Lowering the greenhouse floor below ground level?

[Vlk]

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?

[Daintree]

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

[Forester]

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]

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. 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.

[Vlk]

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. 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.

[brian]

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.

[Vlk]

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. 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.

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.

[Mark in Texas]

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]

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. And sunburnt neck on top.

[Seanny]

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.

[Izamarfeat]

Spammer

[fruttier]

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?

Sorry for the bump, but I am intersted in the result, how was (and is) your crop? Cannot imagine how it is possible to grow tropical fruit in the region with such a cold winter, but you are so enthusiastic and I am sure that you'd have a great crop! What have you done with the greenhouse? It's been a year so I'm sure you have something to say about your experience

[josianecanadian]

spammer

[brian]

here's some graphs showing experiments where soil temperates were measured at various depths:





So even at shallow depths, soil temperate is much more stable than the air, but it isn't until you get way down before it is actually warm enough to keep plants alive in winter in cold regions. 

But the thermal conductivity of damp soil is 50x that of air, so an uninsulated foundation is likely much worse than an above ground one. 

https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

Air, atmosphere (gas)    0.0262
Ground or soil, moist area    1.0


If you have an unheated greenhouse, below ground might make sense if it makes the difference between freezing and not in a marginal zone.  If you are heating it, I think it makes sense to be above ground as cold damp soil will pull heat out faster than much-colder air. 

[hammer524]

My loan was just approved to build a greenhouse. I am going the climate battery/ geothermal approach. My plan is to dig 8ft and install this. Not sure how well this will cool of my greenhouse in the summer but its worth a shot.

https://www.youtube.com/watch?v=hGxy-abbKWk

[brian]

My loan was just approved to build a greenhouse. I am going the climate battery/ geothermal approach. My plan is to dig 8ft and install this. Not sure how well this will cool of my greenhouse in the summer but its worth a shot.

https://www.youtube.com/watch?v=hGxy-abbKWk

Start a thread with pictures as you do it!  Greenhouse construction threads are great content

[hammer524]

My loan was just approved to build a greenhouse. I am going the climate battery/ geothermal approach. My plan is to dig 8ft and install this. Not sure how well this will cool of my greenhouse in the summer but its worth a shot.

https://www.youtube.com/watch?v=hGxy-abbKWk

Start a thread with pictures as you do it!  Greenhouse construction threads are great content

For sure planning on it!!!

[fruitnut1944]

My loan was just approved to build a greenhouse. I am going the climate battery/ geothermal approach. My plan is to dig 8ft and install this. Not sure how well this will cool of my greenhouse in the summer but its worth a shot.

https://www.youtube.com/watch?v=hGxy-abbKWk

That system will work well to heat a greenhouse in winter. But in Phoenix you don't need much heating in winter.

Your issue is cooling in summer. The geothermal will be totally inadequate in that regard in Phoenix. I won't waste money on that system. Especially not borrowed money.

You better put in an evaporative cooling system as well. That is a huge wet wall and large exhaust fans pulling about two air exchanges a minute. Even that might not be adequate.

You'll likely end up adding shade cloth and or a high pressure misting system.

My thought is you don't understand greenhouse heating and cooling well enough to borrow money for a system that you hope works.

Sorry if this seems harsh. Many many people waste money building greenhouses that are poorly designed and have no chance of working properly.

Find some greenhouses in Phoenix that work for what you want to do. See what they're doing. Or find a professional that has experience there.

I could help if you'd tell us what you want to grow, what temperatures you need in winter and summer, and light levels desired. A greenhouse is designed around those goals for your climate.

[hammer524]

No I appreciate it. I got the zero-percent interest loan through a charity. My plan was to add shade cloth and exhaust & intake fans.

There are some good deals on evaporative walls near me but I'm not sure how to set them up. Fruitnut could you help me out and point me towards a good reference in cooling a greenhouse? I care more about the cooling than the heating.

I still haven't spent any money.

My loan was just approved to build a greenhouse. I am going the climate battery/ geothermal approach. My plan is to dig 8ft and install this. Not sure how well this will cool of my greenhouse in the summer but its worth a shot.

https://www.youtube.com/watch?v=hGxy-abbKWk

That system will work well to heat a greenhouse in winter. But in Phoenix you don't need much heating in winter.

Your issue is cooling in summer. The geothermal will be totally inadequate in that regard in Phoenix. I won't waste money on that system. Especially not borrowed money.

You better put in an evaporative cooling system as well. That is a wet wall and large exhaust fans pulling about two air exchanges a minute. Even that might not be adequate.

You'll likely end up adding shade cloth and or a high pressure misting system.

My thought is you don't understand greenhouse heating and cooling well enough to borrow money for a system that you hope works.

Sorry if this seems harsh. Many many people waste money building greenhouses that are poorly designed and have no chance of working properly.

[W.]

My loan was just approved to build a greenhouse. I am going the climate battery/ geothermal approach. My plan is to dig 8ft and install this. Not sure how well this will cool of my greenhouse in the summer but its worth a shot.

https://www.youtube.com/watch?v=hGxy-abbKWk

That Atmos system looks like a variation on the "Greenhouse in the Snow" geoair method. Since Russ Finch's design was based more on keeping plants warm in the winter (though Nebraska gets plenty hot in the summer), I would be interested to know how such a system would function in the desert. I believe that it should, at least in principle. You may need more underground piping to further moderate the hot desert air you will be sucking in during the summer. I agree with fruitnut that you should do a good bit of research and probably crunch a lot of numbers about what your needs will be and whether this greenhouse system can economically meet those needs. I wish you luck on your project.

[hammer524]

Thanks. I've been in contact with the company and I sent them another email to ask if they have seen this sort of design in similar hot environments.

I've also reached out to various local growers asking around for advice. The most trustworthy one came from Arizona Worm Farm and he informed me about this method but he hasn't built one himself.

[fruitnut1944]

You start a GH design with the climate. Phoenix has occasional freezes and 4 months with lows below 50F, mid Nov to mid March. And ~5 months with highs of 100F+. Those are the primary areas that could use help.

If I moved there I'd think about two systems. One built around mango and similar fruits that like heat and don't need chilling. So mango, figs, citrus, grapes, etc. The second group would be crops that need chilling in winter: nectarines, peach, apricot, pluots, and sweet cherries. So a separate structure for each say 20ftx40ft with one 36 inch 1/2 hp exhaust fan.

Design for both would be similar. A greenhouse frame that's covered with poly from mid Nov until mid March or somewhat longer. A heater inside would maintain 50-60F for Mango and 40F for stonefruit. The difference would be a wet wall for the stone fruit and none needed for mango.

The wet wall for stone fruit would be used to hold temperature between 40 and 60F as much as possible day and night during December and January. During that time the structure would be covered with both the poly and the shade cloth. I'd double up the shade cloth. Put both over the stone fruit to moderate temperature for chilling purposes. No shade cloth on mango in winter just the poly for heating purposes.

In May when 100F sets in cover both with 40-50% aluminet shadecloth. Ends open just cover the top and sides. You could have bird netting over both all year long if critters are an issue.

These structures would be a lot cheaper than the 8ft deep geothermal design and could be heated in winter with minimal cost. Most of the year there's no cost to heat or cool. A Greenhouse that's poly covered all year in Phoenix is very difficult to keep cool enough.

[hammer524]

Fruitnut. My aim is to grow tropicals and for them to not need too much babying during the winter. I haven’t experienced a freeze in like two years (I’ve come close). Last summer I was using a 50% shade cloth held up by long T-Post. So I could probably get by fine with taking off the film during summer and putting up aluminet shade cloth. My budget is 8.5k so heeding your advice would allow me to get the proper accessories.

I should probably go the safe route since I am borrowing this money

[brian]

In a hot dry area like Phoenix you should absolutely be using some kind of evaporative cooling.  A misting pump and fittings is relatively cheap, few hundred bucks.  Even with a more humid climate, it works fantastically for me.  It is like magic.  All you need is a large source of very clean water, ideally collected rainwater. 

[hammer524]

I’m gonna go with a simple build and use aluminet shade cloth + fans for cooling. At a later time I can always add on if needed

Thank you everyone for the input