heat capacity (confirmed it's better than concrete). But given the challenges, I won't be doing any digging. – Jinren Jun 25 '20 at 13:50
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Unless you're in the perfect type of sand/clay soil, with no gravel or rock larger than say 1/4", this won't happen. You'll have to excavate to the width of a backhoe. In the case of a mini-excavator, this might be 12".

If you have ideal soil you might get a cable trencher to clean out a slot, but they're really designed to backfill as they go.

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  • I think they made a 12" bucket for my full-size hoe (narrowest thing the bucket pin would allow) but I don't own one, as they are a rare specialist tool so few were made and those would be hard to find. They also made one 3 or 3-1/2 feet wide, but most are 2 feet like mine. – Ecnerwal Jun 23 '20 at 16:52
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Nearly impossible to do it that way. As close as you might find would be to use a ditch-witch or similar trenching tool, and few of those will go 5 feet deep, and if they do, odds are excellent that the trench will collapse before you can insert foam.

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  • ...and as you get to wider (but not very wide) trenches the odds of killing youself in a ditch collapse get huge, should you not stay on the surface. There are massive steel plates with supprts to allow safely working in trenches (i.e. for doing work in roads and for pipelines), but in your backyard it's going to cost a whole lot less to dig a wide enough trench that it's not an entrapment hazard and then fill it back in. – Ecnerwal Jun 23 '20 at 16:49

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