Hi there,
I am about to build a small greenhouse about a week or so from now on and I've been playing with the idea of lowering the greenhouse base/floor below ground level.
Now I am no engineer and I am rather bad at math and physics, so please, bear with me.
I was wondering what would be your take/opinion on this.
So the greenhouse is really small, say 2x3 m, about 2,5 m in height, made of hollow polycarbonate sheets, thicker than they are usually made, which should have higher insulation properties (up to 30% they say). I have no plans of heating it throughout the winter, so it will be a seasonal greenhouse from early spring to late autumn before the temperatures will drop (we don't have a particularly harsh winter, but the temps can drop really low (-27,5 C was last winter's peak, which was unusually harsh, it's usually around -10 to -15), and heating for tropical species would require a lot of power and it wouldn't be sustainable.
With that in mind, my initial plan was to build the greenhouse on a level with the ground (on clear ground, no concrete). Later on, I've gone through an article discussing greenhouse heating and one of the tips was to lower the floor of the greenhouse below ground level - they recommended 70-100 cm, but as far as I remember, it was just saying something about a trench and not an entire area of the floor.
Now I was actually having thoughts about this before reading up on that article, but this made me think about it a little more. There is something about heat retention in the ground and the ground itself has a certain stable temperature at a particular depth. It could have several advantages from my view, such as heat retention in colder days/nights (ground absorbing the heat during the day and then keeping it for some time during the night perhaps?), protection from light frosts (though I doubt I would leave the plants out in that case since they would mostly be tropics), maybe making the greenhouse cooler in hot summer days and making the greenhouse more spacious (in one of the versions, more on that below).
But even though the greenhouse is small, this would mean a lot of additional physical work that I know can be quite demanding (I was digging a small shallow pond last year, so speaking from experience
). So the question is, would it be all worth it in my current plan and setup?
There are several options I could go about this:
A) Building the greenhouse on ground level and then lowering most of the floor inside below ground level, leaving a ground-level rim around the walls, so the construction would be firmly in the ground. This would create more space in the greenhouse, making it higher, as the pots would be below ground level. It would however take some inner area off (say 10 cm fro the walls would be enough), making it slightly smaller in terms of surface.
B) Digging an evenly leveled rectangle slightly larger than the greenhouse and then building the greenhouse inside of it. That means part of the walls would be below ground and I suppose covered by soil. This would mean that the space inside would have the same volume as if it would be build on ground level. It would maybe mean less light as part of the walls would be below ground, but maybe the temperature management would be much more stable? And since it is a greenhouse, there would still be plenty of light which would be coming up from above anyway.
C) Doing some variation of the versions above. Maybe just making a trench in the middle, leaving the sides on ground level - maybe that alone would have some effect?
I've made some simple drawings to illustrate the situation.
Now the question is if any of this would be worth the effort and if it would have any significant valuable positive effect. Or is it just not worth all the work for a small seasonal greenhouse? I might be overseeing some cons and maybe I am overthinking the whole thing, which is quite possible, as I have a tendency to do that.
What do you think?
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