Earth berm greenhouse ideas

I was thinking of doing an earth berm green house this summer. I live in Alaska and was thinking a long narrow structure covered in a very thick layer of dirt with only the south side open. Basically the south side would be all windows. I can't decide if I'd want to isulate the east west and north sides or not. The reason being that I believe such a stucture would increase the soil temps on the dirt I use to cover the stucture, and that could be useful for planting things on or near the structure. What do you guys think?

Hi Dylan,

I think that so long as it is built properly, the very thick layer of dirt you mention should act as sufficient insulation and thermal mass on its own and should not require other insulation. Keep in mind that you will want to isolate your structure from water that falls on that soil. If you haven't yet, maybe just read this and then I won't regurgitate all of its points: https://richsoil.com/wofati.jsp

The biggest concern with a full south side of glazing and the other three sides totally closed off is that in the summer time everything is going to get very toasty, even roasty. Mind you, that's also a big part of the idea with all the thermal mass is to temper all of that heat. I recommend at the least putting in some opportunity for cross-ventilation through the greenhouse and if you don't need it then you don't need it.

Shawn

Howdy Dylan, can you do one that is covered with dirt on three sides? In THIS THREAD, I posted a picture of one that I did in Wyoming. It got up to 120 F in the summer so I had to open up the door and roof vents. It cooled down in the winter but I didn't heat it or insulate it with anything. It added about a month on either side of my growing season.

Yes a vent low on the East wall and high on the west wall is what I was thinking. Appreciate the input.

Neat ideas.
The berm could be covered in black plastic,to keep the soil dry and therefore insulating.
Or would clear plastic be better?
Both could add solar gain.
Either way you could sacrifice some dryness and plant into the berm through slits in the plastic.
Maybe a layer of carpet and/or flat stones to protect the plastic and hold it in place.
Maybe pile soil on one side and both ends of a hoop house.
Drive fenceposts,slide pallets over them,form a three side box of walls.
Pile soil and/or compost against the outside of the walks,plastic sheet over everything, including the mounds of soil.

Hi

You've likely already sourced this, but in case you haven't, you might want to take a look at Mike Oehler's Earth Sheltered Greenhouse Book (Mole Publishing, 2007).

Cheers

This link might help - Earth Sheltered Solar Greenhouse Book

My idea which I have not implemented yet is to place drainage pipe at regular intervals in the ground under the floor, up the back wall and across the top. These would be open above and below the windows so that during the day heat would enter the top and be cooled by the ground [warming the ground] and then fall coming out cool under the windows. At night the cold air  from the glass would fall into the pipes and be warmed by the ground and rise to the top of the greenhouse. Most ground heat storage are active system but I wold like to see if a passive system would work.
Even at 47 degrees north I have found that sloping the south windows is of no benefit because the sun is so low in the sky. With the glass vertical there is less overheating in the summer. I only keep tomatoes peppers and basil in there in the summer close the front then move the planters back as the sun gets lower and put my greens that can take the cold closer to the windows for the winter. My tomatoes are still alive but I have to pick the fruit and bring it in the house to ripen.

I don't think it will be worth the expense in time and materials.

For one thing, you simply won't get much air flow from this.   Gravity flow of air caused by temperature is pretty small to begin with and long tubes with multiple bends will slow it down even further.

The efficiency of heat transfer through earth tubes is related to the air speed, the slower the air, the lower the efficiency.

Finally, to effectively heat with air you need multiple air exchanges per hour.  I.e. if your greenhouse is 500 cubic feet, then you need to move at least that much air every 15-20 minutes.

For example, your typical furnace will do a complete air exchange in your house every 2-3 minutes, however typical furnaces are designed for worse case heating requirements (and then some).  If they were designed for continuous operation, you would only need 3-4 air exchanges per hour.

I would suggest you try to come up with some sort of experiment to measure how much heat transfer you can get from each tube before you put in the time and expense of building the system.  Ideally this would be something that could log airflow as well as input and output temperature on the tubes, but a simpler experiment might be to build two identical scale models from scrap, one with and one without the earth tubes and then see how the internal temperature changes over night.

My guess is that it won't make any measure difference in the interior temperature of the greenhouse.

William Bronson wrote: Neat ideas.
The berm could be covered in black plastic,to keep the soil dry and therefore insulating.
Or would clear plastic be better?
Both could add solar gain.

If the plastic is exposed to the sun then you will end up having to replace it frequently.  Perhaps every 6 months if it isn't UV stabilized, maybe every 6-8 years if it is stabilized.  If you bury it then it should last for decades.  If you bury it deep enough you could plant grass/flowers/etc. on top without having to put holes in the plastic.

I don't like the idea of using plastic for anything personaly. Even uv stabilized plastic is a disposable item. I know that it has it's benefits and Can be very useful, but i think there definitely are different ways to get similar results without leaching pthalates and plactisizers into the soil or having to replace the material.

Hans, I wonder I you could design something that uses water to store and transfer heat into the ground. You would have to have a design that has the water thermosyphon where it needs to go.

Yes using water would be much more efficient transferring heat and thermosiphoning.  My plan using ribbed polycarbonate roofing horizontally is to fasten it to scraps of gas pipeline that I have as vertical supports. The sun heating the pipes would drive air through the overhead down the back and under the floor while drawing warm air down the back and under the floor. This would overcome some of the resistance Peter VanDerWal mentioned.
I recently brought home some 4 inch sewage pipe. I could set that up in a closed loop in my existing greenhouse and see how the heat circulates and if it store any heat in the floor. [This is is an above ground with 8 aluminum framed sliding door units on the south and 6 on the west; the north and east which does not get sun are insulated with used carpet and padding.] The radiation from the pipe should reverse the flow at night and in my case may not store enough heat to last very long but if all but the pipe exposed to the sun are buried and you added reflective insulation to the south windows it may be effective.
A lot depends on your material plans. Are you going to build it like the shed berm at Wheaton Labs? You could add a loop inside with elbows at 3 corners ahd a T at the highest corner to add water cover the open end with an unglued cap with a small hole to prevent pressure build up. If the soil above the bottom pipe is remarkably warmer and the back and ceiling pipes radiate heat at night you have a winner so add more loops.

I was looking at earth berm structures a few years ago and designing a greenhouse attached home for high elevations in Colorado.  Nothing near the colds that AK presents, but here's something I learned.  Soil is a terrible insulator, it takes a considerably thick amount of soil to create insulation.  I would insulate the structure, protect the insulation, and then berm the earth up onto it.  I would also look at incorporating an umbrella insulated structure in some way...Umbrell insulate house  ...and from the search results... earth sheltered umbrella basics

BUT...these are only things I researched while learning of the WOFATI and Mike Oehler.  

I would also put an earth climate battery in the greenhouse.  And insulate around the perimeter of the structure as deep as possible into the soil.  I was going to say go down to at least the frost depth....but..... LOL  INCREDIBLY deep frost depths...so maybe instead of going down with a skirt of insulation around the perimeter of the structure, go out and away from the structure.

Another thing worth considering, is a double layer south window....where you have the two layers of windows separated by a gap of air of say a foot or possibly more?  I built an attached PVC greenhouse this fall using plastic greenhouse film.  It is double layered with only a 6" gap between the layers and it is making quite a difference. I insulated below the frost depth around the perimeter as well.  It has been 14-18 degrees outside at nights, and it hasn't dipped below 40 degrees inside the greenhouse, even with some air leaks that I need to finish covering up.  You could also pump warm air from the greenhouse into the air gap.

An insulating blanket that rolls down at night to cover the south windows would also be worth looking into in your climate.  It will prevent such huge heat losses through the windows at night.  You can make these automated so they roll up with the sun, and roll back down with the dark.

That's about all I know.

I am also skeptical about the viability of a passive system as described. What about adding a solar-powered expansion chamber to the loop?

If a 55 gallon drum or two were set up on the back wall of the green house, attached in-line with the upward internal chimney pipe of the system and situated so that the chimney would be hit by direct sunlight and therefore warmed, it would add momentum to the mass of air in the system. Using two joined drums, painting the exteriors with black barbeque paint or some other black, surface-area-heightening coating would increase heat gain, and shaping the cut pieces of the lids as fins to funnel the air mass and act as heat exhangers would all serve to accelerate the velocity of the air mass.

I think it important to link the heightened functioning of this passive system with solar gain on the structure, as when you're getting the sun is exactly when you will need the hot air from the top of the greenhouse sucked down underground. At night, the air in the system will continue to be warmed, but by the soil, and will rise, as warm air does, to keep the greenhouse warm.

In areas of extreme seasonal shift, you might still need supplementary heat, although you are probably aware of the Geoff Lawton video featuring an active version of such a system operating well in Alberta. I don't recall what their supplementary heat system looked like, if any.

Also, if using supplementary heat in the winter, and if the expansion chamber is heated by some of the exhaust, the system will continue to operate to even out spikes in space heating.

As a caveat, this is just an idea I have some preliminary notes on. I haven't yet been able to implement it.

Another one such that might be of interest came up just now, as I was reading through this thread. The crossbreeze and cooling discussion made me think about a bit of tudor-era technology that I find rather amazing. They cooled some of their work rooms that required it by using unglazed clay flooring. When they needed the room cooled, they would wet the floor and open doors to encourage a crossbreeze. The breeze would cause the water to evaporate off the porous clay, and the phase change of evaporation would lower the temperature of the floor, and the room. I was thinking that perhaps it would be possible to use this idea for largely passive cooling.

In any case, great thread. Keep us updated.

-CK

Dylan Kirsch wrote:Hans, I wonder I you could design something that uses water to store and transfer heat into the ground. You would have to have a design that has the water thermosyphon where it needs to go.

I had some thoughts on this...  We know in natural convection of air and water "heat rises".   This makes it easy to move heat upwards but more of of a challenge to move heat downwards to store heat below ground.  (Of course you can always use pumps or fans and I figure the timing works well to make them solar powered.  Water pumps will be particularly efficient at moving heat.  And there may be some savings if the pump is also used for irrigation/hydroponics/aquaponics.)  To passively put heat into a thermal-mass of earth or water, one way would be to dig a hole and have the sun shine in directly, or even use reflectors to concentrate the sunlight slightly by say a factor of 2-4 , but this might not be ideal.

A relatively simple way to do this would be to essentially make a "Trombe wall", wikipedia diagram (regardless of whether the wall is heated directly or indirectly), where the thermal-mass is located so that it is on the same horizontal level with the greenhouse/plants, and also spans the same height vertically so that there is a thermosyphon effect.   During the day the heat flows up through the plants as the air is heated by the sun, and flows down through the mass as the air is cooled by the mass.  At night the flow reverses and heat flows up through the mass as the air is heated by the mass, and flows down through the plants&greenhouse as the air is cooled by the greenhouse.   Since earth can be used both for thermal-mass and insulation, this design is synergistic with using earthen berms for insulating the colder walls to the north, northeast and northwest, (or even the eastern and western walls as well, at least in the dead of winter in the higher latitudes.)   In colder climates, I suppose the ideal shape wouldn't be a long greenhouse, but rather a series of smaller greenhouses, each having earthen berms to the northwest, north, and northeast.  This would maximize both sun exposure and insulation and ensure there was enough thermal-mass.

A totally different way that is more complicated but would allow the ground-below to be used without needing Trombe wall/berms, nor active pumps/fans, would be to use a device known as a "heat pipe". wikipedia.  Here, a small amount of water is put in an evacuated metal tube, typically copper.  The tube has a liner to wick water upward (to the hottest part of the pipe).  Because the water is under a partial vacuum it will evaporate at a low temperature and absorb a lot of heat in the process through evaporative-cooling.  The water vapor will then flow to the coldest spot in the pipe, (even if this is down in the ground), and condense, releasing heat.  These "heat pipes" can be purchased or they can be made by filling a copper tube with something like 0.5% of water by volume, heating the water until the steam purges the air, and then capping and sealing it with solder.  The hardest part might be making a good wick.  Anyway, the heat pipe is one of the few ways to make heat flow downwards passively.  It will also work in reverse and move heat up when it's colder on top.  If you want to exchange heat with the air directly, then you need a lot of surface area around the pipe, for example with closely spaced aluminum fins, exactly like the radiator tubing used in forced-hot-water systems, cars and air conditioners, and I figure such a product could be purchased or up-cycled for this purpose and plumbed in with the heat pipe.  The reason someone might go to all this trouble is that it might allow more insulation and/or light concentration to be used in a greenhouse without having to vent hot air on a sunny afternoon.  In theory, the more solar heat that can be held in on a sunny day without overheating the plants and without needing to vent air or remove insulation, then the longer the growing season can be extended.  

How about steam?
A trough style solar collector could boil off  water .
Open ended pipe could direct the steam to a buried chamber where it would condense.
The chamber could be cement block, aluminum concrete, stone or brick. It neednt be air tight, in fact it should probably be open on the bottom.

In a way its like the heat pipe, but the heat and the gas make a one way trip.

 

William Bronson wrote:  > How about steam?  A solar collector...

William, yeah that could work.  I think if folks make or buy a higher-concentrating collector they may want to generate electricity too.  I was thinking that if there was a way to use a solar collector with (higher-temperature) thermal storage  ( say 300-500 deg.F), and if this (when backed up by fuel-based heat sources if necessary) was used to power a heat engine, then the electricity could provide artificial lighting!  This seems like it could be a significant benefit on cloudy days, summer, spring, autumn and winter nights, and winter days.  

First I'll try to examine some of the fundamentals.   Plants need a surprisingly narrow temperature range for good growth. example.  Also some plants (broccoli, potatoes, arugula and rosemary) do better in cool temperatures than others, (corn, tomatoes).  So a greenhouse design needs to ensure there is enough heat and insulation to keep it warm (at least during the lighted growth times), and also ensure the temperature is correct for the plants and very even and constant.  Fortunately this seems possible, and in theory might be affordable too, with sufficient thermal mass, and a sufficient rate of heat exchange between the thermal mass and the air around the plant (assuming it's not practical to heat the plant directly).  

In the off-season, when the sun is low in the sky, it hits the ground more obliquely, so the light intensity is less.  The plant's optimal light intensity requirements are less than in summer but more than in winter.  light requirements.  This suggests that ideally, summer sun should be diffused, while spring and autumn light is near optimal, and ideally winter light ought to be concentrated (if used at all).  

So the main question seems to be: What's the most optimal, (and economical) way to maximize both the plant area and length of time in a year where the plants are at room temperature and well-lit?  Assuming that economic resources are more of a design constraint than land area, this suggests that the place to start is with "season extension", since it could be easier to build and operate and get more growth out of two 3-season greenhouses than one 4-season greenhouse.  (And easier to store food in the off-season and/or import food from a warmer climate).  

I think maybe the biggest challenge is what to do about window glazing.  It seems like building materials are either: translucent, insulating or affordable.  Pick two (as they say).  One approach is to have a movable insulation that is installed each evening and removed each morning.  This can be anything from a curtain to blown-in insulation like foam beads, or even soap bubbles!  Another approach is to use reflectors to make solar concentrators.  This way, less window area is needed so more money can be spent per unit area to make it more insulating.  Also the window can be put on the bottom (of all things!) so that there is much less thermal-convection and much lower heat losses.  Another approach is to use 100% artificial lighting so no translucent materials are needed.

I like the idea of using a foamed semi-translucent material, whether plastic (like "bubble wrap") or soap bubbles or a combination.  This technique is the closest I've seen to achieving all 3 goals of being: translucent, insulating and (potentially) affordable.  Due to reflection losses on so many surfaces, the light transmittance will be reduced somewhat, which could be good in summer, but not good in winter.  Perhaps in winter this could be made up for with solar concentrators.  It's fairly easy to concentrate sunlight by a factor of 3.  The flexibility of other design considerations seems to allow that factor-of-3 design advantage to be applied where needed.  

Other ideas and thoughts:

Ground-source geothermal. Pumping 1 gallon per minute from a well can deliver 10,000 BTU/hr (at the yearly mean temperature ~45 deg.F).  This alone isn't warm enough but could reduce the heating requirements significantly, or reduce the insulation and heating requirements needed for passive solar thermal heating.

So if we combine all of our best ideas it might make a good design.  Maybe take UV-stabilized greenhouse film, make it into bubble-wrap.  Take 2 layers of this with a channel in between that is a few inches thick.  Blow soap bubbles in between.  Maybe use a bit of heat from the 45 deg.F well-water to blow the air bubbles.  Make the greenhouse with a strip of this say 1-2 feet wide.  The total amount of plastic would be about the same as a cheap tunnel, just quadrupled up for better insulation, and relying on solar concentration to bounce the light through the smaller window.  Use reflectors, whether polished aluminum sheet metal flashing, or aluminized mylar or mirrors to concentrate the light by say 3 fold.  Put this on the south side of a building with a Trombe wall.  Add some grow lights for a bit of extra light and warmth....  Something like this might work in a colder climate without breaking the bank.  

Some folks are using solar pool covers instead of plastic glazing.  It's basically extremely heavy gauge bubble wrap.  They put it bubble side in so that snow will still slide off.  That plus a layer of poly with a fan inflating the space between them could be the ticket.  

Mike J., good thought.  I'm familiar with those.  They're UV-stabilized and very durable. The clear (colorless) ones may be better since the usual blue ones might block red light.  Plus, all the kids might ask to swim in your greenhouse.  :p  

Yup, people use the clear ones.  Since they aren't rolled up like they are on a pool, they should last a long time.  One person had over ten years on their cover so far.

It would be nice to know the actual R value and the light transmittance of them.  I called one manufacturer and they had no idea since it wasn't designed for that purpose...

Plant stands made of concrete with pipes built in. A boiler heats the water flowing thru the pipes. The boiler heats the space and the mass in the plant stands moderates the heat. A thermostat would have to control the water temp in the pipe/stands.

It might be simpler to put the money into a temperature controlled truck and haul in your vittals?

R value for a window is roughly +1 per glazing.  So single wall is R-1 , double wall is R-2, etc.  Most air-trapping insulation is about R-4 per inch give or take 1 depending on how well it traps the air and how small the air pockets are.  So large soap bubbles are probably R-3 per inch, and small ones R-4 per inch.

Unfortunately, light transmittance even of crystal clear materials is only about 93%, because there is a 3.5% reflection loss at each surface.  This is why soap suds are white.  So if I'm not mistaken, 5 layers is (0.93)^5 = 70% transmittance.  So with larger bubbles, maybe this could be in the ballpark of a R-7 window?  I know it seems like a lot to lose one-third of the light, but I was figuring on making up for it with concentrating reflectors.  Anyway, maybe the size and amount of bubbles could be adjusted by the fan speed?  That would be amazing if it worked because the insulation could be totally adjustable (and automated) and in theory would cost very little for materials.  Could be too good to be true though.  Might be better to just blow bubbles in melted plastic once and be done with it.  Throw some zinc oxide in for UV resistance...  Or maybe just go with the pool cover... I don't know...  Hmm, interesting to think about...

Anyway, it's not too difficult to make a simple model (say on a spreadsheet) and estimate the amount of insulation and thermal mass needed for a given daily temperature swing and amount of glazing area.  

wow lots of good advice here.
if you are not using glass or solid poly plastic the two layer greenhouse plastic works real well it can last 15 or more years and is excellent insulator as far as greenhousees go

There is a lot to like about the two layer greenhouses; they are very affordable.  But the insulation value on them is R-2.  This may keep cold-tolerant plants alive,  but you're not going to be growing all that much in the winter, maybe some leafy greens.