For years, I've been thinking of ways of insulating the green house but was worried about lowering it into the ground because of settling cold air so I love that he has incorporated a cold sink. Also, it can act as an animal shelter.
I doubt I have 8-9 deep soil anywhere near my house but I was thinking in regards to the water table in the spring, it might be a benefit to build up rather than down anyway though I'm not sure how that would affect the heat transfer. I have a big pile of dirt laying around waiting for a use.
On a tiny scale, yes. I have lots of things to overwinter that only need to be kept frost-free, not outright warm. Some day I might scale it up.
One reason for my own interest is that energy costs are already high enough that I don't see heating a greenhouse as being cost-effective. Two, dangerous things can go wrong with a heating system, resulting in fires or other hazards. If the system simply fails, it could result in expensive losses of rare propagation material. Three, a partially-sunken greenhouse presents a lower profile to the wind, thereby making possible a much more wind-resistant structure. It gets expensive to repair or replace greenhouses that have been ripped apart by windstorms, which we seem to get more often these days, not that it takes all that much if the wind hits it just right.
ottawagardener, I thought you had hillside land. Can't you dig into the slope, rather than down, putting the excavated soil back on three sides for earth-insulation, or even four sides if you incorporate a drainpipe?
"Yesterday is history; tomorrow is mystery; today is a gift, that's why it's called the present." E. Roosevelt "If the world is to end tomorrow, I would plant an apple tree today" Martin Luther
Our small greenhouse has a 2' earth berm on the north side made of concrete block with foam insulation on the outside and painted dark brown on the sunny side. Inside the greenhouse there is an twinwall insulated lean-to cover over this area which we through some blankets on when it's really cold. We can winter over biennials for seed production (celery, celeriac, parsley, leeks, chard, kohlrabi, winter radish, etc.). The rest of the greenhouse gets much colder but with blankets spinach does quite nicely.
For my own backyard, I was just thinking of shallow excavation, nothing that would kill me if it collapsed, and I was going to use something like rebar-reinforced cinder-blocks, which should be more than strong enough. Belly-button high would be about right for getting it sunk deep enough to create a well for cold air to flow down into, and to have the benches up high enough to make it reasonably comfortable to work. I'm just thinking of a sunken cold greenhouse, not a full-blown earthsheltered contraption with a reflective wall. Back yard not big enough for that and probably overkill for my purposes and climate.
10 foot PVC (what I use to make low tunnels) isn't long enough or strong enough to span a width wide enough to be useful. I think I'll need a pipe bender. It'll be a little rinky-dink but I like the idea of keeping it simple enough that I can easily disassemble it and move it, or, for that matter, swap out components to repair it.
Not sure what to use for a cover. One of my row covers uses woven PVC, which lasts a lot longer than non-woven films. Doesn't come in wide enough rolls, but maybe I could use two, and overlap.
I should get cracking so that we could grow some tomatoes in it.