Author Topic: Lowering the greenhouse floor below ground level?  (Read 139 times)

Vlk

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Lowering the greenhouse floor below ground level?
« on: June 10, 2021, 09:33:37 AM »
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. :D 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 :D). 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. :D

What do you think?








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Daintree

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Re: Lowering the greenhouse floor below ground level?
« Reply #1 on: June 10, 2021, 10:09:52 AM »
I would go with B. Your greenhouse is too small to use A or C effeciently.  You just wont have enough floor space with C (always having to step up or down or watch for the wall could be a tripping hazard), and with A, the dirt wall could eventually slough off and compromise the greenhouse wall.
Pile the dirt from the hole along the north wall for insulation.
Don't like to dig? I don't either, so I sympathize.  But the greenhouse will be there a long time, so go for the long-term, not the short-term.  Maybe hire a small piece of equipment, some neighbor boys, or a really energetic big dog.  My friend had a dog that would dig huge holes anywhere you pointed and said "get it!". Borrow a dog like that, and you would just have to lift the loose dirt out of the hole!

Carolyn

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Re: Lowering the greenhouse floor below ground level?
« Reply #2 on: June 10, 2021, 10:20:26 AM »
Hi, in Russia, this has long been used for year-round greenhouses. Definitely choose option B. It is important to choose the right place and do good waterproofing. I saw flooded greenhouses in the spring and autumn,it looked deplorable.


Vlk

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Re: Lowering the greenhouse floor below ground level?
« Reply #3 on: June 10, 2021, 10:23:19 AM »
I would go with B. Your greenhouse is too small to use A or C effeciently.  You just wont have enough floor space with C (always having to step up or down or watch for the wall could be a tripping hazard), and with A, the dirt wall could eventually slough off and compromise the greenhouse wall.
Pile the dirt from the hole along the north wall for insulation.
Don't like to dig? I don't either, so I sympathize.  But the greenhouse will be there a long time, so go for the long-term, not the short-term.  Maybe hire a small piece of equipment, some neighbor boys, or a really energetic big dog.  My friend had a dog that would dig huge holes anywhere you pointed and said "get it!". Borrow a dog like that, and you would just have to lift the loose dirt out of the hole!

Carolyn

Thank you for your input! Very good points regarding options A and C. So you think it would be worth doing (option B)? I could still just stay with the version of the greenhouse being on the ground level, which of course is the easiest option as the only digging involved is the one for the anchors of the construction. What would be the advantages of having the floor as in option B compared to having it on the ground level? And how deep should it be in order to make some difference? I think 100 cm is a bit too much, I was thinking more like 50-70 cm max.

A dog digging on command would be great. :D My dog is more of a sleuthhound, but my rabbits really like to dig. Though that would take a lot of time for them to dig something like this. :D
« Last Edit: June 10, 2021, 10:37:19 AM by Vlk »
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Vlk

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Re: Lowering the greenhouse floor below ground level?
« Reply #4 on: June 10, 2021, 10:28:57 AM »
Hi, in Russia, this has long been used for year-round greenhouses. Definitely choose option B. It is important to choose the right place and do good waterproofing. I saw flooded greenhouses in the spring and autumn,it looked deplorable.


Thank you! The setup on the photo is great! But that would require A LOT of digging. :D And also a custom-made greenhouse - I already have a pre-fabricated one that I will just put together. I was thinking about the waterproofing as well. But I don't think it would be an issue really, especially if I would make the floor only about 50-70 cm below ground level. And after all, the plants would be in pots, so they can always be transported elsewhere in case something like this would happen.
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brian

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Re: Lowering the greenhouse floor below ground level?
« Reply #5 on: June 10, 2021, 10:36:54 AM »
My greenhouse is partially below grade (ground) because it is built on a slope.  The dirt inside is level, but because the ground outside is sloped the upper portion of my greenhouse is below grade and the lower portion is above grade.  I simply built a wooden staircase inside so stepping up/down ~2ft is no big deal.  However, on very rainy days the soil on the upper portion of my greenhouse can become waterlogged around the edges.  If the entire thing was below grade it would be much worse.  If you don't have some kind of foundation or retaining wall a dirt perimeter wall will collapse when it rains heavily. 

As far as insulation, I don't think it makes much difference.  Soil doesn't really provide good insulation, and the "higher soil temps underground" effect often only applies many feet below ground (see - geothermal designs, frost line guidelines).  Insulating your greenhouse with foam panels or similar will be more helpful than relying on the soil effect.

If you build a deep trench greenhouse like Forester shows that would retain heat, but it requires a lot of digging, has flood concerns, and has rather limited light because the sun only reaches inside at a very narrow angle, and in winter when you would have plants in this greenhouse the sun is already at an extreme angle so the effect is even worse.  Also, you run the risk of burying yourself alive if the walls collapse!


If you have a lot of space, you can build your greenhouse larger and use the extra space to put 55gal drums of water, or a wall of water containers, to retain heat and release it through the night.

If you really want efficiency, devise some kind of insulated cover you can place over the whole thing at night.
« Last Edit: June 10, 2021, 10:41:55 AM by brian »

Vlk

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Re: Lowering the greenhouse floor below ground level?
« Reply #6 on: June 11, 2021, 07:56:57 AM »
My greenhouse is partially below grade (ground) because it is built on a slope.  The dirt inside is level, but because the ground outside is sloped the upper portion of my greenhouse is below grade and the lower portion is above grade.  I simply built a wooden staircase inside so stepping up/down ~2ft is no big deal.  However, on very rainy days the soil on the upper portion of my greenhouse can become waterlogged around the edges.  If the entire thing was below grade it would be much worse.  If you don't have some kind of foundation or retaining wall a dirt perimeter wall will collapse when it rains heavily. 

As far as insulation, I don't think it makes much difference.  Soil doesn't really provide good insulation, and the "higher soil temps underground" effect often only applies many feet below ground (see - geothermal designs, frost line guidelines).  Insulating your greenhouse with foam panels or similar will be more helpful than relying on the soil effect.

If you build a deep trench greenhouse like Forester shows that would retain heat, but it requires a lot of digging, has flood concerns, and has rather limited light because the sun only reaches inside at a very narrow angle, and in winter when you would have plants in this greenhouse the sun is already at an extreme angle so the effect is even worse.  Also, you run the risk of burying yourself alive if the walls collapse!


If you have a lot of space, you can build your greenhouse larger and use the extra space to put 55gal drums of water, or a wall of water containers, to retain heat and release it through the night.

If you really want efficiency, devise some kind of insulated cover you can place over the whole thing at night.

Thank you very much for your input and for sharing your experience, I really appreciate it!

That is exactly what I was thinking, if, given the circumstances, the size of the greenhouse and my plans to use the greenhouse during the spring/summer/early autumn and not all-year-round, if digging and lowering the floor to say 50-70 cm below ground level would be worth it and would make sense, all things considered. If there wouldn't be any significant advantage for instance in terms of insulation - and it makes an absolute sense of what you are writing - that it would really need to be deep in order for it to make a really significant difference - then I think leaving the greenhouse on the ground level as I initially planned will be the best in my case. And that's great news for me because that means no digging and no extra work. :D I am sure the construction of the greenhouse itself will take a lot of time and effort, so saving some extra strength and time is very welcomed. :D

The idea with the insulated cover is definitely worth considering! I was thinking some thick bubble wrap would do a really good job, having some multiple straps with stones from each end that would hold it in place on top of the greenhouse in case of a strong wind... Would be quite easy to put on and take off!

Thank you everyone for your thoughts! I will go with the original plan then. :)
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Mark in Texas

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Re: Lowering the greenhouse floor below ground level?
« Reply #7 on: June 11, 2021, 08:27:00 AM »

Thank you everyone for your thoughts! I will go with the original plan then. :)

Agree.  Everything above ground will save you a lot of headaches, especially considering the small size. 

Vlk

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Re: Lowering the greenhouse floor below ground level?
« Reply #8 on: June 11, 2021, 10:39:45 AM »

Thank you everyone for your thoughts! I will go with the original plan then. :)

Agree.  Everything above ground will save you a lot of headaches, especially considering the small size.

Indeed! And with the summer heat coming up, digging in the full sun would literally mean headaches. :D And sunburnt neck on top. :D
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Seanny

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Re: Lowering the greenhouse floor below ground level?
« Reply #9 on: June 11, 2021, 12:56:49 PM »
A trench inside your GH allows colder air to drop down below your pots.
In a minor flood the water drain down the trench, keeping pots dry.